Brain tissue segmentation is essential for neuroscience and clinical studies. However, segmentation on longitudinal data is challenging due to dynamic brain changes across the lifespan. Previous researches mainly focus on self-supervision with regularizations and will lose longitudinal generalization when fine-tuning on a specific age group. In this paper, we propose a dual meta-learning paradigm to learn longitudinally consistent representations and persist when fine-tuning. Specifically, we learn a plug-and-play feature extractor to extract longitudinal-consistent anatomical representations by meta-feature learning and a well-initialized task head for fine-tuning by meta-initialization learning. Besides, two class-aware regularizations are proposed to encourage longitudinal consistency. Experimental results on the iSeg2019 and ADNI datasets demonstrate the effectiveness of our method. Our code is available at https://github.com/ladderlab-xjtu/DuMeta.

Smart grid is an emerging technology which is designed to integrate advanced communication technologies into electrical power grids. In smart grid, automatic communications between machine devices is necessary. The number of Machine-Type Communication (MTC) devices will increase exponentially as smart grid technologies are further developed and deployed. It is a critical issue to deal with the massive accesses from an enormous number of MTC devices while guarantee the desired quality of service (QoS). In this paper, we formulate this problem as a queuing problem. Then we propose an adaptive massive access management by applying a probability estimation measurement, which is based on large deviation theory. The results demonstrate that our algorithm can adaptively adjust allocation rate and provide a better QoS. The results also show that our algorithm can improve the spectral efficiency.

An efficient receiving status feedback in multicast communications can improve the system performance considerably. In this paper, we propose a novel compressed hybrid automatic repeat request (HARQ) mechanism for the reliable multicast services in the cellular network controlled Device-to-Device (D2D) communications. Closely located D2D cluster devices transmit the acknowledgement and negative acknowledgement (ACK/NACK) message to a cluster head through D2D links directly, after that the cluster head feeds back the whole D2D cluster receiving status using 2-bit ACK/NACK to the cellular network. Performance analysis and numerical simulation results reveal that the proposed HARQ feedback mechanism is better than the conventional D2D multicast mechanisms in terms of the error probability and signaling overhead.

Multicast service is a key function of the next generation broadband mobile system. The performance of wireless multicasting is severely degraded due to the wireless channel fading. Multiple antennas and cooperative communication are promising techniques that can be used to improve the multicast performance. Most cooperative schemes are based on the decode-and-forward (DF) strategy that requires the cooperating users being able to correctly decode the multicast data first. In this paper, we study the cooperative multiple antenna multicast scheme based on amplify-and-forward (AF) strategy. A cooperative protocol for multicast systems with multiple transmit antennas is proposed. We analyze the performance of the protocol with two multiple antenna transmission schemes: the diversity transmission and multiplexing transmission. The close form expression and two bounds on the multicast outage probability are derived. Thorough numerical evaluation shows that our proposed protocol can take full advantage of the degree of freedom provided by the multiple transmit antennas. And a larger outage capacity is obtained at moderate and high signal-to-noise ratio.

Relay selection enhances the performance of the cooperative networks by selecting the links with higher capacity. Meanwhile link adaptation improves the spectral efficiency of wireless data-centric networks through adapting the modulation and coding schemes (MCS) to the current link condition. In this paper, relay selection is combined with link adaptation for distributed beamforming in a two-hop regenerative cooperative system. A novel signaling mechanism and related optimal algorithms are proposed for joint relay selection and link adaptation. In the proposed scheme, there is no need to feedback the relay selection results to each relay. Instead, by broadcasting the link adaptation results from the destination, each relay will automatically understand whether it is selected or not. The lower and upper bounds of the throughput of the proposed scheme are derived. The analysis and simulation results indicate that the proposed scheme provides synergistic gains compared to the pure relay selection and link adaptation schemes.

Relaying is generally considered as a method to ensure capacity improvement and coverage extension in cellular systems. The additional communication traffic between a mobile station and a relay intermediately inserted into a link between RUE and BS requires additional considerations on HARQ protocol. In this paper, we propose a method to jointly and dynamically balance the maximum number of transmissions between the access link and relay link by taking into account the end to end error probability, HARQ latency and spectral efficiency.

In a mobile relay system, the idle users can be formed as a cluster to support nearby active users. In order to reduce the complexity of the processing in the relay nodes and destination nodes, a novel transmit preprocessing method is proposed for the cluster based multi-user relay system. Block diagonalization (BD) algorithm is adopted twice in the proposed method to remove inter-cluster and intra-cluster interference simultaneously, and the relay nodes are just in simple amplify-and-forward (AF) mode. Compared to the conventional signal processing methods, the proposed method can achieve better tradeoff between sum capacity performance and processing complexity in the relay nodes.

We consider relay selection technique in a cooperative cellular network where user terminals act as mobile relays to help the communications between base station (BS) and mobile station (MS). A novel relay selection scheme, called Joint Uplink and Downlink Relay Selection (JUDRS), is proposed in this paper. Specifically, we generalize JUDRS in two key aspects: (i) relay is selected jointly for uplink and downlink, so that the relay selection overhead can be reduced, and (ii) we consider to minimize the weighted total energy consumption of MS, relay and BS by taking into account channel quality and traffic load condition of uplink and downlink. Information theoretic analysis of the diversity-multiplexing tradeoff demonstrates that the proposed scheme achieves full spatial diversity in the quantity of cooperating terminals in this network. And numerical results are provided to further confirm a significant energy efficiency gain of the proposed algorithm comparing to the previous best worse channel selection and best harmonic mean selection algorithms.

