Near-field multiple-input-multiple-output (NFMIMO) technology has emerged as a promising candidate for spectral efficiency enhancement in 6G. However, conventional centralized antenna arrays confines NF effects and performance gains to very short ranges. To overcome this challenge, this work focuses on a practical antenna deployment strategy termed widely spaced multi-panel. The spatial degrees-of-freedom bounds of the effective channel are presented. A novel codebook is proposed for channel state information acquisition, featuring high performance, low overhead, and implicit near-field exploitation. Simulation results at mid-band frequencies demonstrate considerable spectral efficiency gains. Remarkably, the beneficial range is up to 80 meters under line-of-sight conditions and nearly 99% of users in designated hotspot areas attain full-rank transmission. Additionally, the proposed codebook achieves performance comparable to standardized high-resolution codebooks, while requiring only one-tenth the feedback overhead, highlighting its efficiency and practicality.

Integrated sensing and communication (ISAC) is an important new research topic for 5th-generation communicationAdvanced (5G-A) and 6th-generation (6G) mobile communication systems, where waveform design should be studied. Orthogonal frequency division multiplexing (OFDM) is anticipated to be more favorable in the future 5G-A and 6G ISAC systems due to its good 5G New Radio (NR) backward compatibility. Different resource multiplexing methods, including time-division multiplexing (TDM), frequency-division multiplexing (FDM), and codedivision multiplexing (CDM), for sensing and communication can be assumed in the OFDM-based ISAC system. Due to the low peak-to-average power ratio (PAPR) and good ambiguity function properties, chirp sequence can be used for sensing in the OFDM-based ISAC system. This paper proposes a unified chirp sequence generation method for OFDM-based ISAC system. By parameter configuration, different resource multiplexing methods of sensing and communication, including TDM, FDM, and CDM, can be realized based on the proposed method. Evaluation results demonstrate that the proposed chirp sensing sequence achieves better PAPR performance for both TDM and FDM methods and lower sidelobe level of the ambiguity function for TDM compared to OFDM with Zadoff-Chu (ZC) sequence, which indicates that the proposed chirp sequence with TDM and FDM can be considered as a suitable sensing sequence for scenarios with coverage limitations.

Digital twin technology, envisioned as a cornerstone of sixth-generation (6G) networks, demands robust capabilities for dynamic and real-time environmental reconstruction. Wireless sensing plays a pivotal role in enabling digital twin systems, yet significant challenges persist. High-resolution wireless imaging typically requires large array apertures, which are often accompanied by prohibitive deployment costs and implementation complexity. This paper addresses these limitations by proposing a user equipment (UE)-assisted synthetic aperture radar (SAR) imaging scheme. The proposed approach exploits UE mobility to emulate a synthetic aperture, enabling high-resolution imaging while reusing existing wireless communication infrastructure. Advanced image reconstruction algorithms, including back projection (BP) and Fourier transform (FT)-based methods, are employed to process echo signals and accurately reconstruct target scenes. To validate the feasibility of the scheme under realistic channel conditions, typical configurations aligned with current network deployments are adopted, and ray-tracing-based channel modeling is applied to representative local areas within urban environments such as Munich and Paris. Preliminary simulation results show that, with a carrier frequency of 3.5 GHz and a bandwidth of 100 MHz, the proposed scheme achieves meter-level resolution in reconstructing building envelopes and capturing distinct structural details. These findings demonstrate the feasibility, scalability, and cost-effectiveness of the UE-assisted SAR imaging framework, paving the way for practical wireless sensing and digital twin applications in future 6 G urban environments.

Integrated sensing and communication (ISAC) is one of the most important research topics towards 5th generation (5G)-Advanced and 6th generation (6G) mobile communications. Diverse use cases could be supported from the analysis of measurement results obtained by sensing. Reference signal (RS) is necessary for the measurements, which should be designed to provide good sensing performance at low overhead. Positioning RS (PRS) defined for user equipment (UE) positioning in 5G new radio (NR) is a good starting point for sensing, whose performance on estimation accuracy and maximum unambiguous performance are validated to be good. However, the overhead of NR PRS is too high, limiting its usage for sensing. In this paper, inspired by the multiple pulse repetition frequency (PRF) method in radar systems, we propose a new sensing RS pattern composed by multiple comb RSs with co-prime comb sizes. On the one hand, the comb RS pattern has good backward compatibility with communication systems. On the other hand, the co-prime comb sizes can effectively eliminate the distance ambiguity caused by the comb RS pattern. The evaluation results show that proposed sensing RS pattern could achieve same maximum unambiguous distance with NR PRS but reduce the overhead from 7.14% to 2.21%.

Reconfigurable intelligent surfaces (RISs) for coverage enhancement in millimeter-wave bands are investigated through system-level simulations. A large-scale RIS model is developed to approximate the forwarding beam pattern in both far-field and near-field conditions, and a pragmatic low-complexity RIS assignment procedure is considered. The impacts of RIS in regular cellular scenarios and those with blind-spot areas are compared. This study identifies that a substantial portion of users benefit from RIS deployment; RIS-induced interference is generally negligible, except in rare cases; and strategically deploying small- to medium-sized RISs effectively addresses blind-spot coverage.

Integrated sensing and communication (ISAC) is an important new research topic for sixth-generation (6 G), where the waveform design should be studied. The chirp-based waveform is widely used in radar systems due to its low peak-to-average power ratio (PAPR) and good autocorrelation properties and is a candidate waveform for ISAC systems. To realize the chirpbased waveform in ISAC systems without additional hardware cost, an orthogonal frequency division multiplexing (OFDM) compatible sensing waveform is proposed in this paper. Firstly, a frequency domain processing (FDP) module is proposed to realize chirp signal under the OFDM transmitter structure. Then, to satisfy different communication requirements, the sensing symbol pattern is designed on top of the FDP module to realize multiple orthogonal chirps with flexible patterns. Evaluation results demonstrate that the proposed chirp-based waveform based on the unified waveform structure can achieve 4 dB and 13 dB signal-to-noise ratio (SNR) gain over traditional OFDM waveform with Zadoff-Chu (ZC) sequence or random payloads under the same range and velocity estimation accuracy.

Sub-Terahertz (sub-THz) communication is a potential technology to enable 6 G wireless communication with extreme experience due to its rich spectrum resources. Working on sub-THz frequency, radio frequency (RF) impairments including phase noise (PN) and non-linear power amplifier (PA) become more severe and will heavily degrade communication performance. Considering the new characteristics in such high frequency, various waveform schemes have been proposed to achieve better performance under RF impairments, among which the enhanced waveforms based on discrete Fourier transform spreading orthogonal frequency diversion (DFT-s-OFDM) are highly regarded for the low peak to average power ratio (PAPR) characteristic. This paper focuses on 6 G candidate waveforms of orthogonal frequency division multiplexing with cyclic prefix (CP-OFDM), DFT-s-OFDM and enhanced DFT-s-OFDM including unified non-orthogonal waveform (uNOW), DFT-s-OFDM with frequency-domain spectral shaping (DFT-sOFDM with FDSS), and DFT-s-OFDM with FDSS and spectral extension (DFT-s-OFDM with FDSS and SE). In this paper, 6G candidate waveforms are evaluated for different sub-carrier spacing (SCS) considering PN and non-linear PA in sub-THz. Specially, waveforms are compared and analyzed from various aspects including PN robustness without compensation, PN compensation performance based on phase tracking reference signal (PT-RS), and output power back-off (OBO). In addition, under non-linear PA, this work has in-depth discussions on the changing trend of each RF requirement as OBO increases for different 6 G candidate waveforms. The results indicate that uNOW outperforms other waveforms for BLER performance considering both PN compensation and OBO, making it the promising 6 G candidate waveform.

To meet the stringent spectrum efficiency requirements of International Mobile Telecommunications (IMT)-2030 (6 G), it is essential to explore the application potential of higher frequency bands and extremely large antenna arrays (ELAA). This shift introduces near-field effects, where electromagnetic waves deviate from the far-field plane wavefront assumption and must be modeled as spherical waves under certain distance conditions. Consequently, introducing beam focusing codebooks for the near-field is crucial. The typical fifth-generation New Radio (5G NR) high-precision Type II codebook, constructed from a two-dimensional discrete Fourier transform (DFT)-based grid of beams, enables the channel state information (CSI) feedback of a set of orthogonal beam selections along with corresponding amplitude and phase information. However, it is not optimized for near-field characteristics. To satisfy the 3rd Generation Partnership Project (3GPP) Releases’ requirement for backward compatibility, we introduce the plane wave expansion (PWE) method of focused beams. This method allows for the generation of focused beams through the weighted superposition of multiple DFT beams. Based on this approach, we define a set of additional feedback contents and formats within the NR Type II codebook framework, enabling a more accurate representation of the near-field line-of-sight (LoS) CSI for feedback. Numerical simulation results substantiate the performance of the proposed NR Type II codebook enhancement scheme, achieving a performance within 2dB of the optimal beam focusing scheme with appropriate configuration.

