Efficient and accurate monitoring of land surface eco-environmental situation (LSEES) is critical to promoting the sustainable development of global society. This study utilizes satellite data from EOS/MODIS to derive the land surface eco-environmental index (LSEESI) through the covariance-based principal component analysis method. Four strategies are used to evaluate the performance of this methodology. The stability, reasonability, comprehensive representation, and regional adaptability of this model are approved. LSEESI is also compared with the remote sensing ecological index (RSEI) and shows that LSEESI better indicates the LSEES (R2= 0.674 for LSEESI, 0.437 for RSEI). Application of the LSEESI model in Yangtze River Delta during 2001-2021 shows the conclusions as follows: 1) Overall, the LSEES in Yangtze River Delta is stable or improving, and the annual average LSEESI increased from 0.572 to 0.593. 2) There were significant spatial differences in LSEES in Yangtze River Delta. Areas with relatively poor LSEES were mainly in Suzhou-Wuxi-Changzhou urban agglomeration, Hangzhou-Jiaxing-Ningbo urban agglomeration, and Shanghai. Regions with deteriorating LSEES were also mainly concentrated in the above urban agglomerations around Lake Taihu. 3) The contribution of temperature, precipitation, and NTL to LSEES was 0.07, 0.38, and 0.55, respectively, suggesting that LSEES change in Yangtze River Delta in recent 21 years might have been influenced primarily by human activity, with only some parts of Anhui Province affected mainly by climate change. This study demonstrated that the proposed LSEESI model can effectively monitor and quantitatively evaluate LSEES change, and provide the information necessary for monitoring and managing eco-environmental systems.

In this paper, we study a multi-user multi-relay interference-channel network, where energy- constrained relays harvest energy from sources' radio frequency (RF) signals and use the harvested energy to forward the information to destinations. We adopt the interference alignment (IA) technique to address the issue of interference, and propose a novel transmission scheme with the IA at sources and the power splitting (PS) at relays. A distributed and iterative algorithm to obtain the optimal PS ratios is further proposed, aiming at maximizing the sum rate of the network. The analysis is then validated by simulation results. Our results show that the proposed scheme with the optimal design significantly improves the performance of the network.

In this paper, we consider an energy harvesting (EH) two-way (TW) dual-relay network, including one non-EH relay and one EH relay equipped with a finite-sized battery. In the network, a space-time transmission protocol with space-time network coding is designed, and an optimal transmission policy for the EH relay is proposed by using a stochastic solar EH model. In this optimal policy, the long-term paired-wise error probability (PEP) of the system is minimized by adapting the EH relay's transmission power to the knowledge of its current battery energy, channel fading status, and causal solar EH information. The designed problem is formulated as a Markov decision process framework, and the conditional capability of the contribution to PEP by the EH relay is adopted as the reward function. We uncover a monotonic and limited difference structure for the expected total discounted reward. Furthermore, a non-conservative property and a monotonic structure of the optimal policy are revealed. Based on the optimal policy and its special structures, the expectation, lower and upper bounds, and asymptotic approximation of the PEP are computed and an interesting result on the system diversity performance is revealed, i.e., the full diversity order can be achieved only if the EH capability index, a metric to quantify the EH node's capability of harvesting and storing energy, approaches to infinity; otherwise, the EH diversity order is only equal to one, and the coding gain of the network is increasing with the EH capability index at this time. Furthermore, a full diversity criterion for the EH TW dual-relay network is proposed. Finally, computer simulations confirm our theoretical analysis and show that our proposed optimal policy outperforms other compared policies.

In this paper, a multiuser wireless powered communication network is considered where all users harvest energy from power beacons by wireless power transfer to support their uplink information transmission. A frequency-division duplex transmission scheme is adopted, where downlink power transfer and uplink information transmission are separated in different frequency bands. Compared with the time-division duplex scheme considered in most of existing literature, the frequency-division duplex scheme has more freedom to optimize the charging time of different cells respectively, and more suitable for distributed deployment of power beacons. The transmission slots and power allocation problem can be formulated as an optimization problem, which is proved to be convex. We derive the optimal slots allocation algorithm with the purpose of fair sum-throughput maximization. Moreover, the asymptotic expression of throughput is obtained, which provides useful insight on the deployment of power beacons. Simulation results demonstrate the proposed scheme can achieve significant performance gains compared to existing scheme from the literature.