In the next generation cellular system, relay has been considered to increase coverage extension and cell-edge throughput. Cooperative relay can provide transmission diversity gain to improve outage probability, overall throughput and energy efficiency. Cooperative beamforming has been considered as one of the cooperative relay if the relay nodes can get the feedback information from the destination. In this paper, a new cooperative relay scheme is proposed for hop-by-hop HARQ in cooperative relay. The scheme is to adopt selective random cyclic delay diversity (CDD) to the retransmission in HARQ, which can get rid of the relative bad relay nodes and form a better frequency selective channel to exploit diversity gain when channel coding is used. Performance evaluation and analysis have shown that the selective random CDD can achieve higher throughput and lower transmission delay than the conventional cooperative beamforming.

In this paper we investigate the channel estimation and data transmission scheme for bi-directional relaying applied in cellular systems. We propose a novel pilot-aided transmission strategy to realize the efficient data exchange between Base Station (BS) and Mobile Station (MS) via a fixed Relay Node (RN). Based on the characteristics of the BS-RN link, the transmitted signals at the BS are pre-equalized. At the RN, part of the CSI is estimated and used to equalize the relayed signals. We then propose a semi-orthogonal pilot structure for channel estimation. With the proposed transmission scheme and pilot structure, the pilot number is reduced by half and the channel estimation at the MS has much low complexity. Simulation results show that the BER performance of the proposed scheme at the MS is a little better than the conventional scheme.

In this paper, we address a sub-optimum distributed power allocation (SODPA) scheme for two-hop parallel relay networks, which includes a source-destination pair and multiple relay nodes. The system employs Decode-and-Forward (DF) as relaying protocol. In the proposed scheme, each relay is assumed to be able to decode the received signal if it satisfies the given SNR threshold and may make decision individually on whether to forward the source code to destination. Compared with the work in, the proposed scheme requires only the statistical CSI at source node instead the exact instantaneous CSI. Therefore, the SODPA reduces the amount of feedback information significantly. Simulation results show that the proposed scheme can reduce the total transmit power effectively.

The cooperative transmission is an efficient way in the wireless network which benefits from the spacial diversity. In this paper, we introduce and analyze a novel three-node relay protocol with high transmit efficiency, termed as the Adaptive Cooperative Coding (ACC). This technique is implemented by transmitting the rate-compatible punctured convolutional code (RCPC code) sequence and its complementary punctured convolutional (CPC) sequence during the two-hop transmission. Compared with the conventional cooperative transmission, the ACC scheme remarkably improves the achievable data rate and is adjustable to variable channel conditions. Furthermore, we deduce the theoretical upper bound of the ACC protocol, from which the lowest packet error probability can be achieved by selecting a proper coding rate for the first hop.

A low-density parity-check (LDPC) coded adaptive modulation and coding (AMC) strategy is presented for orthogonal frequency division multiplexing (OFDM) systems. Since system performance is improved as the maximum iteration times for LDPC decoding increase, iteration times for decoding reflect channel quality. We first propose an AMC strategy to utilize the information of iteration times for decoding to adopt higher order modulation and coding schemes (MCS) than the conventional strategy, so as to improve system throughput. Since it may bring worse reliability, hybrid automatic repeat request (H-ARQ) is used in a second strategy. Simulation under typical urban channel is carried out, and the results show the proposed strategies can bring higher throughput, and the strategy with H- ARQ can improve throughput and guarantee reliable transmission simultaneously.

Link adaptation technique is applied in an amplify and forward (AF) - orthogonal frequency division multiplexing (OFDM) relaying system, and adaptive bit and power allocation (ABPA) is investigated for such system. ABPA problem is presented first, and the optimal solution is proposed with aid of Greedy algorithm. Because channel transfer functions for two hops are independent, which limits throughput of a conventional AF-OFDM system, we take sub-carrier coupling (SQ into account. Two solutions are proposed for the problem with ABPA and SC; the first adopts a concatenated method to realize SC and ABPA with two steps, and the second utilized a joint way to adjust SC and allocated bits and power. Numeral simulation results show that the proposed ABPA solution can improve throughput greatly while guaranteeing bit error rate (BER) requirement very well. When SC is involved, more improvement can be obtained; the joint solution outperforms the concatenated solution.

In this letter, we study the user pairing algorithms for uplink virtual multiple-input multiple-output (V-MIMO) systems, and propose a new double proportional fair (D-PF) algorithm for user pairing of the uplink V-MIMO systems. The proposed D-PF algorithm uses the PF criterion to decide the first user, and a modified PF criterion is suggested to choose the pairing user. Simulation results show that the proposed D- PF algorithm outperforms the conventional determinant formula (DF) algorithm in terms of both throughput and fairness performances. Moreover, if compared with the conventional single PF (S-PF) algorithm, the proposed D-PF algorithm achieves a fairly good trade-off between the throughput and fairness, being able to trade with 5% throughput loss for 15% fairness enhancement.

Random access channel (RACH) is usually used for initial channel access, bandwidth request, etc. In this paper, we analyze the throughput and access delay performance of the RACH in an orthogonal frequency division multiple access (OFDMA) system. A closed-form expression of the RACH throughput is presented and its mean access delays under both binary exponential and uniform backoff policies have also been derived. Moreover, to further improve the utilization of RACH in an OFDMA system, we propose a novel message transmission scheme in a shared RACH. The message in the proposed scheme can be sent implicitly in the cyclic-shifted preambles which make the transmission much more reliable in a contention based RACH. Meanwhile, performance of the preamble detection has also been improved by the redundant information conveyed in messages. Finally, simulation results will demonstrate performance gains of the proposed scheme.

