As positioning solutions integrate multiple components to improve accuracy, the number of parameters that require calibration has increased. This article studies a calibration-free positioning technique using Wi-Fi ranging and pedestrian dead reckoning (PDR), where every parameter in the system is optimized in real time. This significantly decreases the time and effort required to perform manual calibration procedures and enables the positioning solution to achieve robust performance in various situations. Additionally, this article studies an efficient way of performing irregular Wi-Fi ranging procedures to improve battery life and network performance of mobile devices. The positioning performance of the proposed method was verified using a real-time Android application on several mobile devices under a large indoor office environment. Without any calibration, the proposed method achieved up to 1.38-m average positioning accuracy for received signal strength (RSS)-based ranging scenarios, which differs only by 30 cm from the benchmark assuming perfect calibration. In addition, the proposed method achieved up to 1.04-m accuracy for round trip time (RTT)-based ranging scenarios with a 40-MHz bandwidth configuration, which differs only by 10 cm from the benchmark.

Given that the accuracy of range-based positioning techniques generally increases with the number of available anchor nodes, it is important to secure more of these nodes. To this end, this paper studies an unsupervised learning technique to obtain the coordinates of unknown nodes that coexist with anchor nodes. As users use the location services in an area of interests, the proposed method automatically discovers unknown nodes and estimates their coordinates. In addition, this method learns an appropriate calibration curve to correct the distortion of raw distance measurements. As such, the positioning accuracy can be greatly improved using more anchor nodes and well-calibrated distance measurements. The performance of the proposed method was verified using commercial Wi-Fi devices in a practical indoor environment. The experiment results show that the coordinates of unknown nodes and the calibration curve are simultaneously determined without any ground truth data.

TEST 02 - Elsevier's Scopus, the largest abstract and citation database of peer-reviewed literature. Search and access research from the science, technology, medicine, social sciences and arts and humanities fields.

Recent achievement in self-interference cancellation algorithms enables potential application of full-duplex (FD) in 5G radio access systems. FD communication promises to double the spectral efficiency by enabling the same time-frequency uplink and downlink transmissions. Yet for cellular access network with FD small cell base stations (BS) serving multiple user equipment (UE), additional BS-to-BS and UE-to-UE interference due to FD operation could diminish the performance gain if not tackled properly. Existing works all strive to achieve close to 2× FD gain at network scale, in this paper, we demonstrate that much higher FD gain (≫ 2×) can be achieved under realistic nonfull buffer traffic due to significant latency reduction. We develop practical interference mitigation strategies such as adaptive power control, joint scheduling and inter-cell interference coordination to cope with the newly introduced interference in FD cellular systems. We validate the flexible traffic adaption of FD system under bursty traffic model by our LTE-based system level simulator, where significant FD gain of upto 13v in mean throughput and 19× in cell-edge throughput can be achieved under different traffic loads over an existing LTE half-duplex system.

This article investigates novel adaptive self-interference cancellation solutions and the total integrated cancellation performance of a mobile single-antenna inband full-duplex transceiver. First, novel self-adaptive digital self-interference cancellation algorithms are described, with an emphasis on tracking of time-varying self-interference coupling channel in a mobile device as well as on structural ability to suppress also nonlinear self-interference with highly nonlinear mobile power amplifiers. This leads to an advanced self-adaptive nonlinear digital canceller which utilizes a novel orthogonalization procedure for nonlinear basis functions, together with low-cost LMS-based parameter learning. The achievable self-interference cancellation performance is then evaluated with actual RF measurements using mobile device scale RF components, in particular a highly nonlinear PA. The measurements also incorporate a novel self-adaptive RF cancellation circuit in order to realistically assess the total integrated cancellation performance. The reported results show that highly efficient self-interference cancellation can be achieved also in a mobile device, despite a heavily nonlinear PA and limited computing and hardware resources. The proposed cancellation solutions, when integrated together, show that 100 dB of self-interference can be cancelled using a 20 MHz LTE waveform, while the SI can be attenuated by over 110 dB with a narrower bandwidth of 1.4 MHz, all measured at 2.4 GHz ISM band. Furthermore, these results are achieved using a highly nonlinear transmitter power amplifier and fully adaptive canceller structures which can track a rapidly changing coupling channel in a mobile full-duplex device.