We consider the uplink of a single-cell multiuser MIMO-OFDM system with MRC receiver and large antenna arrays at the base station. Assuming that the channels are perfectly estimated by the uplink pilots, we analyze the uplink spectral efficiency, taking the pilot overhead into consideration. The closed-form expression for the uplink spectral efficiency is derived and the optimal number of users in such a system is discussed. Our studies also show that the per antenna performance gain decreases as the total antenna number increases. Numerical results are presented to verify our analysis.

The energy efficiency (EE) has become an increasingly important issue in wireless communications because of the increasing energy cost and concern over the environmental issues. In this paper, the EE is focused and analyzed in device-to-device (D2D) communication underlaying cellular networks when the resources are shared by D2D and cellular users in three modes, namely, Non-Orthogonal Sharing mode (NOS), Orthogonal Sharing mode (OS), and Cellular Mode (CM). In addition, energy efficient power allocation schemes in the three modes are discussed under the maximum transmission power constraint. Numerical simulations are performed in a single-cell environment and the EE performance in these modes are simulated by varying three system parameters: a) the location of the cellular user, b) the distance between D2D users, c) the distance between the D2D pair and the Base Station (BS). The results demonstrate that it is more preferable that the D2D users communicate with each other directly in terms of the energy efficiency.

In this paper, the energy efficiency (EE) is focused and analyzed in device-to-device (D2D) communication underlaying cellular networks when the resources are shared by D2D and cellular users in three modes, namely, Non-Orthogonal Sharing mode (NOS), Orthogonal Sharing mode (OS), and Cellular Mode (CM). In addition, energy efficient power allocation schemes in the three modes are given under the maximum transmission power constraint. Numerical simulations are performed in a single-cell environment and the EE performance in these modes are simulated by varying three system parameters: a) the distance between D2D users, b) the distance between the D2D pair and the Base Station (BS), c) the location of the cellular user. The results demonstrate that the cross effect of the EE between NOS and OS exists and it is more preferable that the D2D users communicate with each other directly in terms of the EE.

In this paper, we propose an improved utility-based radio resource allocation algorithm in orthogonal frequency division multiple access (OFDMA) wireless networks. The queue state information (QSI), channel state information (CSI), QoS requirements and transmitted packets are synthetically considered in the utility function, thus it can meet the needs of networks with multi-service better. Based on the proposed utility function, the problem is designed as the sum utility maximization subject to both base station (BS) power and each subcarrier power. A heuristic algorithm based on Lagrange Multipliers method is used to solve the formulated optimization problem. Numeric simulation results show that compared with traditional approaches which do not take the QSI and transmitted packets into account, our proposed approach achieves better performance in terms of balancing throughput, delay and fairness.

Antenna selection provides a practical way to decrease system complexity and the hardware cost of radio frequency (RF) chains in MIMO system. In this paper, We propose an antenna selection scheme in the regenerative MIMO relaying scenario,which can achieve full diversity order of MIMO single relay network, and derive the diversity-multiplexing tradeoff(DMT) performance of the scheme. The algorithm has a lower complexity , and the simulation results show that its performance is relatively good.

An amplify-and-forward relay network composed of a source (S), N relays and a destination (D) is considered, where the relays are untrusted in the sense that they may eavesdrop on the transmission from S to D, that is they may decode messages of the source. As a part of the system, these untrusted relays are willing to help the communication from S to D. To prevent the relays from decoding the source message, a secure spacetime code with full diversity is designed at the source node. In this paper, no secret information is exchanged between S and D in advance. Training symbols are transmitted by S and D respectively. Based on the assumption of channel reciprocity, S and D can obtain the equivalent channels between them, which are unavailable to the relays. By exploiting the equivalent channels and random source antenna selection, random phase rotation is designed for each space-time code block at S to prevent the relays from eavesdropping. Simulation results are presented to verify the performance of the proposed secure spacetime coding scheme.