QR decomposition based M algorithm (QRD-M) is a type of maximum likelihood signal detecting (MLSD) scheme to provide a tradeoff detecting method between the system performance and complexity in multi-input multi-output (MIMO) communications. But for the conventional QRD-M algorithm, only one parameter, the maximum number of the survived branches, is used to make tradeoff, which cannot provide more valuable tradeoff options with better performance to complexity ratio. In this paper, an improved QRD-M algorithm with two independent parameters is proposed. It can provide more and better tradeoff options, which is proved through complexity analysis and numerical simulations.

Although frequency domain equalization (FDE) is very efficient to resist frequency selective fading in single-carrier communications, its performance will be greatly degraded due to high Doppler impact in a fast moving environment. To enhance the system performance of single-carrier communication system with FDE (SC-FDE), the self- cancellation technique is migrated from orthogonal frequency division multiplexing system (OFDM). It can obtain significant performance gain in scenarios with high Doppler in numerical simulations.

This paper, patent pending, put forwards a novel low complexity channel estimator for a special case of comb-type pilot pattern with pilot interval I = 2, which can combat the effect of residual timing offset on interpolation-based channel estimation. The channel estimation algorithm based on comb-type pilots is divided into pilot signal estimation and channel interpolation. The pilot subcarriers estimation can be based on least square (LS) or minimum mean square error (MMSE) algorithms which can achieve better performance than LS at the cost of high complexity. Furthermore MMSE need to know in advance the priori knowledge of noise variance and channel covariance. So considering engineering implementation, LS algorithm for pilot subcarriers estimation and linear interpolation for non-pilot subcarriers estimation are adopted in this paper. Then the effect of residual timing offset on such channel estimator is analyzed. Based on the unique characteristics of the special pilot pattern focused by this paper, a quite simple correction method is put forward. And simulation results show that the proposed method can effectively improve the system performance when existing residual timing offset.

Constructing PCGCs with unequal-code-rate irregular component LDPC codes is proposed in this paper. Parameters of component codes are optimized by differential evolution based on EXIT chart. In the optimization results, code rate of component code 1 is relatively higher and the other one is lower so that these two component codes play different roles in different decoding stages. Numerical simulation results show that the proposed PCGCs with optimized parameters outperform PCGCs constructed by equal-code-rate regular/likely regular component codes and optimized irregular LDPC code.

Sphere decoding (SD) has been proposed as an efficient algorithm in digital communications. Its complexity is polynomial for both an infinite and finite lattice which makes the real-time implementation of a maximum-likelihood (ML) decoder feasible. The choice of an initial radius for the SD has a great impact on the trade-off between the performance and complexity. In this paper, we present a new initial radius selection scheme for the SD based on the constellation modulation type and the target bit error rate (BER). The proposed scheme accurately define an initial radius of the SD, which reduces the radius redundancy and ensures that at least one lattice point is inside a sphere. Complexity analysis shows that the proposed scheme has much lower computational complexity than that of the conventional one with zero decoding failure rates meanwhile ensures no performance loss.

Amplify and forward (AF) is an effective mode for relaying systems with ease for implementation. In a conventional orthogonal frequency division multiplexing (OFDM) based AF system, relay stations simply amplifies the received signals and forward them to the destination station. Due to the fact that channel transfer function for the first hop (source-relay) and the second hop (relay-destination) may vary a lot, system capacity is limited. This contribution introduces a method named sub-carrier coupling, which couples sub-carriers for the two hops in a certain rule to improve system capacity. Firstly, system with a single relay station and its capacity is discussed, and the coupling scheme to maximize the system capacity is proposed. Then system with multiple relay stations is investigated, and two solutions are proposed. The solution with sub-carrier coupling can bring to much higher capacity since it determines the sub-carrier coupling scheme and assigns relay stations to forward signal for all sub-carriers jointly. Numerical simulation is carried out to show the efficiency of the proposed solutions.

A channel estimation method based on sample-shift of time-domain signal is proposed for OFDM systems. Through artificial cyclic sample-shift of time-domain signal, the equivalent coherent bandwidth of OFDM systems can be broadened. Hence, the channel estimation errors, caused by the simple linear interpolation in frequency-domain, can be reduced due to the increase of correlation between neighboring pilot sub-carriers. The theoretical analysis and simulation results show that equivalent channel coherent bandwidth can be broadened effectively and the channel estimation performance can be improved by sample-shift significantly, especially over high SNR range. When the value of cyclic sample-shift is given as the sampling-interval normalized RMS of channel, the near optimal performance can be achieved.

A novel turbo codec with hybrid modulation is proposed in this paper. Instead of brutal puncturing, we apply different modulation alphabets to the original not-punctured turbo coded bits, where the systematic bits are modulated by a lower order constellation and the parity bits are mapped by a higher order constellation, respectively. Numerical results show that the proposed scheme without puncturing can noticeably improve the bit error rate (BER) and frame error rate (FER) performance of turbo codes in Rayleigh flat fading channels compared with the conventional turbo codes with puncturing and uniform modulation with the same spectral efficiency.

Channel quality indicator (CQI) is essential for the transmitter to select the proper modulation and coding scheme (MCS). However, the feedback of the CQI also decreases the spectrum efficiency. This paper proposes a reduced CQI feedback signaling for the multi-stream downlink multiple-input multiple-output (MIMO) system. In addition, power loading algorithms are applied to further increase the throughput of the system. The proposed scheme decreases the overhead of the CQI feedback remarkably, while achieves good throughput performance.