The validity of the far-field assumption is being challenged by emerging 6 G technical trends characterized by higher carrier frequencies, larger antenna apertures, and denser networking. This paper investigates near-field line-ofsight multiple-input-multiple-output (MIMO) with a focus on the practically desirable implementation using distributed antennas. Firstly, the feasibility conditions are addressed taking into account the impact of spatial aliasing. The aliasing-free and the partially aliased cases are distinguished, where the achievable spatial degrees of freedom are shown to be subject to array apertures and antenna spacings, respectively. Secondly, a codebook-based precoding for channel feedback is proposed exploiting the inherent structure of the channel. Finally, numerical simulations show that the proposed codebook achieves a performance comparable to the standardized high resolution codebook in 5G NR, but with only one-tenth of the feedback overhead.

The sixth-generation wireless system (6G) is poised to incorporate sensing capability with communication, serving as a foundational infrastructure to facilitate emerging usage scenarios. Meanwhile, reconfigurable intelligent surface (RIS), as an innovative network deployment device, has emerged as a promising technology within the 6G radio network topology, offering a large array size at low cost and power consumption. This paper introduces RIS to further enhance high-resolution wireless imaging alongside typical communication functionalities, implemented by a ring-type codebook (RTC) suitable for near-field beam focusing and a back projection (BP) algorithm for real-aperture imaging. The near-field beam generated using RTC can offer additional range-dimension resolution capability and enable the system to dynamically adjust the granularity of beam scanning based on the different use case requirements. Numerical simulation results are provided to substantiate the viability of the proposed RIS-aided wireless imaging scheme, showcasing its ability to facilitate centimeter-level two-dimensional (2D) wireless imaging and provide additional range-dimension resolution capability for a narrowband system.

To provide sensing capability in communication systems, integrated sensing and communication (ISAC) is an important research topic towards 5GA and 6G. Many sensing services and results are related to the estimated angles of targets. Virtual aperture (VA) technique in conventional radar systems can improve sensing performances of angle by special designs of transmit and receive antennas, which is different from those in communication systems. To exploit VA in ISAC systems without additional hardware cost, a unified antenna structure and a new VA scheme under the structure are proposed. Firstly, the antennas with half-wavelength spacing in communication systems are divided into multiple transmit antenna groups and one receive antenna group for sensing. Then, time-domain multiplexing (TDM)-based orthogonal signals are considered for multiple transmit antenna groups to realize VA. Beamforming is exploited in each transmit antenna group for beamforming gain. The estimation algorithm is also proposed to obtain expected larger VA. Thanks to the unified antenna structure, the total antennas can be flexibly exploited for either beamforming gain during transmission period or larger virtual aperture during estimation period. Results show that proposed VA scheme under unified antenna structure achieves 5 dB signal-to-noise-ratio (SNR) gain at$0.1^{\circ}$angle root-mean-square-error (RMSE) and improves angle RMSE from$0.1^{\circ}$to$0.03^{\circ}$at 0 dB SNR than conventional VA.

The reconfigurable intelligent surface (RIS), as an innovative device for network deployment, has emerged as a promising technique within the sixth generation (6G) radio network topology. In pursuit of achieving higher beamforming gain within the near-field region due to the high operation frequency and large array size, near-field beamforming schemes such as the ring-type codebook (RTC) have been proposed. Nevertheless, the user equipment (UE)-specific narrow beams generated by the existing schemes are not suitable for RIS to effectively forward control channels, which may require wider beams to cover a group of UEs. In this paper, the wide beam generation scheme based on the RTC codeword selection mechanism is proposed. While combined with the proposed beam shaping methods, RIS can dynamically switch between wide and narrow beams using RTC, providing flexibility in adapting to the forwarding of both data and control channels.

To provide extreme coverage towards 5G Evolution and 6G, high altitude platform station (HAPS) is an important component in non-terrestrial network (NTN) due to its deployment flexibility. Multi-beam transmission should be used in HAPS for high throughput, where each beam covers one cell with multiple users. However, intra-beam and inter-beam interference will be severe with traditional multi-user multiplexing schemes, which may affect the HAPS performance. In this paper, an enhanced non-orthogonal multiple access (eNOMA) scheme is firstly proposed to overcome intra- and inter-beam interference. Then a unified multi-user multiplexing scheme is further proposed to dynamically adapt between spatial-division multiple access (SDMA), non-orthogonal multiple access (NOMA) and eNOMA for various interference scenarios. The evaluation results show that the proposed scheme achieves maximum 48% spectrum efficiency (SE) gain over existing schemes under three user configuration.

The sixth generation (6G) wireless communication system puts forward higher requirements for positioning performance to achieve centimeter-level (cm-level) high-precision positioning performance and integration of sensing and communication (ISAC). The received signal strength (RSS) fingerprinting-based positioning technique is a widely used scheme to locate user equipment (UE) in cellular networks. However, traditional RSS fingerprinting-based scheme do not consider the ISAC performance, and the positioning accuracy is limited since the RSS values of neighboring locations may be similar. In this paper, we propose a reconfigurable intelligent surface (RIS)-aided positioning scheme that exploits the diversity provided by RIS to improve the accuracy of the RSS fingerprinting-based indoor positioning, which requires the RSS from only one access point (AP). Furthermore, by using the proposed near-field multi-focal focusing method, it can ensure the communication performance of the target UE and support the high-precision positioning of other UEs, simultaneously. Simulation results confirm the effectiveness of our proposed scheme, achieving cm-level positioning performance at a cost of reducing the average throughput by only about 10%.

The sixth generation (6G) wireless communication system requires extreme capacity and coverage, which can be achieved through the introduction of higher frequency and ultra-large-scale reconfigurable intelligent surface (RIS) array. This also enables the expansion of the near-field range to tens or even hundreds of meters. However, near-field beamforming presents different challenges than far-field beamforming, as it involves two parameters (angle and distance) rather than just angle. This requires a larger number of codewords for near-field beams and frequent beam switching due to the narrow beamwidth. To address these challenges, we propose an enhancement to the ring-type codebook (RTC) using non-uniform quantization of focal length and beamwidth control methods. Our proposed methods effectively reduce the beam management overhead and SNR fluctuation by 34% and 4dB respectively, while also improving the robustness of the beams. Overall, our proposed enhancement to the RTC can significantly improve the performance of near-field beamforming in the 6G wireless communication systems, enabling them to satisfy the extreme capacity and coverage requirements of the future.

The 6th generation mobile networks require large bandwidth on sub-Terahertz bands to provide more than 100 Gbps throughput. To provide enough beamforming gain to combat the path loss on high-frequency bands, large-scale antenna arrays should be used, which generate narrow and directive beams. It may require hundreds or thousands of beams to cover a cell and introduce challenges for the beam alignment during the initial access stage. As a result, both the overhead and latency of beam sweeping may become unacceptable. We propose the Hash-based beam alignment method to address this problem, including a Hash-based beam transmission scheme for the synchronization signal block (SSB) transmissions and a corresponding UE beam pair selection method. It utilizes multiple transceivers to transmit multiple beams simultaneously, where the beam order follows a pattern generated by a Hash function. Considering UE may only receive discrete SSBs randomly, we design a weighted voting algorithm for UE to fast select the optimal beam pair for the following data transmissions. Simulations demonstrate that the proposed methods can reduce about 50% of the beam alignment latency or find a better beam pair with about 3 dB higher beamforming gain given a limited searching time.