This paper presents a novel RF circuit architecture for self-interference cancellation in inband full-duplex radio transceivers . The developed canceller is able to provide wideband cancellation with waveform bandwidths in the order of 100 MHz or beyond and contains also self-adaptive or self-healing features enabling automatic tracking of time-varying self-interference channel characteristics. In addition to architecture and operating principle descriptions, we also provide actual RF measurements at 2.4 GHz ISM band demonstrating the achievable cancellation levels with different bandwidths and when operating in different antenna configurations and under low-cost highly nonlinear power amplifier. In a very challenging example with a 100 MHz waveform bandwidth, around 41 dB total cancellation is obtained while the corresponding cancellation figure is close to 60 dB with the more conventional 20 MHz carrier bandwidth. Also, efficient tracking in time-varying reflection scenarios is demonstrated.

Full Duplex or Simultaneous transmission and reception (STR) in the same frequency at the same time can potentially double the physical layer capacity. However, high power transmit signal will appear at receive chain as echoes with powers much higher than the desired received signal. Therefore, in order to achieve the potential gain, it is imperative to cancel these echoes. As these high power echoes can saturate low noise amplifier (LNA) and also digital domain echo cancellation requires unrealistically high resolution analog-to-digital converter (ADC), the echoes should be cancelled or suppressed sufficiently before LNA. In this paper we present a closed-loop echo cancellation technique which can be implemented purely in analogue domain. The advantages of our method are multiple-fold: it is robust to phase noise, does not require additional set of antennas, can be applied to wideband signals and the performance is irrelevant to radio frequency (RF) impairments in transmit chain. Next, we study a few protocols for STR systems in carrier sense multiple access (CSMA) network and investigate MAC level throughput with realistic assumptions in both single cell and multiple cells. We show that STR can reduce hidden node problem in CSMA network and produce gains of up to 279% in maximum throughput in such networks. Moreover, at high traffic load, the gain of STR system can be tremendously large since the throughput of non-STR system is close to zero at heavy traffic due to severe collisions. Finally, we investigate the application of STR in cellular systems and study two new unique interferences introduced to the system due to STR, namely BS-BS interference and UE-UE interference. We show that these two new interferences will hugely degrade system performance if not treated appropriately. We propose novel methods to reduce both interferences and investigate the performances in system level. We show that BS-BS interference can be suppressed sufficiently enough to be less than thermal noise power, and with favorable UE-UE channel model, capacities close to double are observed both in downlink (DL) and uplink (UL). When UE-UE interference is larger than DL co-channel interferences, we propose a simple and "non-cooperative" technique in order to reduce UE-UE interference.

In order to meet the constantly increasing demand for ubiquitous, mobile access to the internet, next-generation mobile broadband communications systems based on OFDMA, such as IEEE 802.16 m, require a significant performance increase over previous generation systems, such as IEEE 802.16e-2005, particularly in cell-edge and average spectral efficiency. In this paper, we address the downlink adaptive frequency reuse (AFR) and multi-user MIMO (MU-MIMO) techniques which are considered to be the most promising candidates for meeting the requirements on cell-edge and average spectral efficiency of next-generation mobile broadband systems.

MIMO (multiple input multiple output) techniques are widely employed to improve the performance of wireless systems. These techniques are used to overcome multipath fading and/or improve the peak throughput of wireless systems. It is well known that there is a fundamental tradeoff between diversity gain and multiplexing gain [1]. Orthogonal space time codes such as the Alamouti code (also known as space time block codes (STBC)) exploit multiple antennas as a diversity source, and thus improve packet error rate (PER) and the average throughput. However, space time block codes are not designed to increase the peak data rate of the system. On the other hand, spatial multiplexing (SM) techniques offer higher peak throughput by transmitting parallel streams of data from different antennas. In order to successfully decode the parallel streams, the channel must exhibit a small eigenvalue spread. Otherwise, the streams will interfere with one another and it is difficult to decode the information data. The performance improvement from SM is therefore highly dependent on the channel characteristics. It is possible to use multiple encoders and rate control per layer to improve the SM performance. However, there are instances of channel where STBC is more beneficial than SM with a single encoder. In order to resolve the shortcomings of STBC and SM, a hybrid technique can be applied where depending on the instantaneous channel conditions either STBC or SM is selected. This technique is commonly referred to as adaptive MIMO switching (AMS) [5]-[11]. One important aspect of the technique is the switching criteria, namely the PHY abstraction method which estimates a packet error event as a function of the instantaneous channel condition, transmission profiles, and receiver characteristics. We show that the proposed algorithm outperforms other techniques such as determinant and Demmel condition number based techniques in flat fading channel. In selective fading channels where a codeword sees a finite number of multiple channel qualities, estimating the resulting PER is very challenging. Existing techniques rely on an estimate of the average of the channel qualities seen by each codeword. In this paper, we propose a PHY abstraction and switching algorithm hereby referred to by weighted sum of instantaneous qualities (WSIQ) whereby the channel qualities are ordered in order to reduce the variance of channel qualities and then a weighted sum of the qualities is applied. The weighting vector is chosen to minimize the variance of the linear sum. We have provided simulations to show the superiority of WSIQ even when channel statistics are unknown. In addition, computationally efficient techniques suited for practical implementation are proposed.