Physical (PHY) layer security has recently become a hot issue in wireless communication. In this paper, an approach to a generalized anti-eavesdropping space-time network coding (GAE-STNC) for cooperative communications is proposed to achieve the physical layer security and overcome the problem of imperfect synchronization while still guaranteeing full diversity. Based on the assumption of channel reciprocity, the basic idea is to exploit the channel state information (CSI) between the legitimate transmitters and the receiver, which is used to generate the secret key. With this secret key, the transmitters introduce pseudo random interferences by adding an anti-eavesdropping matrix to the initial system. Since the eavesdropper's channel is typically independent of the legitimate channel, the channel between the legitimate transmitters-receiver pair, the signal received by the eavesdropper is interfered. While the receiver can effectively decode the signal by utilizing the global CSI, the eavesdroppers can not decode the received signal correctly. Then, two specific schemes derived from the GAE-STNC are proposed. Numerical analysis and simulation results are presented to illustrate the proposed GAE-STNC schemes.

For downlink multi-user multiple-input multiple-output (MU-MIMO) system, the precoding scheme of maximizing signal-to-leakage-and-noise (SLNR) at base station (BS) relies on channel covariance matrix. However, the CSI feedback defined in LTE/LTE-A does not directly correspond to channel covariance. Instead, precoding matrix indicator (PMI) and channel quality indicator (CQI) are fed back from user equipment (UE) to the base station. It is possible to reconstruct channel covariance matrix based on PMI/CQI. However such reconstruction is quite rough and may cause MU-MIMO performance degradation.

The best beam directions for Q coexisted 60GHz radio systems are jointly determined to maximize the sum rate. Capturing the directional characteristic of 60GHz indoor interference channel, a joint discrete optimization problem is properly established with the objective function evaluated from the noisy channel estimates. In addition to the conventional exhaustive search (ES) method, we propose a simulation based sequential stochastic approximation (SSA) algorithm with segmented indicator to solve the problem in an adaptive way. Although only the sub-optimal solution can be guaranteed in practice, the proposed algorithms are demonstrated to be more efficient and practical to implement than beam search in brute force, especially in dense network. Also, they exhibit good flexibility and feasibility for different usages in tradeoff between performance and complexity.

This letter addresses the joint maximum likelihood carrier frequency offset (CFO) and channel estimation for asynchronous cooperative communication systems. We first present a space-alternating generalized expectation-maximization (SAGE) based iterative estimator (SAGE-IE). Then a low-complexity approximate SAGE-IE (A-SAGE-IE) is developed. Our proposed algorithms decouple the multi-dimensional optimization problem into many one-dimensional optimization problems where the CFO and channel coefficients of each relay-destination link can be determined separately. Simulations indicate that, even though timing offsets are present, the proposed estimators can asymptotically achieve the Cramer-Rao bround (CRB) for the perfectly timing synchronized case.

This paper develops a cooperative scheme among cognitive radios (CRs) to increase the spectrum sensing performance. CRs adaptively transmit local binary decisions to a fusion center (FC), which in turn uses a simple maximum-likelihood detector to decide the presence of absence of the primary user (PU). The diversity order of the probability of false alarm and miss-detection is studied under both Neyman-Pearson and minimum-error-probability criteria. In particular, it is shown that under the Neyman-Pearson criterion, the probability of miss- detection can achieve diversity order up to the number of CRs for a bounded probability of false alarm. Under the minimum-error-probability criterion, both the probability of false alarm and miss-detection can achieve diversity order up to the number of CRs. Compared to existing cooperative sensing approaches, this novel scheme is robust to fading effects in both PU-to-CR and CR-to-FC links. Simulated tests verify the analytical claims, showing considerable performance gains compared to non-cooperative and non-adaptive hard-decision schemes.

We propose a novel algorithm for the problem of nonorthogonal joint diagonalization of a set of structured matrices based on successive Jacobi-like transformations. Though the elementary transformation matrices we use are not optimal in the sense of the global criterion, they are ensured to be nonsingular, and can be computed in closed form. The algorithm is efficient in virtue of its low computational complexity and fast convergence. The performance of the new algorithm is compared in simulations to the similar algorithms of the recent literature.