5G New Radio (NR) takes multi-carrier waveform orthogonal frequency division multiplexing with cyclic prefix (CP-OFDM) with high spectral efficiency (SE) as the core waveform, and only supports single-carrier waveform discrete Fourier transform spreading OFDM (DFT-s-OFDM) with high power efficiency (PE) when uplink coverage is limited. 5G Evolution (5GE) and 6G have diversified scenarios, including high-frequency bands and integrated space-terrestrial systems, in which the non-linearity of power amplifiers (PA) restricts the system design. Therefore, high PE or low peak to average power ratio (PAPR) becomes one of the most significant key performance indicators (KPI) for 5GE and 6G waveform design, in addition to the high SE requirement. In this paper, we propose a unified non-orthogonal waveform (uNOW) based on DFT-s-OFDM enhancement with time-domain compression and expansion (TD-CE) as well as frequency-domain spectral shaping (FDSS) to improve PAPR and SE performance, simultaneously. Firstly, we describe the uNOW transmitter structure and two receiver structures, i.e., the linear minimum mean squared error (LMMSE) time-domain equalization (TDE) for time-domain compression and enhanced LMMSE frequency-domain equalization (FDE) for time-domain expansion. Then, the basic principle of uNOW is analyzed, i.e., how TD-CE and FDSS affect the time-domain pulse-shapes to reduce PAPR. Simulation results show that when only TD-CE or FDSS is considered, both PAPR gain and throughput gain are achieved compared with DFT-s-OFDM. When considering both TD-CE and FDSS, the PAPR and throughput gain achieved by uNOW can be significantly improved. Therefore, uNOW can be considered as a promising waveform candidate for 5GE and 6G.

Reconfigurable intelligent surface (RIS) is a research hotspot for the fifth generation evolution (5GE) and the sixth generation (6G) wireless communication systems to extend high-frequency coverage. Due to the increase of the carrier frequency of the wireless communication system and the large scale of the RIS array, the near-field region is significantly expanded. Using the plane wave assumption-based discrete Fourier transform (DFT) codebook for the RIS near-field beamforming may result in a large signal-to-noise ratio (SNR) loss. Furthermore, for practical RIS with a large number of elements, it is more cost-effective to implement only discrete phase shifts with a small number of control bits for each element, for example, 1-bit for two-level (0 or$\pi$) phase shifts, which generates the mirror grating lobe with the same power as the main lobe and results in power leakage. In this paper, a ring-type codebook is proposed based on the Fresnel principle, which can achieve SNR enhancement as well as mirror grating lobe suppression for 1-bit beamforming in near-field region. Compared with DFT codebook, the proposed codebook improves the average throughput by 14% and achieves a main grating lobe suppression ratio of about 17.9dB.

Beamformer design for large-dimension reconfigurable intelligent surface (RIS) is investigated. The challenge arises from the expanded near-field of RIS where its beam-forming gain is, in general, distance-dependent due to near-field diffraction. To gain more insights into this effect, we first analyze the performance of conventional collimated beamforming using Fresnel integrals and compare it with that of focused beamforming. Then, a two-step beamforming scheme is proposed. The principle of the proposed scheme is to divide the RIS into multiple subarrays followed by steering the sub-beams of the individual subarrays to focus. Providing proper subarray size, the proposed scheme retains the low complexity of collimated beamforming and achieves a near-optimal performance, as shown both analytically and numerically.

Non-terrestrial network (NTN) is an important technique to provide extreme coverage towards 5G advanced and 6G. High-altitude-platform-station (HAPS), as one key factor in NTN, is attracting a lot of attentions. To achieve better resource utilization and performance, a unified design for integrated HAPS-Terrestrial networks is necessary, where the interference among these two systems may be important. Current methods usually consider fixed resource allocation among the two systems without considering the distribution of traffic load and hence have low resource utilization. In practice, the traffic load may change dynamically or semi-statically due to the wide coverage in HAPS, which should be considered in the design of integrated HAPS-terrestrial systems. In this paper, we proposed an interference coordination method based on the distribution of traffic load as well as the deployment of HAPS and terrestrial networks. The evaluation results show that the proposed method achieves higher throughput than existing fixed resource allocation method.

Non-terrestrial network (NTN) is an important technique to provide extreme coverage. Current fifth-generation (5G) new radio (NR) techniques mainly focus on the terrestrial networks. How to support NTN by 5G NR techniques is studied in 3rd generation partnership project (3GPP) since Release 15. After extensive studies, the first release of NR NTN is finished in Release 17. Uplink time synchronization is an important topic for both terrestrial networks and NTN, without which the interference among multiple users as well as the uplink and downlink misalignment will happen. Different from 5G NR, open-loop timing advance (TA) including common TA and UE-specific TA is firstly introduced in NR NTN. This paper introduces the latest progress in Release 17 and gives the solutions, procedures, and evaluation results for uplink synchronization in NR NTN.

Non-linearity of power amplifiers (PA) is the restriction of waveform design for 5G Evolution (5GE) and 6G in high frequency bands with large bandwidth. The low peak to average power ratio (PAPR) single carrier waveform discrete fourier transform spreading orthogonal frequency division multiplexing (DFT-s-OFDM) can be used as a candidate waveform for 5GE and 6G. To satisfy the extremely high data rate requirement in 6G, how to keep the low PAPR of single carrier waveform and improve its spectral efficiency (SE) will be the challenge. In our previous study, a novel non-orthogonal waveform (NOW) design was proposed by combining DFT-s-OFDM with FTN to improve SE with low PAPR. However, due to the spectrum extension caused by FTN, large out of band emission (OOBE) and signal-to-noise ratio (SNR) loss were observed. In this paper, we propose an enhanced non-orthogonal waveform (eNOW) to improve the SE of DFT-s-OFDM by designing the frequency domain power allocation to efficiently suppress OOBE and reduce SNR loss. Simulation results demonstrate that the OOBE and SNR loss of eNOW can be significantly reduced than that of NOW by design the frequency domain power allocation. With eNOW scheme, OOBE, PAPR and throughput gain can be achieved, simultaneously.

High frequency bands with large bandwidth (e.g., millimeter-wave and terahertz frequencies) is promising for 5G Evolution and 6G, but the system design is restricted by the non-linearity of power amplifier (PA). Multi-carrier waveform of orthogonal frequency division multiplexing with cyclic prefix (CP-OFDM) may not be suitable for 5G Evolution and 6G due to its high peak to average power ratio (PAPR). Single carrier waveform represented by discrete fourier transform spreading OFDM (DFT-s-OFDM) with low PAPR could be a promising candidate waveform for 5G Evolution and 6G. Considering the extreme high data rate requirement of 5G Evolution and 6G, how to keep the low PAPR of DFT-s-OFDM and improve its spectral efficiency (SE) will be the challenge especially when high-order modulation is not applicable. In this paper, we propose a non-orthogonal waveform (NOW) to improve the SE of DFT-s-OFDM and derive the optimal time domain compression factor in terms of SNR based lossless compression. Furthermore, we investigate the impact of different compression factors to the system performance of throughput and PAPR. Simulation results demonstrate that the proposed scheme can achieve both throughput gain and PAPR gain over the traditional orthogonal waveforms DFT-s-OFDM and CP-OFDM, by flexibly adjusting the compression factor. When the compression factor is smaller than the optimal value, SNR loss to different extent may be observed.

Due to the availability of wide bandwidth, high frequency band is promising for 5G evolution and 6G. With the increase of antenna ports and bandwidth, existing subband-level precoding consumes high feedback overhead and restricts the precoding granularity. However, channel sparsity can be observed in transformed angular-delay domain in mmWave systems with massive antennas. In this letter, to utilize the sparsity, we propose a transform domain precoding (TDP) to design precoder and feedback. Realistic factors including hybrid beamforming, frequency domain windowing and power allocation are analyzed and evaluated by link-level simulations. Results show that 60%~89% overhead reduction and 2.38%~21% spectrum efficiency (SE) enhancement can be achieved by TDP.

Uplink non-orthogonal multiple access (NOMA) is a promising technique to meet the requirements of the fifth generation (5G) and beyond systems. Various NOMA schemes have been proposed in both academia and industry. However, most existing schemes assume equal average received power, which limits the performance. We propose three enhancements of uplink NOMA to achieve the requirements of massive connectivity and high reliability in 5G, where unequal average received power is exploited as part of the multiple access signature. First, the optimal sequences targeting to generalized Welch-bound equality (GWBE) are obtained for unequal average received power. Then user grouping with multi-level received powers is proposed for better successive interference cancellation (SIC) at the receiver. Finally, sequence grouping based on the cross-correlation properties of sequences is proposed to reduce inter- and intra-group interference. Simulation results show that by incorporating multi-level received powers and sequence grouping into existing NOMA schemes, for an NOMA system with 400% overloading and fixed signature allocation, 3 dB and 10 dB signal-to-noise ratio (SNR) gains at 0.1 block error rate (BLER) target can be achieved compared with existing NOMA schemes and orthogonal multiple access (OMA), respectively. Besides, 0.01 BLER target can be achieved while an error floor exists in existing NOMA schemes. Under random sequence selection, collision probability is reduced by multi-level powers. In addition, GWBE sequences achieve lower BLER than existing sequences and the gain is large especially for low BLER requirements. This shows that the proposed scheme can support larger connectivity and higher reliability.