A novel adaptive LDPC coded OFDM transmission technique is proposed in this paper. Assuming the channel state information (CSI) is known to the transmitter, the irregular LDPC coded bits are ordered and allocated into subcarriers adaptively. Then two power allocation schemes, fixed ratio truncating (FRT) and fixed ratio inverse water-filling (FRIWF), are proposed to reallocate the power of each subcarrier. Performance of the strategy is analyzed by density evolution and numerical simulation. "Equal SNR search" (ESS) algorithm is also proposed to evaluate the performance of FRIWF scheme. Noticeable gain in terms of SNR can be achieved over frequency-selective fading channels.

Frequency domain equalization (FDE) is a very efficient way to resist frequency selective fading in single-carrier communications, but its performance will be greatly degraded when suffering serious Doppler impact due to time varying channel state. Based on the single-carrier system with FDE (SC-FDE), a segmentation FDE scheme is proposed to resist high Doppler in this paper without the price of system bandwidth efficiency. Through numerical simulations, its significant performance improvement in high Doppler is proved.

Conventional filter bank's orthogonal condition can be decomposed into two conditions in frequency domain and in time domain. Novel bi-orthogonal transmit and receive filters with different length and impulse response are designed respectively to satisfy these two orthogonal conditions. When the proposed bi-orthogonal filter design method is employed in the filter-bank based uplink transmission scheme, the computational complexity at UE side can be greatly reduced due to the short length of transmit prototype filter.

To enhance the bandwidth efficiency in cyclic prefix (CP) assisted OFDM system, we propose a complexity reduced blind channel equalization scheme based on the finite alphabet property of the transmitted signal and we also present its simplification scheme in this paper. According to the numerical simulation results, our proposals can perform as well as the conventional pilot assisted frequency domain channel estimation (PFDCE) in high Eb/N0, but a little worse in low Eb/N0

A DFT spread generalized multi-carrier (GMC) based frequency division multiple access (FDMA) scheme, i.e. DFT-S-GMC, is proposed for uplink transmission of wideband mobile communication systems. The scheme achieves spreading gains in the frequency-domain by discrete Fourier transform (DFT), and the frequency division multiplexing (FDM) and FDMA by inverse filter-bank transform (IFBT). By theoretical analysis and simulation results, it is shown that the proposed scheme has good peak-to-average power ratio (PAPR) performance and is more robust to the multi-access interference (MAI) than the existed main stream uplink schemes, e.g. OFDMA and DFT-S-OFDMA scheme

Cyclic Prefix (CP) has been applied in the single-carrier systems to resist the inter-block interference (IBI) due to the frequency selective fading channel and equalize the channel by one-tap frequency domain equalizer. However, there still exists the residual inter-symbol interference (ISI) due to imperfect channel equalization, which will severely degrade the system performance. In this paper the residual ISI is analysis in CP-SC with linear minimum mean square error (LMMSE) equalizer. With the initial estimates of LMMSE equalizer, the interference cancellation (IC) technique is adopted to alleviate the residual ISI so that the system performance can be considerably improved.

In general, cyclic prefix (CP), zero padding (ZP) or fixed symbol padding (FP) can be seen as the guard interval (GI) for data transmitting. By inserting enough long GI, we can remove inter-block interference (IBI) completely, and the signal distortion due to multi-path environment can be simply compensated by frequency domain equalization (FDE). But on the other hand, the system bandwidth efficiency will be decreased due to the redundancy of GI and additional channel estimation pilots. This paper proposes a new scheme to estimate channel state with the help of our special type GI and presents the FDE scheme based on the proposed GI system

The cellular OFDM systems with the frequency reuse factor as one suffers strong intercell interference. Time-domain scrambling OFDM systems suppress the intercell interference and provide the frequency diversity, but consume much power at the receiver. A novel OFDM system switch is proposed for radio network control based on instantaneous channel quality and multicell environments. Alternative systems are switched to optimize the overall system throughput and avoid unnecessary power consumptions

The adaptive frequency domain interference cancellation and channel equalizer for MIMO-CP-CDMA systems is proposed to cancel the residual interference stage by stage. At each stage, the proposed scheme performs the adaptive forward filtering and interference cancellation in frequency domain so that the signal to interference plus noise ratio can be improved and the error propagation of the interference cancellation can be avoided. The achieved SINR of each code channel is utilized to compute the normalized correlation coefficient. Simulation results show that the proposed scheme can achieve the satisfactory performance and avoid the error propagation

High peak-to-average power ratio (PAPR) is an inherit drawback of orthogonal frequency-division multiplexing (OFDM) systems, and it limits the use of OFDM technology. In all the PAPR reduction methods, digital clipping is probably the simplest one and it is very effective in the solution of the PAPR problem in OFDM systems. However, clipping introduces additional distortion, which degrades the performance of channel estimation and signal detection. In this paper, the performance of channel estimation using the simple least square (LS) method in OFDM systems with a clipping operation is analyzed, a decision-aided channel estimation method to mitigate the clipping distortion is developed, and the potential gain in channel estimation of the decision-aided method is derived theoretically. Simulation results show that, in clipped OFDM, the theoretical potential gain in channel estimation can be obtained by a few iterations of an iterative decision-aided channel estimation and signal detection process, and, simultaneously, the performance of signal detection is effectively improved

This paper considers the design of iterative receivers for space-frequency block-coded orthogonal frequency division multiplexing (SFBC-OFDM) systems in unknown wireless dispersive fading channel with outer channel coding. The iterative joint channel estimation and symbol detection algorithm is derived. A simple decision-error judgment criterion is proposed and an improved training scheme is applied in the system to suppress the error propagation. Both theoretical analysis and computer simulation show that this algorithm gives better BER performance and saves the overhead of OFDM systems