Prolonging lifetime is of paramount importance for an energy-constrained wireless network to ensure uninterrupted information exchange. Cooperative communication technology, which can achieve high energy-efficiency by exploiting spatial diversity of distributed nodes, has become an attractive way for lifetime extension of wireless networks. In this paper, a novel relay selection method is developed for lifetime extension in an Amplify-and-Forward (AF) cooperative network where the transmission power is subject to a signal-to-noise ratio (SNR) requirement at the destination. In the proposed method firstly the average transmission power for every relay to forward the message is derived through the knowledge of channel statistics and then a utility function is designed to introduce it into the relay selection procedure so as to offer a relay more opportunity to be selected when its required transmission power is lower than usual. Simulation results show that the proposed method achieves improved performance in extending the lifetime by both reducing the average transmission power and lowering the residual battery energy.

In cognitive radio systems, secondary users should continuously sense the licensed spectrum in case a primary user starts to transmit. Whether or not the secondary users can sense the spectrum during the secondary communication is an attractive problem. In this paper, we consider a particular wireless cooperative communication system acting as a secondary system, and demonstrate that by exploiting the cooperative property of the cooperative communication, the secondary system dose sensing while communicating. Compared with the conventional periodical sensing scheme, the proposed method achieves better sensing performance without any overhead. The theoretical analysis is given, and the sensing performance of the secondary system is investigated. Simulation results show that the sensing performance of the proposed method is improved significantly as opposed to that of the conventional spectrum sensing.

A full frequency diversity COFDM (FD-COFDM) scheme based on sub-channel multiplexing is proposed to improve the performance of COFDM without sacrificing spectral efficiency. Since the channel suffers the frequency selective fading, the received signal contains strong inter-symbol interference (ISI). To obtain the diversity- and code-gains, an iterative receiving algorithm is presented. By jointly using diversity combining, interference cancellation and soft decoding, the proposed iterative receiving algorithm properly decodes the information bits in the strong ISI environment. Consequently, all the subcarriers of FD-COFDM participate in diversity so that the system can achieve full frequency diversity in most practical channels. The new receiving algorithm is relatively simple compared to the maximum likelihood estimation algorithm, and is therefore a better alternative to balance the algorithm complexity and system performance.

For orthogonal frequency division multiplexing (OFDM) communication systems, the frequency offset in mobile radio channels distort the orthogonality between sub-carriers, which results in intercarrier interference (ICI) and seriously degrades the performance of systems. In this paper, based on ICI coefficients analysis, a novel precoder design scheme is proposed for mitigation ICI in OFDM mobile systems. In this technique, precoder matrix is designed by linear counteraction and inserted in transmitter. Computer simulation results show that this new scheme can effectively reduce ICI and significantly improve the carrier-interference power ratio. Compared with existing ICI mitigation schemes with channel estimation, the proposed scheme has lower computational complexity, and compared with self-cancellation scheme, the bandwidth efficiency can be improved. The proposed scheme has also better convergence stability for time-varying frequency selective fading channel.

Inter-cell interference is a serious problem in multicell OFDMA systems. In this paper, an iterative interference price based power allocation algorithm with inter-cell interference coordination is proposed for downlink multi-cell OFDMA systems. In the algorithm, each base station announces a price for interference and performs power allocation based on the price information received from other base stations with the objective of maximizing the net utility of the local cell. Through price information updating and transmit power reallocation in every iteration, the algorithm can intelligently coordinate the inter-cell interference in the system. Besides, the algorithm can be implemented distributedly with price information shared among neighboring cells. Considering the heavy feedback overhead from the users to the base stations, a simplified algorithm which considers only the major interference is also proposed. Simulation results show that when the inter-cell interference is serious, the proposed algorithms can effectively increase the transmission rate compared with iterative water-filling.

In this paper, we consider asynchronous two-way wireless relay networks with amplify-and-forward protocol (AF- TWRN), where the distributed space-time code (DSTC) is utilized without synchronization among relays to assist the communication between two terminals. We analyze the pairwise error probability (PEP) behavior of DSTC for AF-TWRN under frequency-selective channels. From our analysis, it is seen that of the paths from one terminal to relays and from relays to another terminal, those with smaller diversity order result in an overall system performance bottleneck. Considering both the timing errors among relays and the multipath fading, we then propose a family of distributed convolutional space-time codes (DCSTC) for AF-TWRN such that the full diversity order can be achieved. Finally, various numerical examples are provided to corroborate the analytical studies.