Three-dimensional (3D) MIMO is a promising enhancement of MIMO technology to improve system capacity by exploiting the spatial characteristics in both azimuth dimension and elevation dimension. As one of the key aspects in practical systems, the channel state information-reference signal (CSI-RS) design specific to future LTE-Advanced are discussed, and a precoding scheme of CSI-RS considering elevation beamforming is proposed. It is shown that the proposed scheme has the advantages of better performance, small standardization efforts due to its reusability of existing codebook and CSI-RS configurations, and transparency to the legacy UEs.

In order to support a higher peak data rate, carrier aggregation (CA) is implemented into Long Term Evolution Advanced (LTE-A) systems to provide sufficient bandwidth for data transmission. In Time Division Duplexing (TDD) CA systems, aggregation of Component Carriers (CCs) with different TDD configurations is necessary due to the coexistence with legacy TDD systems. To achieve the peak data rate of Secondary Cell (SCell) in case of cross-carrier scheduling, the Physical Downlink Shared CHannel (PDSCH) hybrid automatic repeat request (HARQ) timing of SCell should follow its own PDSCH HARQ timing. Meanwhile, Primary Cell (PCell) still uses its own PDSCH HARQ timing. However, using different PDSCH HARQ timing between PCell and SCell may lead to Physical Uplink Control CHannel (PUCCH) resource mapping collision, which sacrifices downlink (DL) peak data rate and increases transmission latency for the reason of PDSCH HARQ feedback failure. In this paper, a new implicit PUCCH resource mapping method is proposed to avoid PUCCH collision in inter-band TDD CA systems. ENB and user equipment (UE) re-allocate PUCCH resources for each DL subframe on SCell. By using this proposal, PUCCH collision for cross-carrier scheduling in TDD CA systems can be completely eliminated without introducing significant PUCCH overhead increase.

In Long Term Evolution- Advanced (LTE-A) Release 11, an enhanced physical downlink control channel (ePDCCH)was introduced to increase the capacity of the physical downlink control channel (PDCCH), in order to support Multiple User-Multiple Input Multiple Output (MU-MIMO) and interference mitigation in heterogeneous networks. Design of the search space, in which control information is mapped, for the ePDCCH is an essential issue. Conventional search space designs suffer from low resource utilization efficiency and a high blocking probability. This paper presents a novel search space design. The proposed design introduces proper resource division in conjunction with a search space split. Simulation results show that the proposed design effectively improves the resource utilization efficiency and reduces the blocking probability.

This paper focuses on investigating the search space design of Enhanced Physical Downlink Control CHannel (EPD-CCH), which is introduced in Long Term Evolution-Advanced (LTE-A) system to increase the capacity of the downlink control channel. Since EPDCCH is frequency-division-multiplexed with downlink data channel, resource utilization efficiency is a very important issue in addition to blocking probability. This paper proposes the innovative methods of locating the search spaces of EPDCCH with different transmission schemes, i.e. distributed transmission and localized transmission, considering the UE complexity, the capacity of control channel and the impacts on Physical Downlink Shared CHannel (PDSCH). By simulation results, it is proven that the proposed methods could get higher resource utilization and lower blocking probability while maintain the same UE complexity in blind decoding.

Carrier aggregation was introduced to Long Term Evolution Advanced in order to improve the performance especially in terms of a higher peak data rate. When employed, sets of user equipment will have a set of serving cells, which includes a Primary cell (PCell) and Secondary cells (SCells). For inter evolved Node B (eNB) handover, target eNB is selected only consider the channel condition of the PCell. Thus, a suitable set of serving cells cannot be selected. In order to address this problem, this paper proposes three handover methods considering channel conditions for multiple cells. Methods 1 and 2 adjust the handover margin according to the difference in channel conditions between the PCell and SCells for the serving eNB. Method 3 decides handover based on equivalent channel conditions for the PCell and SCells in both the serving eNB and neighboring eNBs. Simulation results show that the proposed methods provide higher cell-edge throughput and a much smoother handover at the cost of a slightly increased number of handovers.

Carrier aggregation (CA) has been proposed within 3GPP to aggregate multiple component carriers (CCs) in LTE-Advanced system to achieve higher peak data rate. In this paper, we propose CA between FDD and TDD carriers in order to utilize FDD and TDD spectrums more efficiently. By comparing the differences in control channel between LTE FDD and TDD specifications, three problems in the case of CA between FDD and TDD carriers are identified, and we then propose the respective solutions and analyze their pros and cons. It is shown that CA between FDD and TDD carriers can provide system flexibility and performance benefits for the operators which own both FDD and TDD spectrums.

In LTE-Advanced, macro-pico heterogeneous networks (HetNet) are extensively discussed. Cell range expansion (CRE) and inter-cell interference coordination (ICIC) are introduced to improve the HetNet system performance. In this paper, semi-static ICIC and dynamic ICIC methods are discussed, and the problems facing conventional dynamic ICIC methods are analyzed and investigated. Joint decision and multiple feedback schemes are proposed to enhance the system performance through appropriate selection of normal/mute transmitting status and accurate scheduling for dynamic ICIC. Simulation results show that the proposed schemes achieve better average system performance and edge user performance than the semi-static ICIC and conventional dynamic ICIC methods, even when a reduced feedback method is used in the proposed schemes. The performance gain is as high as 9% for the average throughput and 30% for the edge user throughput with a proper bias and muting ratio.

This paper focuses on investigating appropriate management of Component Carriers(CCs) in carrier aggregation(CA) system, which is identified as one of the most distinct features of 4G systems including Long Term Evolution Advanced (LTE-A). An LTE-A user equipment (UE) is allowed to concurrently utilize multiple carriers, which leads to scalable increase in user throughput. However, in certain circumstances, aggregating entire available carriers for a UE is not meaningful due to probably low channel quality or high traffic load in some of the CCs. Therefore, how to make good use of multiple carriers in real deployment scenarios is an important issue. Based on the analysis of resource allocation across multiple carriers in Layer-2, two CC management methods in Layer 3 are proposed. The proposed method is shown by simulation results to be effective in providing comparable user throughput with lower implementation complexity, as compared to solely ingenious resource allocation.

3rd Generation Partnership Project (3GPP) is specifying Long Term Evolution Advanced (LTE-A) targeting for higher average cell and cell-edge spectrum efficiency over LTE, and multi-user MIMO (MU-MIMO) is identified as a key technique to reach the International Mobile Telecommunication Advanced (IMT-A) performance requirements. In this paper, we present the investigation on the MU-MIMO enhancement issues in LTE-A mainly from the viewpoint of MU-MIMO transparency, i.e., necessity of indication of co-scheduled UEs in the same resource block, and demodulation reference signal (DM-RS) configuration. Besides qualitative analysis from various system impacts, e.g., signaling overhead, scheduling flexibility and channel estimation accuracy, extensive numerical results are provided by both link-level and system-level simulations. In addition, we further propose an adaptive data mapping scheme for transparent MU-MIMO to improve the channel estimation accuracy. Moreover, a novel system-level simulation methodology taking into account dynamic change of channel estimation error is provided in this paper. Finally, based on the investigation results, we propose transparent MU-MIMO with quasi-orthogonal DM-RS configuration in LTE-A.

Due to the limited feedback amount, the codebook and feedback design is very important. In order to achieve better tradeoff between overhead and performance, the double codebook structures are proposed and discussed, where a final precoder W is determined by two precoders W1and W2, which are selected from two different codebooks. In this paper, we give an overview of several double codebook structures and evaluate them based on system level simulations (SLS). We consider two frameworks to determine a final precoder: W1W2and W2W1where W1and W2track long-term/wideband and short term/subband channel properties, respectively. It is shown that under 4Tx assumption, double codebooks achieve gains compared with Rel.8 codebook when considering the multi-user (MU) MIMO and do not suffer in the case of single-user (SU) MIMO. Both W1W2and W2W1provide nearly the same gains over that for the Rel. 8 codebook with uniform linear array (ULA) deployment. Furthermore, W1W2framework performs better than W2W1framework with cross-polarized array (CPA) deployment.