When MIMO channel correlation information is available at the transmitter, closed-loop precoding can be introduced to improve system performance. In this paper, we proposed a novel closed-loop precoding algorithm based on code distance allocation for transmit correlated spatial multiplexing MIMO systems. We show that pairwise error probability of transmit correlated spatial multiplexing MIMO systems is determined by the minimum transmit code distance allocated to each eigenmode subchannel. We give the structure of code distance allocation matrix to optimize transmit code distance. Our proposed structure of code distance allocation matrix can simultaneously optimize minimum code distance allocated to each subchannel. Computer simulations show that our proposed procoding algorithm can improve the performance of transmit correlated spatial multiplexing systems

Bell Lab vertical layered space-time architecture (V-BLAST) is an approach for exploiting high MIMO channel capacity. A power allocation algorithm has been proposed to improve bit error rate performance when using ordering successive interference cancellation (OSIC) detection. However, the BER performance of proposed power allocation algorithm degrades greatly with the increase of transmit antenna correlation. In this paper, we proposed a closed-loop precoding algorithm based on OSIC detection. Our proposed precoding algorithm combines decorrelation with power allocation algorithm to improve the BER performance of transmit correlated V-BLAST systems. Computer simulations demonstrated that our proposed precoding method has better performance than mere power allocation algorithm

OFDM symbols exhibit a property of high peak-to-average power ratios (PAPR), which degrades system performance. Partial transmit sequence (PTS) is a kind of methods to depress the PAPR effectively. To obtain good trade-offs between complexity and performance on PAPR reduction using PTS schemes, a trellis structure based PTS factor search method is proposed and investigated in this paper. The trellis search is with a variant constraint length L/sub C/, 1/spl les/L/sub C//spl les/V-1, where V is the number of PTS sub-blocks. The method is to decide a PTS factor by searching all possible paths obtained by varying L/sub C/ consecutive factors. Using different constraint length, trellis factor search PTS exhibits different PAPR reduction performance with different computation burden. The trellis search can be viewed as a general PTS factor search model including the full search and the iterative search appeared in previous papers.

In this paper, effects of cyclic prefix (CP) are investigated for orthogonal frequency division multiplexing (OFDM) systems over time-varying channels. Although employing a CP as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), it reduces the transmission efficiency of the system, especially for channels with long channel-impulse response (CIR), such as hilly terrain channels. On the other hand, the time-varying channel also gives rise to ICI. We show that to achieve a signal-to-interference ratio of 20 dB, for slow fading channels, half the length of CIR is enough for CP. For fast fading channels, the Doppler frequency become domination factor to affect OFDM systems

In cellular OFDM systems with the frequency reuse factor as one, all the same subcarriers are used in every sector of adjacent cells. This induces very strong intercell interference, particularly for the user equipment at the cell edge. A novel OFDM transceiver with time-domain scrambling is proposed in this paper to suppress the intercell interference and provide the frequency diversity. Numerical results proved that the proposed scheme considerably improves the spectrum efficiency and the overall OFDM system throughput in both single-cell and multicell environments. By using long scrambling in the time domain, an improvement in estimates of channel tap delays for frame synchronization and fast cell search can be also realized.

Orthogonal frequency division multiplexing (OFDM) is the projected modulation of choice for fourth-generation broadband multimedia wireless systems. However, for mobile applications, channel variations during one OFDM symbol introduce intercarrier interference (ICI), which degrades the performance. This gets more severe as mobile speed, carrier frequency or OFDM symbol duration increases. In this paper, we first analyze the ICI caused by time-variant channel in terms of the complex weighting coefficients, which give the contribution of each transmitter subcarrier to each demodulated subcarrier. Then, we propose a high efficiency intercarrier interference cancellation scheme to reduce the effects of Doppler frequency shift. Finally we derive the signal-to interference power ratio (SIR) for the proposed OFDM system. From the comparison between the normal OFDM and the proposed system, we can see that such an ICI cancellation scheme gives more than 5 dB SIR improvement. The simulation results confirm the analysis and show the SIR improvement.

We investigate the difference between extrinsic and posteriori information fed back from outer decoder to inner detector in a turbo-BLAST system for a quasistatic channel. With the help of extrinsic information transfer (EXIT) chart and simulation, it is observed that a faster convergence rate is obtained to feed back posteriori information, and especially, a better steady state performance is attained when antenna number is small.

A novel blind speech user interference cancellation receiver is proposed for high speed downlink packet access (HSDPA). The hard-decision HSDPA signals with known spreading codes and the soft-decision speech user signals with unknown spreading codes are estimated by a full Walsh transform correlator instead of the conventional RAKER. By regenerating and subtracting the speech user interference from the received signal, the desired HSDPA estimates may be further improved by multistage processing. Numerical results prove that the system performance can be considerably enhanced.

Clipping is a popular method to overcome large peak-to-mean envelope power ratios (PMEPRs) in orthogonal frequency-division multiplexing (OFDM) systems, but it results in nonlinear distortion, which degrades signal detection performance. In this paper, clipping effects on signal detection with a simple channel estimation in OFDM are investigated, signal-to-noise and distortion ratio (SNDR) results of detected signals are derived, and symbol error rates (SERs) of clipped OFDM in frequency selective Rayleigh fading channels are evaluated. The evaluated SER results are supported by simulations.