In cooperative communication networks, the performance of the distributed space-time code (DSTC) will be severely degraded if the timing synchronization among relay nodes are not perfect. In this paper, we propose a systematic construction of the so called distributed linear convolutional space-time code (DLCSTC) for multipath fading channels that does not require the synchronization assumption. We derive sufficient conditions on the code design such that the full cooperative and multipath diversities can be achieved under the minimum memory length constraint. Then we design DLCSTCs that both have the trace-orthonormality property and achieve the full diversity. We also study the diversity property of the DLCSTC with suboptimal receivers. We show that the proposed codes can also achieve the full diversity for asynchronous cooperative communications with ZF, MMSE and MMSE-DFE receivers under frequency-selective channels. Finally, various numerical examples are provided to corroborate the analytical studies.

In this paper, distributed power allocation schemes are studied for amplify-and-forward (AF) cooperative relay networks, where only partial channel state information (CSI) is available at the source and relays. Aiming at minimizing the total transmit power while providing a target outage probability, a scheme is first investigated in which the source decides transmit power and a relay-forwarding threshold, and each relay makes individually the transmit decision based on the threshold and its own CSI. Then a single relay power allocation scheme is proposed to simplify the implementation complexity, in which only one relay is selected to forward messages. Simulation results illustrate the performance improvement of the proposed schemes.

This letter presents an iterative detection scheme for orthogonal frequency division multiplexing (OFDM) systems with imperfect channel state information (CSI). In deriving the proposed scheme, channel estimation error is combated by using the theory of variational inference. The proposed scheme deals with the channel impulse response parameters and detects the information symbols iteratively and jointly. The effectiveness of the new approach is demonstrated by Monte-Carlo simulations.

In wirelessad-hocnetworks, network coding can take advantage of the bi-directional traffic flows to increase the achievable throughput. In this letter, on the one hand, cooperative diversity based on distributed cyclic delay diversity is integrated into the physical network coding scheme to combat fading in wireless channel. On the other hand, to reduce the system overhead for the coherent detection at each destination terminal, differential modulation based on linear constellation precoding is proposed. Theoretical analysis and numerical results both confirm that our two-way relaying scheme can achieve full spatial diversity. Furthermore, the proposed scheme can significantly improve the performance over traditional one-way wireless relay scheme and the existing two-way wireless relay schemes due to its enhanced spectral efficiency and lower implementation complexity.

In most cooperative communication works, perfect synchronization among relay nodes is assumed in order to achieve the cooperative diversity. However, this assumption is not realistic due to the distributed nature of each relay node. In this paper, we propose a family of distributed space-time trellis code (DSTTC) that does not require the synchronization assumption. It is shown that the proposed DSTTC has minimum memory order, and the construction of such DSTTCs is equivalent to using the generator matrices that have full row rank, regardless of sub-matrices shifting problem. Here, a sub-matrix corresponds to the generator matrix of a relay node under frequency-selective channels. We derive sufficient conditions on the code design such that the full cooperative and multipath diversities can be achieved. By further studying the diversity product, we design the DSTTCs which can achieve full diversity and the maximum coding gain through the exhaustive computer search. The newly proposed codes exhibit good properties, e.g., high energy efficiency and low synchronization cost, and can be applied to distributed wireless networks. Finally, various numerical examples are provided to corroborate the analytical studies.

In cooperative communication networks, the performance of distributed space-time code will be severely degraded if the timing synchronization among relay nodes is not perfect. In this letter, we propose a systematic construction of the so called distributed linear convolutional space-time code (DLCSTC) for multipath fading channels that does not require the synchronization assumption. We derive sufficient conditions on the code design such that the full cooperative and multipath diversities can be achieved under the minimum memory length constraint. Then we design DLCSTCs that both have the traceorthonormality property and achieve the full diversity. We show that the proposed codes can also achieve the full diversity for asynchronous cooperative communications with ZF, MMSE and MMSE-DFE receivers under frequency-selective channels. Finally, various numerical examples are provided to corroborate the analytical studies.