This paper focuses on coordinated scheduling to combat uplink inter-cell interference for LTE/LTE-Advanced. In OFDM system, inter-cell interference is the dominant interference which limits system capacity, especially the performance of cell edge users. Meanwhile inter-cell interference in OFDM system has an inherent characteristic of interference fluctuation, which reduces the efficiency of channel dependent scheduling and the accuracy of MCS selection. In conventional inter-cell interference coordination methods, such as path-loss based coordinated scheduling, fractional frequency reuse and OI (overload indicator) based inter-cell power control, either the estimation of the interference is not accurate or the problem of interference fluctuation is not solved. To combat these problems, we propose a coordinated scheduling method by utilizing multiple OI thresholds. In our method, different OI thresholds are previously determined on different resource blocks, having a complementary pattern among neighbor cells. Based on the exchanged OI information among neighbor cells, one user can be scheduled to an appropriate resource block, on which the neighbor cells can endure inter-cell interference caused by this user. So high interference generated users and high interference endurable users are scheduled on same resource block, resulting in more accurate inter-cell interference estimation and reduced interference fluctuation.

CoMP JP (Coordinated Multi-Point Joint Processing) is regarded as a promising technique to improve both cell edge user throughput and cell average user throughput in LTE-A downlink. CoMP JP can be categorized as CoMP JT (Joint Transmission) and COMP DCS (Dynamic Cell Selection). Compared with CoMP JT schemes, CoMP DCS provides a good trade-off between the transmission algorithm complexity, backhaul overhead and system performance. Conventional DCS scheme can benefit cell edge users by instantaneous selecting the best severed eNB for cell edge users based on limited signaling overhead in uplink radio interface. DCS with muting scheme further apply muting to the strongest neighbor cell for decreasing interference to cell edge users. However the system spectrum efficiency may be degraded, since frequency reuse one is not maintained and radio resource (resource block and power) may not be fully used. An enhanced DCS with muting method is proposed here to further improve the frequency and power efficiency, by using adaptive muting mode selection based on capacity calculation and flexible power allocation based on muting mode selection status. Performance evaluation shows the proposed algorithm can provide about 5.5% cell average throughput gain and about 10% cell edge throughput gain respectively compared with conventional DCS schemes.

Single-carrier frequency division multiple access (SC-FDMA) has been selected as the uplink access scheme in the UTRA Long Term Evolution (LTE) due to its low peak-to-average power ratio properties. Nevertheless, in order to achieve such a benefit, it requires a localized allocation of the resource blocks (RBs), which naturally imposes a severe constraint on the scheduler design. In this paper, two improved recursive maximum expansion scheduling algorithms for SC-FDMA are proposed. Compared with conventional recursive maximum expansion (RME) scheme in which UE can only expand the resource allocation on neighboring RBs with the highest metrics, in proposed improved recursive maximum expansion (IRME) scheme, higher degree of freedom in RB expansion is achieved by allowing RB expansion within certain ranking threshold. Moreover, to further increase the flexibility in resource allocation, multiple surviving paths are introduced in proposed improved tree-based recursive maximum expansion (ITRME) scheme. The simulation results show that, compared with conventional RME scheme, the proposed IRME scheme can exhibit the gain about 15% in terms of spectral efficiency with a linear increase of computational complexity, and the performance can be further improved to 18% with proposed ITRME scheme if further higher complexity is permitted.

Coordinated Multi-Point (CoMP) transmission is considered for Long Term Evolution - Advanced (LTE-A) as an effective method to improve both average and cell-edge throughput. However, most of current research on CoMP with Multi-User (MU)-MIMO are under ideal assumption. In this paper, we study CoMP under realistic conditions, including limited feedback and complexity. Taking both performance and complexity into consideration, a hybrid cooperating set selection scheme is proposed. Furthermore, Channel Quality Indicator (CQI) estimation based on instantaneous precoder and CQI update based on statistical distribtion are proposed to improve CQI accuracy for Modulation and Coding Scheme (MCS) selection. Finally, the system level simulation results of CoMP with MU-MIMO are provided, which show that, compared with single cell transmission, CoMP MU-MIMO with our proposed scheme can achieve considerable performance gain in terms of both average and cell-edge throughput.

Quality of Experience (QoE) reflects the customer's perception of service, and is extremely important for operators. In VoIP service, variety of factors will affect customer's QoE, e.g. source data quality and the transmission channel condition. In cellular systems, although higher level of source coding will improve QoE, more radio resource will be consumed and thus degradation of system capacity will be caused. On the other hand, lower level of modulation and coding can ensure the data delivery, but will lead to more consumption of radio resource. In this paper, to solve these problems in multi-user cellular systems, the source coding quality and channel coding levels are jointly considered according to customer's QoE. In the source and channel coding adaptation, customers with over qualified QoE will sacrifice some radio resource to improve the under guaranteed QoE of others, or increase the system capacity. The simulation shows that the algorithm can effectively balance the utility of limited radio resource and increase customer's satisfaction ratio and system capacity.

This paper investigates flexible spectrum usages for LTE network that consists of overlaying macrocell and femtocell. In such a networking environment, shared spectrum usage and partitioned spectrum usage are two options to be employed between two radio tiers. After recognizing the pros and cons of these two usages, we propose hybrid spectrum usage to take advantage of their merits. In our proposal, the femtocells embedded in macrocell are differentiated to inner and outer femtocells, which operate in partitioned spectrum usage and shared spectrum usage, respectively. Analysis and performance evaluation are given to illustrate and justify our proposed method on improving spectrum utilization for wireless overlay network.

This paper investigates spectrum resource coordination for the mixed macrocell and femtocell deployment. Though sharing spectrum provides larger bandwidth for both macrocell and femtocell, cross-tier interferences exist and become drawbacks for them to achieve higher capacities. After analyzing the interferences when using shared spectrum, we propose to employ hybrid spectrum sharing with respect to different femtocells such that the merit of sharing spectrum can be kept and the major interferences can be reduced. Then, we present a method to mitigate the remaining interference when employing hybrid spectrum sharing. Performance evaluation is given to justify our proposed approach on improving overall spectrum utilization.

In order to reduce handover delay caused by handover failure, radio resource is reserved for handover users not only at target cell, but also at prepared cells. This has been agreed in 3GPP LTE. However, these reserved resources cannot be assigned for other users until the finishing of the handover, which results in inefficiency of radio resource utilization. This paper presents a new resource reservation scheme for handover users to achieve fast handover failure recovery and efficient radio resource utilization. By proposed scheme, a resource pool at prepared cells is shared among multiple users, and the number of prepared cells is dynamically set according to the service type of users. Numerical analysis and simulation results show that the proposed scheme can improve successful handover probability and reduce handover delay.

The widespread deployment of Wireless LANs are underpinned by two most popular variants of IEEE 802.11 standards, 802.11b and 802.11g. The 802.11g extends 802.11b to support higher data rate via new OFDM-based physical layer, but both of them operate at 2.4GHz band and perform medium access with the same MAC protocol. When the 802.11b and 802.11g devices co-exist in the overlapping coverage area, the interoperability between them needs to be dealt with. The 802.11g defines a protection mechanism to avoid frame collisions between 802.11b and 802.11g devices. Since the legacy 802.11b devices cannot notice a busy channel by sensing the OFDM-modulated 802.11g frames, extra frames (e.g., CTS-to-self frames) are introduced for channel reservation in the protection mechanism. Those frames are understood by both 802.11b and 802.11g devices and transmitted first before sending 802.11g data frames. Nevertheless, they are viewed as overhead because they reduce the available medium resource for data delivery. In this paper, we propose an enhanced protection mechanism to reduce the number of extra frames used for channel reservation. The effectiveness of our proposed solution to improve system performance in the mixed 802.11b and 802.11g WLAN is verified by simulation.