The use of multiple antennas at the transmitter and receiver results in enormous capacity increase. As a space-time architecture, VBLAST (vertical Bell laboratories layered space time) has demonstrated very high spectral efficiency. Originally, the BLAST detection scheme, called OSIC (ordered successive interference cancellation), was based on a successive interference cancellation. OSIC suffers from the error propagation problem. In this paper, we present a modified detection algorithm, called PD/spl I.bar/OSIC (partial decision OSIC), which aims at improving the performance of OSIC. It increases the number of candidate symbols, remains all candidate symbols and uses ML detection to detect all substreams at last stage. The simulation results show that the proposed algorithm obtains far better performance than OSIC.

An improved channel estimator with inter-symbol interference (ISI) free window measurement for OFDM systems is proposed by exploiting the inherent correlation between the cyclic prefix and portion of the received OFDM signal. This scheme make use of real time measured size of ISI free window to estimate the maximum delay spread of channel, then a transform domain channel estimator is adopted to further reduce the noise variance. Due to the ISI-free window can be estimated before the application of FFT to the received signal, it is a parallel processing, and very suitable for practical systems. Simulation result demonstrated that the proposed method can track the variation of delay spread of wireless channel, and improve the MSE of channel estimation significantly, thus give at least 1-db improvement in TU channel and 0.5-db in HT channel.

Multiple-input multiple-out (MIMO) wireless transmission has been proved to be a promising technique to support high data rate transmission. Among proposed schemes, space-time codes are designed to improve transmission reliability, while BLAST architectures are proposed to improve spectrum efficiency. To optimize both channel capacity and error probability, linear dispersion codes (LDCs) were proposed. However, original LDCs are designed for quasi-static fading channels, which don't guarantee optimal error probability in fast fading channels. In this paper, we discuss performance of LDCs both in quasi-static and fast fading channels. And we propose a modified LDCs transmission scheme, in which we introduce an interleaver in each antenna branch to turn quasi-static channels into equivalent fast fading channels. Then we present codes optimization criterion to search for LDCs which optimal both in quasi-static and fast fading channels. Computer simulations show that the performance of proposed LDCs design method is better than original LDCs.

The system performance can be improved by interference cancellation. Due to the flat fading at each narrowband subcarrier and the inter-symbol interference resistance in MC-CDMA, the interference cancellation can be much simpler than that in WCDMA. In MC-CDMA multicode downlink transmissions, the user equipment has the knowledge of all the assigned spreading codes and it makes interference cancellation more feasible. In this paper an advanced equalized parallel interference cancellation (EPIC) receiver that equalizes the data estimates of previous two stages is proposed for MC-CDMA systems. By predefining the weights on power of two, the scaling and soft combination required by the proposed EPIC can be implemented by two summations and one bit-shift operation at each stage instead of one summation and two multiplications. The additional required complexity can be reduced into negligible and the proposed EPIC has approximately same complexity as the conventional scheme. The numerical results proved that the proposed receiver could significantly improve the system performance.

Space-time transmission techniques, which make full use of the rich scattering channel with multiple transmit and receive antennas, can greatly increase the spectral efficiency. The conventional V-BLAST (Bell laboratories layered space-time) detection algorithm estimate symbols with the expected SNRs ordering. In this paper, a novel BLAST detection algorithm based instantaneous error ordering (IEO-BLAST) is proposed. The proposed BLAST detection algorithm obtains the additional diversity with instantaneous error ordering and reduces the effect of the error propagation. Therefore, the performance of the BLAST architecture is greatly improved. In addition, the simplified version of IEO-BLAST detection algorithm (SIEO-BLAST) is derived. And the performance of simplified algorithm has little degradation. The selection capacity (SC) is defined to represent the performance of the algorithm with successive interference cancellation.

The system performance can he improved by interference cancellation and transmit diversity. Due to the flat fading at each narrow band subcarrier and the inter-symbol interference resistance in MC-CDMA, the interference cancellation can be much simpler than that in WCDMA. It is ideally suited to the application of transmit diversify with a very simple linear combination on a sub-carrier basis. In MC-CDMA multicode downlink transmissions, the user equipment has the knowledge of all the assigned spreading codes and it makes interference cancellation more feasible. In this paper an advanced equalized parallel interference cancellation (EPIC) receiver that equalizes the data estimates of previous two stages is proposed for MIMO MC-CDMA systems using space time transmit diversity. By predefining the weights on power of two, the scaling and soft combination required by the proposed EPIC can be implemented by two summations and one bit-shift operation at each stage instead of one summation and two multiplications. The additional required complexity can be reduced into negligible and the proposed EPIC has approximately same complexity as the conventional PIC. The numerical results proved that the proposed receiver could significantly improve the system performance.

Clipping is a popular and simple method to overcome large peak to mean envelope power ratio (PMEPR) in OKDM system, but it results in a nonlinear distortion, which degrade the performance of channel estimation and signal detection. In this paper we estimated OFDM channel using a simple LS method based on a scheme that only part of the OFDM tones arc used for pilot, in which the clipping distortion is unknown for the receiver, and then we analyze the clipping effects on channel estimation. According to the estimated channel, we detected the OFDM signal and analyze the clipping effects on signal detection. We derived the BER solution of clipping OFDM in Rayleigh fading channel and the numerical solution is very close to simulation results.