Exploiting the desirable properties of cyclic group coding, we develop a full-rate differential distributed space- frequency coding (DSFC) in the frequency domain for broadband non-regenerative wireless relaying systems. The pairwise error probability analysis demonstrates that our scheme guarantees a diversity order of Gdles min{ML1, ML2}, where M is the number of the relay nodes, and L1and L2 are the number of taps of source-to-relay and relay-to-destination channels, respectively. A novel subcarrier allocation method without channel state information (CSI) at any node is proposed to improve the diversity product. Moreover, for an arbitrary number of relays, a competitive non-regenerative DSFC proposed by Zhang et. aL, is extended to differential transmission incorporated with the proposed subcarrier allocation method for fair comparison. Both theoretical analysis and simulation results confirm that the proposed DSFC outperforms Zhang's scheme significantly in the differential case.

This paper analyzes the performance of limited feedback (LFB) beamforming in multiple-input multiple-output (MIMO) distributed antenna systems (DAS). The averaged effective signal-to-noise ratio (ESNR) is derived for both the case where all antenna ports are used for transmission (blanket transmission, BT) and the case where only one of the ports are selected (port selection, PS). Based on the derived ESNR, we propose two adaptive LFB beamforming schemes for MIMO DAS, which switch between BT and different PS schemes. Through simulations, we confirm the analytical studies and investigate the performance of different LFB beamforming schemes.

In this paper, we study the asynchronous cooperative diversity schemes employing the amplify-and-forward (AF) protocols over the frequency-selective channel. We consider three time-division-multiple-access (TDMA) based cooperation protocols and derive the corresponding pairwise error probabilities (PEP). From our analysis, it is seen that of the paths from the source to relays and from relays to the destination, those with smaller diversity order result in the bottleneck for the overall system performance. We propose a new type of the distributed space-time trellis code (DSTTC) that can achieve full diversity for the asynchronous transmission by utilizing a specifically designed generator matrix. The optimal DSTTCs with the minimum memory order are found by the exhaustive computer search. Finally, various numerical examples are provided to corroborate the analytical studies.

In most cooperative communications works, perfect synchronization among relay nodes is assumed in order to achieve cooperative diversity. However, this assumption is not realistic due to the distributed nature of each relay node. In this paper, we propose a family of distributed space-time trellis code (DSTTC) that does not require the synchronization assumption. It is shown that the proposed DSTTC can achieve full cooperative and full multipath diversities for the asynchronous transmission by utilizing a specifically designed generator matrix. We derive sufficient conditions on the DSTTC with the minimum memory order. Finally, various numerical examples are provided to verify the analytical studies. The newly proposed codes exhibit good properties, e.g. high energy efficiency and low synchronization cost, and can be applied to distributed wireless networks.

In recent literature, it has been shown that in a relaying network utilizing multiple relay precoding, the signal-to- noise ratio (SNR) at each destination node will scale linearly with the number of relays K, which is referred to as the distributed array gain (DAG) K. Different from the prior studies which assume perfect channel state information (CSI) at each of the relay nodes, this work considers the impact of limited feedback on the performance of multiple relay precoding. We show that the conventional limited feedback scheme fails to obtain the DAG K, which is due to the phase ambiguity introduced by the channel quantization function. Based on the analysis, a novel feedback and precoding procedure is proposed. It is proved that the proposed procedure can obtain the DAG K with only one additional feedback bit for quantizing each relay-destination channel compared with the conventional scheme. Simulation results are also provided to verify our analysis.

This paper is concerned with timing synchronization of high rates ultra-wideband (UWB) signals operating in a dense multipath environment, where access must tackle inter-frame interference (IFI), inter-symbol interference (ISI) and even multi-user interference (MUI). A training-based joint timing and channel estimation algorithm is proposed, which is resilient to IFI, ISI, MUI, and pulse distortion. A low-complexity detection scheme similar to transmit-reference (TR) scheme comes out as a by-product, which is robust against moderate timing errors. A lower bound on the probability of correct detection is derived. The results show that the proposed algorithm achieves a significant performance gain in terms of mean square error and bit error rate, compared to the Timing via Dirty-Templates (TDT) algorithms.