In this paper we focus on efficient retransmission strategy for MBMS that employs hierarchical modulation. Hierarchical modulation offers a mechanism to enable the transmission of several data streams on one single frequency channel, with different transmission qualities, which requires more efficient HARQ strategy for MBMS. Constellation rearrangement and symbol bits rearrangement based HARQ are two kinds of methods to improve the efficiency of MBMS transmission from the perspective of constellation diversity. To further improve the performance, we proposed a novel retransmission method, including two constellations designed for transmission and retransmission, the transmitter algorithm to select the constellation to be used based on feedback information and the receiver algorithm to combine the retransmission packets. The merit of the proposal is to obtain the benefits from constellation rearrangement and symbol bits rearrangement simultaneously via once retransmission so as to save retransmission number, resulting in improved spectrum efficiency. Simulation results show our algorithm can provide about 21.5% cell throughput gain and 23% cell edge throughput gain than other HARQ methods.

We address the user selection scheme in multi-user wireless communication system, utilizing zero-forcing beamforming for spatial division multiple access. Recently, it has been shown that multi-user scheduling can help to achieve significant throughput gain in addition to the MIMO spatial multiplexing gain. And the capacity-greedy user selection algorithm can achieve near-optimal performance with much lower complexity compared with exhaustive search, due to reduced trial times of possible user combinations. However the computational complexity may be still too high for feasible applications, which depends on the system bandwidth, the number of active users and the number of antennas. In this paper, a novel capacity-greedy user selection scheme is proposed for the multi-user zero-forcing beamforming system, which involves low-complexity approximation for multi-user zero-forcing beamforming capacity. Simulation results verify that the proposed scheme can reduce the computational complexity of user selection greatly, with slightly performance loss.

For E-UTRA, semi-persistent scheduling has been adopted for Voice over Internet Protocol (VoIP) service. Semi- persistent scheduling is suitable for voice traffic which has constant rate characteristics and small packet size. However, the compressed VoIP packet after ROHC (Robust Header Compression) still has size variation. The variation of compressed VoIP packet size causes problem of packet segmentation propagation. In this paper a complementary resource allocation scheme is proposed. Complementary resource is notified to UEs along with predefined persistent pattern at the beginning of VoIP session. Complementary resource is allocated when VoIP packet with the sizes bigger than the size of predefined resource generated and blind decoding is implemented at the UE side. Simulation results show that the proposed scheme can reduce L1/L2 control signaling significantly while support large number of VoIP UEs compared with conventional solutions.

With the wide deployment of wireless local area networks (WLANs), the problem of configuration and maintenance of access points (APs) arises. The traditional manual configuration method greatly increases the cost of configuration and maintenance of the WLANs system and hinders the scalability of WLANs. The plug-and-play (PnP) of APs, which means that APs auto-configure themselves based on the network environment, is a good solution to this problem. The PnP of APs can reduce both the complexity and cost of configuration and maintenance for WLANs and also improve the system performance. The selection of working mode and channel is one of the major challenges for the PnP of multi-mode APs. In this paper, we propose a mode and channel selection scheme that combines analytic hierarchy process (AHP) with grey relational analysis (GRA). The goal of our proposed scheme is to select the most suitable mode and channel for multi-mode AP to improve the performance of WLANs and offer better service to users. In the proposed scheme, AHP is responsible for deciding the weights of criteria factors according to their contributions to the final goal. GRA combines the weights of criteria factors with the values of criteria factors to rank all alternative modes and channels combination and make a decision. It is seen from the simulation results that our proposed scheme can select the most suitable working mode and channel for AP in different scenarios.

This paper focuses on bidirectional relaying schemes for multihop communications. For a two hop communication system, traditional 4-step bidirectional relaying (TBR) method allocates 4 slots, resulting in low efficiency; Coded bidirectional relaying (CBR) method, making use of network coding, needs only 3 slots, can provide throughput improvement up to 33% compared with TBR method; Two-way relaying method performs data bidirectional relaying with 2 slots, by employing interference cancellation algorithms in the relay station, can further improve spectrum efficiency. However due to different channel condition between RS to BS and RS to MS and broadcast transmission utilized from RS to BS and MS, CBR and two-way relaying methods can not fully exploit the channel capacity from RS to BS and RS to MS simultaneously. In order to solve the problem, we propose two enhanced relaying schemes: the first one is hierarchical modulation (HM) based scheme, in which HM is adopted to allocate individual MCS for each hop and a novel network coding pattern is designed to avail the introduction of HM to relaying. To further improve the performance of the first method, another method as hybrid constellation based relaying scheme is proposed, in which each end node jointly utilizes a hybrid constellation and the prior data it previously transmitted to retrieve the information from the other end node. Performance evaluation results show our algorithms solve the problem successfully and can improve system spectrum efficiency by 5-12% compared with traditional CBR method. Finally it is concluded that the enhanced bidirectional relaying schemes are promising relaying schemes candidates for multi-hop communications.

Codebook based multiple-input multiple-output (MIMO) precoding can significantly improve the system spectral efficiency with limited feedback and has been accepted as one of the most promising techniques for the evolved UTRA (E-UTRA). Compared with single-user (SU) MIMO, multi-user (MU) MIMO can further improve the system spectral efficiency due to increased degree of freedom in transmission. In order to reduce the feedback overhead and computational complexity, feedback of single rank 1 (corresponding to transmission of single stream) channel quality indicator (CQI) is required in E-UTRA currently. Then, the main challenge is how to obtain CQIs of other ranks at Node B for rank adaptation with single CQI feedback. Therefore, in this paper, an efficient CQI update scheme at Node B arising from statistical characteristics of CQI of various ranks is proposed. Simulation results show that our proposed scheme can yield significant improvement on spectral efficiency compared with conventional CQI update schemes.

The merit factor (MF) introduced by Golay has long been accepted as the standard criterion to evaluate the binary sequence's anti-multipath property in sonar, radar, and communication systems for its simplicity and seemly infallibility. First we present two binary sequence pairs (of lengths 32 and 64) of identical MFs with great discrepancies between the respective binary sequence pairs' uncoded symbol-error rate (SER) performance and theoretical analysis (based on the MF) to shed new light on the hitherto unnoticed determining factor of the sequence's anti-multipath property. We then propose the weighted merit factor (WMF) based on a non-uniform weighting of the out-of-phase aperiodic autocorrelation function (ACF) which leads to better matches between the cited sequence pairs' analytical and experimental results. Ensuing theoretical analysis demonstrates that the WMF provides optimal measurement of self-generated interference for the constant amplitude complex-valued sequences and the nonconstant modulus ones. The MF is shown to be a biased metric for the binary sequences when the multipath delay spread exceeds one chip period. We conjecture that this conclusion continues to hold true for the constant amplitude complex-valued sequences.

Multi-carrier based radio access schemes have been widely accepted as one of the PHY candidates in many standards, for their capabilities not only to combat multi-path propagation, but also to support multiple user access aided by multi-user scheduling. The performance of multi-user wireless system with multi-user scheduling depends on the fluctuation of independent channel response across different users. However, the channel selectivity may be limited in some channel conditions, e.g., light-of-sight (LOS) or spatial correlation, which may limit the multiuser diversity gain. Multi-degree random cyclic delay diversity (MD-RCDD) (Zhengang Pan et al., 2006) scheme is introduced to obtain opportunistic beamforming gain with intensified frequency fluctuation of channel response. However, the available delay sets in MD-CDD are randomly generated, not fully utilizing the spatial resources. Therefore, multi-degree adaptive cyclic delay diversity (MD-ACDD) scheme is proposed in this paper to further optimize the system performance, in which the available delay sets are channel-dependent, and generated based on the feedback information of all users. Moreover, the detailed working procedure and overhead analysis are presented, and simulation results show that proposed scheme can achieve significant performance gain compared with conventional MD-RCDD.

Voice over Internet protocol (VoIP) over a wireless local area network (WLAN) is becoming an important application. However, the performance of voice over WLAN (VoWLAN) is compromised severely in the presence of heavy coexisting traffic from other applications even if the IEEE 802.1 le MAC protocol is utilized. This problem poses a severe challenge to the harmonious coexistence of VoIP and other application in WLAN. This paper proposes and investigates a scheme that modifies network allocation vector (NAV) update process of voice terminals. The extensive simulation in the paper demonstrates that the voice capacity of WLAN with coexisting applications of voice service and best effort service can be largely improved by the proposed simple solution compared with EDCA protocol. Specifically, in our proposed method, only the MAC protocol at the voice stations (STAs) should be modified, which makes the proposed solution more readily deployable over the existing network infrastructure.