Space-time transmission techniques, which make full use of the rich scattering channel with multiple transmit and receive antennas, can greatly increase the spectral efficiency. In this paper, a novel parallel multistage equalizer with partial decision feedback (PDF) is proposed for the layered space-time frequency-selective channel. It is comprised of a partial parallel interference canceller, a MIMO decision feedback equalizer and a linear combiner. Performance analysis shows that the proposed receiver is equivalent to the partial parallel interference cancellation (PPIC) using MMSE for initial estimation and tentative decisions. By using past- and post-symbol estimation to regenerate intersymbol and co-antenna interference, the proposed receiver can be further improved with partial decision feedback. Moreover, the proposed equalization schemes could be implemented by negligible complexity increase comparing with the other well-known approaches. Simulation results show that the proposed equalizers with PDF significantly outperform the conventional MIMO decision feedback equalizer without interference cancellation (IC) and MIMO equalizer with ordering successive interference cancellation (OSIC).

Space-time transmission techniques, which make full use of the rich scattering channel with multiple transmit and receive antennas, can greatly increase the spectral efficiency. In this paper, parallel multistage decision feedback equalizer (DFE), which is composed of parallel interference cancellation, multiple input single output decision feedback equalizer (MISO-DFE) and linear combination of the soft outputs of MISO-DFE and the previous soft decisions, is proposed for a single-carrier system over layered space-time frequency-selective channels. In addition, an algorithm is proposed to obtain the soft and hard decisions for initialization. And the proposed parallel multistage DFE can be implemented with lower complexity than MIMO-OFDM. Simulation results show that the parallel multistage DFE outperforms the MIMO-OFDM and various previous equalizers for a single-carrier system.

Transmit diversity techniques use multiple transmit antennas to enhance the spectral efficiency and data rate. Space time transmit diversity (STTD) combats the fading effects in frequency-non-selective channel efficiently. However, the interference enhanced by multi-antenna transmission and multipath environments will severely worsen the system performance and ruin the transmit diversity due to the symbol delays of EDGE systems in frequency-selective channels. In this paper a novel maximum likelihood sequence estimation (MLSE) receiver with recursive interference cancellation (IC) is proposed for EDGE STTD systems in practical environments. The proposed recursive ICMLSE receiver suppress the enhanced co-antenna and co-channel interference, restores the transmit diversity and multipath diversity, and significantly improve the system performance.

In this paper, two kinds of space-time block coded transmit diversity are proposed for OFDM system in mobile channels. One is in time domain and at least one frame (one symbol) delay is needed for the decoding algorithm; the other is in frequency domain so it also can be termed as space-frequency coded (SFC) transmit diversity more concisely, and no delay is needed for the decoding algorithm. We simulate the performance of these kinds of space-time block coded transmit diversity, the space-time trellis coded transmit diversity with 4- and 16-state are also simulated to give out a performance comparison with space-time block coded transmit diversity in this paper.

The Gauss-Seidel iterative algorithm for linear asynchronous decorrelation or minimum mean square error (MMSE) multiuser detector can be viewed as a kind of linear soft decision feedback multiuser detection. Based on this idea, a linear decorrelating or MMSE detector can be implemented by using a much simpler pipeline structure compared with the conventional scheme of computing the inverse of the corresponding correlation matrix. Through this pipeline structure, a lot of computations can be reduced.

An equalized parallel interference cancellation is proposed for uplink CDMA systems. The hard decisions of the RAKE receiver and 1-stage conventional parallel interference cancellation are combined and equalized by the proposed receiver for further performance improvements. By using digital two-bit-comparator, the hard decisions of the proposed detector at the current stage are compared with that of the previous stage, and then are separated into reliable and unreliable groups. The multiple access interference induced by reliable data estimates is regenerated and entirely subtracted from the soft outputs of the RAKE receiver. In contrast, the multiple access interference induced by unreliable data estimates is weighted and fractionally subtracted. The simulation results show that the proposed receiver significantly improves the system performance with negligible increase in complexity and memory.

High speed downlink packet access (HSDPA) is underway, supporting the evolution of third-generation systems to meet the rapidly developing needs for high data rates. High order modulation has been proposed for the adaptive modulation and coding sets to increase the transmission data rate for users with high signal-to-noise-plus-interference ratio. However, high order modulation is significantly less robust to noise, interference, and channel estimation errors. An advanced HSDPA receiver using equalized parallel interference cancellation is presented to suppress the interference. Link level simulations for WCDMA multicode transmissions in frequency-selective Rayleigh fading channels indicate significant performance advantages over conventional RAKE receiver with only 2-3 times complexity increase.

The effects of fading channels and multiple access interference limit the performance of downlink WCDMA systems. In this paper, a linear minimum mean-square error multiuser receiver with space-time transmit diversity is proposed for downlink WCDMA systems. The fading effect and multiple access interference can be jointly mitigated by the proposed scheme. The performance analysis under Gaussian approximation has also been carried out. Simulation results show that substantial performance gain can be achieved compared with conventional algorithms.

Multicode CDMA with V-BLAST architecture is an attractive scheme to achieve high data rate transmission. We propose an advanced detection scheme, order partial parallel interference cancellation with decision feedback (DF-OPPIC), to combat co-antenna interference and multiple access interference in the system. Compared with conventional V-BLAST detection algorithms, the scheme has better performance on the frame error ratio in flat Rayleigh fading channel and multipath Rayleigh fading channel. Moreover, due to avoiding calculating pseudo-inverse and optimal detection order at every time of iteration, the scheme has lower arithmetic complexity.

Multicode CDMA with V-BLAST architecture is an attractive scheme to achieve high data rate transmission. We propose an advanced detection scheme, order partial parallel interference cancellation with decision feedback (DF-OPPIC), to combat co-antenna interference and multiple access interference in the system. Compared with the original reference, the scheme has better performance on the frame error ratio in flat Rayleigh fading channel and multipath Rayleigh fading channel. Moreover, due to the avoidance of calculating pseudo-inverse and optimal detection order in every time of iteration, the scheme has lower arithmetic complexity.