This paper derives a maximum likelihood Doppler frequency estimator for orthogonal frequency division multiplexing (OFDM) systems in time-varying multipath channels. The proposed scheme is a frequency-domain approach that utilizes pilot subcarriers, which are commonly implemented in most practical systems. Time-varying fading causes intercarrier interference (ICI) in OFDM systems. Thus, in the proposed estimator, the effect of ICI is taken into consideration with a proper model for accurate results. The estimator can be implemented using a finite impulse response (FIR) filter bank whose coefficients can be pre-calculated and stored in order to lower the computational complexity. We evaluate various methods to improve the estimation accuracy and analyze their complexity-performance tradeoffs. We also derive the Cramér-Rao bound and provide simulation results to quantify the performance of the proposed algorithm.

Smart antenna technology can greatly improve the coverage radius of wireless systems by focusing the transmitted signal (beam) toward desired users. However, beamforming has an important side effect as it creates a region, herein referred to as the complementary region, where some users in the network cannot sense the directional signals (beams). We have referred to this as the hidden beam problem in Tarokh et al.. From a physical layer perspective, beamforming reduces cochannel interference (CCI). However, in systems that use medium access-control techniques such as the carrier-sense multiple access (CSMA) protocol in wireless local area networks, during a busy period of the channel, the users in the complementary region may wrongly decide that the medium is idle and transmit packets. This may cause CCI, unnecessary retransmissions, subsequent backoff, and increased network latency. We introduced the concept of complementary beamforming (CBF) to address this issue in Tarokh et al.. In this paper, we propose two new methods, called "subspace complementary beamforming " (SCBF) and "complementary superposition beamforming" (CSBF). The SCBF uses dummy data to ensure a controlled level of received energy in all directions of eigenvectors unused by downlink beamforming. The technique works in multipath fading channels. The approach achieves similar results to those reported in Tarokh et al.. However, it enables the second technique (CSBF) which can also send data content in the complementary beam. This allows passive nodes in the network to receive "broadcast" information, while the active nodes are engaged in the exchange of user-specific data. It is shown that CBF effects can be achieved simply by increasing the transmit power of only one of antenna elements when space-division multiple access is not applied. Simulations are provided in order to quantify overall network performance improvements when using CBF, confirming significant throughput and delay performance enhancements.

Multiple antenna systems can greatly improve the performance of wireless systems by providing better link quality, capacity and immunity to interference. A very simple structure for smart antennas is a fixed beamforming network (FBN) with beam selection. In Y.S. Choi and S.M. Alamouti (2004) the performance of antenna and beam selection/combining techniques are compared. The channel model in that paper is a statistical, clustered propagation model in a correlated Rayleigh fading channel. In this paper we derive the approximate density function of the signal-to-interference-and-noise power-ratio (SINR) of beam selection with eigen-beamforming. We also analyze the outage probability and capacity of beam selection with a fixed beamforming network (FBN) with correlated fading between the antennas. We compare the performance of this system with antenna selection avid independent fading between the antennas. We also investigate the impact of erroneous beam or antenna selection. We show that beam selection outperforms antenna selection in the presence of interference.

Time-domain channel estimation and detection techniques are presented for multicarrier signals in a fast and frequency-selective Rayleigh fading channel. As a consequence of the time-varying channel, the orthogonality between subcarriers is destroyed in conventional frequency-domain approaches, resulting in interchannel interference, which increases an irreducible error floor in proportion to the normalized Doppler frequency. An important feature of the proposed technique is the ability to exploit the time-selective channel as a provider of time diversity. This enables us to achieve performance superior to any other structure without increasing bandwidth or incorporating redundancy, in order to reduce the complexity of the estimator, we apply the theory of optimal low rank approximation to a minimum mean squared error channel estimator and present a theoretical calculation of mean squared error and simulations to confirm that the estimator is robust to changes in channel characteristics.