Codebook based multiple-input multiple-output (MIMO) preceding can significantly improve the system spectral efficiency with limited feedback and has been accepted as one of the most promising techniques for the Evolved UTRA (E-UTRA). Compared with single-user (SU) MIMO, multi-user (MU) MIMO can further improve the system spectral efficiency due to increased degree of freedom in transmission. In order to reduce the feedback overhead, feedback of single channel quality indicator (CQI), e.g. rank 1 CQI, is required in E-UTRA currently. Then, the main challenge is how to obtain CQIs of other ranks at node B for rank adaptation with single CQI feedback. Therefore, in this paper, an efficient CQI update scheme at node B based on statistical characteristics of CQI of various ranks is firstly proposed. To further increase the accuracy of CQI at node B for data transmission, an adaptive CQI feedback scheme is then proposed in which single CQI with the rank same as previously scheduled is fed back. Simulation results show that our proposed CQI update scheme can achieve some performance gain compared with conventional method with fixed backoff. Moreover, with proposed adaptive feedback scheme, much additional performance gain can be obtained and the performance can approach the upper bound.

This paper presents an AP selection strategy in the office environment covered by WLAN. While most of the selection metrics balances the active load among neighboring BSSs, the quality of voice applications greatly depends on the call blocking probability which is decided by all the potential voice STAs. For this reason, we define a QoS-aware information element that is advertised from APs. On this basis, voice stations are able to select the AP which is associating with less number of voice stations so as to achieve a lower call blocking probability. In order to validate the benefit, we perform simulation using OPNET. As a result, the proposed method effectively balances the load among given BSSs, and greatly increases the number of voice calls that could be admitted into WLAN.

To overcome the disadvantage in traditional manual configuration method for WLANs, such as the high configuration and maintenance cost and low efficiency of system performance, we propose the plug-and-play (PnP) of multi-mode APs that APs auto-configure themselves based on the network environment. In order to implement the PnP, the Multi-mode AP must obtain its neighboring APs' configuration and environment information. Existing neighboring APs discovery method can not provide the precise neighboring APs information to satisfy the requirement of PnP of AP. In this paper, we propose three kinds of neighboring AP discovery and information exchange methods, which are passive discovery method, active discovery method and station assistant discovery method. Using these three neighboring AP discovery methods, AP can discover all neighboring APs and obtain needed information. We further propose two whole process flows, which combine three discovery methods in different manner, to achieve different goal. One process flow is to discover the neighboring AP as fast as possible, called fast discovery process flow. Another one is to discover the neighboring AP with minimal interference to neighboring APs, called the minimal interference process flow. The efficiency of the two process flows is shown in the simulation results.

Significant throughput gains can be obtained in multi-user wireless system by exploiting the combination of multi-user scheduling in media access control (MAC) layer and MIMO spatial multiplexing using precoding in physical (PHY) layer. In this paper, we propose a multi-stage hybrid scheduling scheme for codebook-based precoding system, which applies different scheduling methods at different scheduling stages for selecting user equipments (UEs). In detail, we employ different scheduling algorithms when the appropriate UE or UE sets are selected within the same precoding vector, within the same precoding matrix, and within the codebook, respectively, so that we could finely adjust the balance between capacity and fairness. Computer simulation results show that the proposed scheme effectively fills the performance gap between pure maximum carrier-to-interference (C/I) power ratio and pure proportional fairness (PF) methods, and provides an important means at the MAC layer to lever between aggregate cellular throughput and geometry-specific per-user fairness, in order to meet the requirements of more precise quality of service (QoS) provision for future mobile communication systems.

Mobile Ubiquitous Network (MUN) is an integrated network that consists of cellular network, gateways (GWs), as well as local networks. In such heterogeneous environment, GWs deserve much attention since they act as bridges to connect local networks and cellular network to realize global communication. In this paper, we study the issue of GW discovery and especially focus on the scope of GW information advertising. That is, how far for each GW to broadcast its GWINFO to meet the target of good connectivity with low overhead. Generally speaking, cellular link between GW and Base Station (BS), is bottleneck of the whole universal route, and distribution of local nodes decides the total overhead of GW discovery. Therefore, both of them are well studied and considered in the proposed method. Simulation results verified that the method can improve the network performance in terms of data transmission ratio, end-to-end packet delay, average GW discovery time, as well as normalized routing load.

Optimized link state routing (OLSR) protocol has been presented as a proactive unicast routing protocol tailored for mobile ad-hoc network (MANET). With some modifications, the multicast optimized link state routing protocol (MOLSR) is applied to multicast data dissemination. However, both OLSR and MOLSR protocols presuppose the presence of symmetric links between node pairs in the network. In reality, MANETs may consist of asymmetric links connecting heterogeneous nodes. Motivated by the efficient utilization of the limited radio resources, we make extensions of MOLSR to MOLSR-ASYM aiming to take the best use of the asymmetric links. Our simulation results demonstrate that with the marginally increased signaling cost, our proposed approach is effective to improve the multicast routing in providing more and shorter routes for route selection and data transmission.

This paper aims to address the problem of inter-cell interference fluctuation, which is inherent on the uplink direction of future cellular systems employing OFDMA or single carrier as multiple access technology. We present here a novel uplink adaptive resource allocation algorithm based on inter-cell interference measurement to achieve both multi-user diversity gain and the cancellation of inter-cell interference fluctuation. The phenomenon of inter-cell interference fluctuation in future cellular systems is explained and its negative effect on traditional resource allocation methods is analyzed. The proposed algorithm includes two independent sub-algorithms: 1) user grouping based resource allocation scheme, in which radio resource is limited to be reallocated within a group of users generating close inter-cell interference; 2) power compensation based resource allocation scheme, in which inter-cell interference fluctuation is compensated by adjusting transmitting power of UE. And performance evaluation results show that our algorithms smooth the inter-cell interference successfully and can improve system spectrum efficiency by 10-15% compared with existing traditional adaptive resource allocation schemes.

It is of paramount importance to obtain accurate node positions in ad hoc networks for various location-based services. Hop-count based localization provides a low-cost and efficient approach to yield the node locations with moderate accuracy. In this paper, an enhanced hop-count based localization algorithm, Segmentation-aided and Density-aware Hop-count (SDH), is proposed to improve the estimation accuracy. In the proposed scheme, the path from one node to the reference nodes (RNs) is first divided into several three-hop segments; each segment distance can be estimated by density-aware hop- count localization. Based on the proposed SDH localization algorithm, we further propose a relative location estimation method for one mobile node to reach the interested node without knowing the location map of whole network. The advantages of the proposed scheme over other conventional schemes are verified by numerical analysis and simulation.

Link adaptation, scheduling, and hybrid automatic repeat request (HARQ) are indispensable technology for wireless networks to provide high speed packet data services. While prior scheduling algorithms based on the instantaneous data rate to determine the user to be served in each scheduling interval are useful, they do not fully take into account transmission errors and the impact of retransmission techniques to recover those errors. We propose a more accurate measure of users' effective Signal-to-Noise Ratio (SNR) that accounts for the reliability as well as retransmissions when scheduling transmissions to users. Extensive simulation results comparing performance of the proposed method to conventional methods are presented.

Network coding has attracted much interest recently since considerable research indicates that it can offer significant capacity gains. This paper applies network coding to mobile ad hoc networks, especially for large scale content distribution, where each network node is able to generate and transmit encoded blocks of incoming information. Furthermore, it introduces a novel scheduling approach named popularity aware scheduling (PAS) for such network coding based content distribution systems. Here, popularity refers to how much one new data block is wanted by neighboring nodes. The value is calculated based on the number of blocks that already cached by current node which generates the new block (NB-PAS), or correlation between the new block and neighbors' cached blocks (CB-PAS). Then the popularity value is used to make the content distribution more efficient by assigning different levels of channel access priorities to blocks at MAC (media access control) layer. The higher the popularity value, the higher priority is assigned. Performance evaluation indicates that the cooperation of network coding and MAC layer scheduling is quite necessary and efficient from the point view of content dissemination speed, and information transmission efficiency.

In future wireless and mobile networks, the users will have the capability to access to multiple networks via multi-mode terminal. Therefore there is a need to have mechanisms in place to decide which radio access network (RAN) is the most suitable for the user and for every application that the user requires. The decision is complex because it has to be made according to the multiple attributes both in network side and user side. To address these issues, we propose a user-centric intelligent solution by making the modifications in the classical multiple attribute decision making (MADM) technique and introducing the utility function in the selection procedure. The proposed solution achieves the well trade-off among network condition, user preference, and service application and benefits both the users and RANs.