A novel realization scheme of space-time block code (STBC) is proposed by unequal power distribution between different transmitting (Tx) antennas at slow fading scenario. With optimal power distribution between the Tx antennas, the scheme can achieve a better performance than conventional STBC with equal transmitted power. Then we analyze and simulate the performance difference.

The high-order modulation increases the transmission data rate, but apparently is more sensitive to amplitude and phase errors. For CDMA downlink transmissions, the common pilot channel reserves 10% of the total transmission power at base stations with the result of perfect channel estimation at mobile stations. Therefore the multiple access interference and co-channel interference can be successfully suppressed by parallel interference cancellation (PIC). Motivated by this, a high speed packet access (HSPA) scheme using PIC and flexible high-order modulation is proposed in this paper for frequency division duplex downlink CDMA communications. The proposed HSPA scheme has been evaluated by system level simulations in frequency selective Rayleigh fading channel with soft-decision channel decoding. The numerical results showed that the target coded bit error rate could be achieved by the HSPA receiver using 16-QAM with reasonable increase in signal-to-noise ratios, but the data rates can be doubled compared with the RAKE receiver using QPSK modulation.

Two bit estimators, the weighted average bit estimator and shrinkage bit estimator, used in DS-CDMA partial parallel interference cancellation (PPIC) proposed by Divsalar et al. (see IEEE Trans. on Comm., vol.46, no.2, 1998) , are compared. It is shown that the two bit estimators improve the performances by decreasing the power of the residual interferences, remaining in the decision statistic of each stage after interference cancellation, but in different capabilities. We also justify the assumption that the residual interference is Gaussian, under which the two bit estimators are derived by Divsalar et al. Furthermore, theoretical optimal weights of the above two estimators minimizing the BER are derived.

The performance comparison and analysis between space-time trellis code (STTC) and space-time block code (STBC) is given. It is observed that the 2STBC scheme has a fixed performance, while in contrast the 2STTC can improve its performance by increasing states. Furthermore, we prove that coding advantage is not always reliable for analyzing the performance of a space-time code (STC).

A reduced complexity multistage detector is introduced for DS-CDMA systems, which is derived from the soft sigmoid decision PIC (parallel interference cancellation) detector. From the derivation, we can see that this reduced complexity detector has a clear partial interference cancellation explanation about the soft sigmoid PIC detector. Derivation and computer simulations show that the proposed detector can reach the performance of the soft sigmoid PIC detector easily without the variance estimation of the residual noise at each stage or the sigmoid decision function which is not easily implemented in practice.

Due to multiple access interference (MAI) and cochannel interference enhanced by multi-antenna transmission and multipath channels, the performance of multiple-input single-output (MISO) systems can be even worse than the conventional single-input single-output (SISO) systems. We present a novel MISO precoder for W-CDMA time division duplex (TDD) downlink transmissions. The MAI and cochannel interference can be entirely eliminated by using the conventional RAKE receiver at the mobile station. The proposed MISO precoder has been evaluated with two transmit and one receive antenna in the frequency selective Rayleigh fading channel. The simulation results proved that the system performance can be significantly improved compared with the conventional RAKE receiver and linear decorrelator.

Space-time spreading (STS) is considered as one of the transmit diversity schemes in the current cdma2000 standard. Space-time transmit diversity (STTD) has been proposed and specified in WCDMA standardization. In this paper, the performance of STS and STTD have been evaluated and compared in forward link CDMA2000 systems. The performance analysis and simulation results show that these two schemes have approximately the same performance on the frame error ratio in multipath fading channels even with imperfect channel estimation. But STS has more implementation complexity and higher peak to average ratio compared with STTD.

Space-time trellis codes can achieve the best tradeoff between bandwidth efficiency, diversity gain, constellation size and trellis complexity. We propose some optimum low rate space-time trellis codes. Performance analysis and simulation show that the low rate space-time trellis codes outperform space-time block codes concatenated with convolutional code at the same bandwidth efficiency, and is more suitable for the power limited wireless communication system.

In this paper, a multiuser receiver, which cancels the multiple access interference (MAI) of the stage-based fractional number of users, is proposed for CDMA systems. In each stage, MAI of fractional number of users is regenerated and subtracted from the received signal. The fractional number is defined according to the number of successive stages. The simulation results show that the performance can be significantly improved by the proposed scheme compared to the conventional multistage detector.

A multi-sensor multiuser receiver is proposed in this paper for asynchronous CDMA systems. The receiver consists of multiple sensors, RAKE receivers, channel estimators and a multiuser detector. The proposed receiver improves the signal-to-noise ratio, effectively counteracts the effects of fading and suppresses the multiple access interference. We compare the performance of the multi-sensor RAKE receiver, the conventional multistage detector and our generalized multistage detector with different numbers of sensors by simulations. Finally, the bit-error-rate results for Rayleigh fading channels with different system capacity are presented versus the signal to noise ratio.

A novel iterative multiuser receiver is proposed for code-division multiple-access mobile communication systems. The receiver performs multiuser detection, channel decoding and feedback in each pass of the iterative process. The feedback of the previous data estimates is utilized as the improved initial estimates for the next iteration. Finally, two alternative multiuser detection algorithms (parallel interference cancellation based multiuser detection and our new approach) are simulated in the proposed iterative multiuser receiver. The simulated bit error probability are presented and compared with a conventional scheme in Rayleigh fading channels.