Data transmission from source to sink is the most common functionality in wireless sensor networks (WSNs). When transmitting to a mobile sink, the connection of the last hop to the mobile sink breaks very often, so the route to the sink needs to be reestablished frequently. In such a scenario, sink mobility management is essential to support service continuity in WSNs. In this paper, an efficient route maintenance scheme is proposed to handle the sink mobility. In the proposed scheme, different states are introduced to the sensor nodes, and some special nodes close to the mobile sink report their state changes. With the help of exchanged state information, the sensor nodes around the mobile sink can take proactive and timely actions to achieve fast and seamless route reestablishment. The advantages of the proposed scheme over other conventional schemes are verified by numerical analysis and simulation

This paper presents an access point (AP) selection strategy for the (re)association procedure in large-scale WLAN. The proposal considers the difference of node topology among candidate basic service sets (BSSs), wherein the effects from hidden terminals are emphasized due to its significant impact on throughput degradation. Comparing with existing mechanisms, the proposal combats load imbalance problem that the popular signal strength measurement based selection strategy may cause. Meanwhile, it estimates the BSS load status in more accurate way by differentiating hidden terminal from others, so a higher throughput is resulted. More significantly, the proposal uses the advertised information of the Beacon/Probe Response frame in IEEE 802.11e, without any hardware modification. Such back-compatibility facilitates the implementation over currently deployed WLAN. The performance is validated via simulation which shows that the proposal can achieve up to 20% throughput enhancement over existing methods particularly over the uplink.

Energy is a critical and challenging issue for battery-constrained devices, especially for mobile ad hoc stations, which are responsible for relaying data packets for neighbor nodes. Therefore, considerable research has been devoted to the research of energy consumption efficiency. Care has been taken not only to reduce the overall energy consumption but also to balance individual battery usage, since unbalanced energy consumption will result in earlier node failure for overloaded nodes, and then lead to network partition and reduced network lifetime. This paper presents a novel on demand routing algorithm named energy aware reliable routing (EARR) to avoid the over usage of hot spots, reduce the route-reconstructions due to residual energy shortages, and finally prolong network lifetime. In the method, only nodes with sufficient residual energy to complete the task will take part in the propagation of route request (RREQ), therefore only valid path has the potential to transmit data. Simulation results verified that EARR can improve the network performance in terms of average route discovery time, normalized routing load, as well distribution of energy consumption. Besides, it is more robust to energy estimation error than existing methods.

Energy is one of the most precious resources for wireless sensor networks. Dynamic power management (DPM) is an efficient technique to reduce the energy consumption and increase the lifetime of wireless sensor networks. DPM for the event-driven applications in wireless sensor networks has been studied extensively, but power saving for the time-driven applications has not been studied deeply. This paper proposes two novel DPM schemes for the time-driven applications in wireless sensor networks. In the proposed two schemes, sensor nodes reduce redundant data reporting and hence save energy based on neighbor distance and data mining information, respectively. The power saving performance and effectiveness of the proposed schemes are evaluated by confirmed numerical analysis and simulation.

The secondary market spectrum policy adopted by FCC opens the opportunities of flexible spectrum leasing and transfer between wireless network providers (NPs). When trading and allocating spectrum with the emerging market-oriented approach, the wireless NPs need to consider the economic influence when determining the needed spectrum. This paper proposes flexible spectrum allocation (FSA) methods for the NPs to choose by taking into account the NP's operation revenue and provided QoS. Different criteria and corresponding optimization methods are designed for the NPs to determine appropriate amount of spectrum to apply and the respective amount of spectrum to allocate to its managed multiple radio access technologies (RATs). The system architecture incorporating the proposed methods for the NPs to coordinate spectrum sharing is also presented in this paper

Broadcasting is an indispensable operation for multi-hop mobile ad hoc network (MANET) to provide control or routing functionalities. In MANET, since a packet relayed by any node can reach all its neighbour nodes, broadcasting by flooding is inefficient and results in many redundant relays. In this paper, we analyze the deficiencies of some previous broadcasting protocols and propose two enhanced lightweight protocols, namely hop count-aided broadcasting (HCAB) and self-adaptive probability broadcasting (SAPB). The proposed protocols can reduce broadcasting redundancy significantly and guarantee high broadcasting coverage as well. More importantly, the proposed protocols are lightweight, which means each node makes relay decision without any knowledge of its neighbours. Thus, the proposed protocols have low overhead and are attractive to capacity limited mobile devices. Simulation results show that the proposed protocols eliminate unnecessary relay efficiently and outperform the existing lightweight broadcasting protocols

WLAN is becoming a promising system for future mobile ubiquitous network. However, the performance is largely restricted by MAC inefficiency, and AP remains as a bottleneck in infrastructure WLAN. A high efficient MAC, which decreases frame collision and balances the performance between AP and stations, is desirable. Therefore, we proposed an interactive pair scheduling (IAPS) scheme compatible with IEEE 802.11. IAPS uses ACK/CTS exchange to extend channel reservation for reverse data frames, hence achieving contention-free access and performance balance between uplink and downlink. Moreover, we defined MPT, the maximum pair scheduling times, to combat the unfairness due to long-term pair scheduling in LAPS and also benefit the service differentiation. The performance of LAPS is evaluated in an IEEE 802.11 based infrastructure WLAN. Simulation results show that LAPS increases the total throughput and benefits the performance balance especially when hidden terminals exist. Moreover, LAPS efficiently improves the throughput of VoLP in presence of mixed VoLP and best effort traffic.

With the rapidly increasing demand for high-speed mobile multimedia and the need to support diversified requirements of users by packet communication systems, wireless quality of service (QoS) resource management becomes more and more indispensable, especially for the IMT-2000 system and beyond. In previous researches, QoS controls carried out independently at base stations have been proposed. However, in a CDMA packet communication system where channel capacity varies dynamically due to the variant interference level, QoS satisfaction and the system capacity may not be optimised while considering the entire system. In this paper, the essential factors of wireless QoS control are denoted and a wireless QoS resource management for packet mobile communications with base station power resource cooperation control is proposed. Computer simulation indicates that the QoS satisfaction rate and the system capacity are improved remarkably by the proposed scheme with base station power resource cooperation control. The improvement becomes ever larger when real-time and nonreal-time traffic distributed in an unbalanced manner, which further elucidates the effectiveness of the proposed resource cooperative control scheme.

With the rapidly increasing demand for providing highspeed mobile multimedia and supporting diversified requirements of users, wireless quality of service (QoS) resource management becomes more and more indispensable, especially for the upcoming IMT-2000 system and beyond. In this paper, a wireless QoS architecture and two QoS resource allocation algorithms for CDMA cellular systems are proposed to meet these requirements. Based on the proposed QoS architecture, an adaptive QoS resource management is proposed to assign radio resource corresponding to its availability and the users' maximum and minimum QoS requirements. A service-rank-based QoS resource management is further proposed to satisfy more high-rank users as much as possible while maintaining the minimum requirement of low-rank users. Computer simulations indicate that the proposed QoS resource management algorithms exhibit superior performance with respect to dropping probability, rate of satisfied users and necessary transmission time, which indicate higher satisfaction rate of users and better performance.

It is highly desirable to develop an efficient and flexible dynamic channel assignment algorithm in order to realize an integrated traffic TDMA mobile radio communication network. In this paper, an integrated traffic TDMA system is studied in which transmission of voice and data are assumed to occupy one and n time slots in each TDMA frame, respectively. In general, two types of channel (time slot) assignment algorithms are used: the partitioning algorithm and the sharing algorithm. However, they are not well-suited to the multimedia traffic consisting of various information sources that occupy different number of slots per frame. In this paper, assuming that voice is much more sensitive to transmission delay than data, an algorithm based on the sharing algorithm with flexible time slot management scheme is proposed as follows. To reduce the blocking and forced termination probabilities of a voice call, part of the slots of data calls are transferable to voice calls during congestion periods. At the same time, to reduce the handoff-failure of a multi-slot data call, the required number of data slots can be reduced adaptively according to the available number of slots during its handoff. These data calls whose number of slots are reduced either for voice calls or its handoff are allowed to get restored when possible. Computer simulations confirm the good performance of the proposed algorithm when compared to conventional channel assignment algorithms.