In most cooperative communications works, perfect synchronization among relay nodes is assumed in order to achieve cooperative diversity. However, this assumption is not realistic due to the distributed nature of each relay node. In this paper, we propose a new distributed space-time coded transmission (DSTCT) scheme for asynchronous cooperative communications under frequency-selective channels. First, a novel transmission protocol, in which each relay is labelled in delay-ascending order, is proposed. Second, the pairwise error probability and the design criterion of DSTCT are derived. Finally, based on the proposed transmission protocol and the design criterion, a new DSTCT scheme for asynchronous cooperative communications is proposed. Theoretical analysis and simulation results show that this scheme can achieve both full cooperative diversity and full multipath diversity.

In this paper, we study the performance of the multiple-input multiple-output macrodiversity transmission with limited feedback. The downlink scenario, where multiple transmitters serve the user simultaneously, is considered. We find it necessary to modify the model of the quantized channel by Jinal (2006) such that the phase ambiguity in the vector quantization procedure can be characterized. Using the modified model, we show that the conventional limited feedback methods cannot obtain the macrodiversity gain even when the size of the codebook is asymptotically large, and that the macrodiversity gain can be attained by adding only one bit of phase feedback. Our analysis can apply to both random vector quantization and any fixed codebook setup, and is verified by the simulation results.

In most cooperative communications works, perfect synchronization among relay nodes is assumed. However, this assumption is not realistic due to the distributed nature of each relay node. In this paper, we consider the downlink in a MIMO cellular network utilizing two-hop transmission protocols with fixed infrastructure relay stations (RS) to enhance coverage and data rates. We propose a family of distributed space-time trellis code (DSTTC) for two RSs without the synchronization assumption. It is shown that the construction of the proposed DSTTCs is equivalent to using the generator matrices that have full row rank regardless of sub-matrices shifting problem, where a sub-matrix corresponds to the generator matrix of a RS under frequency-selective channels. Finally, various numerical examples are provided to verify the analytical studies.

In this paper, a multiuser detection (MUD) scheme employing joint adaptive filter and space-time signal processing is discussed and experimentally investigated for direct-sequence (DS) ultra-wideband (UWB) multiple-input multiple-output (MIMO) systems. In dense multipath environments, by taking advantage of the space-time filter structure in conjunction with simple transmit and/or receive diversity approaches, the investigated MUD scheme is capable of significantly improving the performance in combating multiuser interference (MUI) compared with the temporal MUD method based on single-antenna systems. The effects of transmit and/or receive diversity are also examined via the Monte-Carlo simulations.

In order to improve the spectral efficiency of the V-BLAST system employing adaptive modulation, a novel hybrid detection order is proposed that detects saturated, invalid and unsaturated valid subchannels in turn. Compared with the existing forward and reverse orderings, hybrid ordering can achieve higher spectral efficiency and smaller gap between the simulated BER and the target BER due to the flexible detection order. In addition, we demonstrate that the higher equivalent channel gain does not imply higher spectral efficiency at the scenario of adaptive modulation.

In order to improve the downlink capacity of a multiuser multiple input multiple output system exploiting spatial multiplexing, this paper proposes two grouped multiuser diversity schemes. Aiming at the suppression of co-channel interference, both schemes select a group of users to share the same time-frequency channel unit in two steps. The first scheme iteratively selects the users based on a traditional multiuser scheduler - the PFS method. The second scheme first groups the users according to the cross-correlation of their channel response matrixes, then selects the group that maximizes the system capacity. Simulation results show that both schemes can provide fair scheduling and enhance the capacity performance. Moreover, the second scheme achieves higher capacity with more computational complexity comparing with the first one.

This paper presents the evaluation work of the air interface technologies developed for beyond third generation (B3G) mobile communications. Strategy of the evaluation is discussed and a set of performances selected for evaluation is justified. A software platform developed for the evaluation of potential B3G systems is then presented. Preliminary results show that OFDM+MIMO are probably one of the best choices of the air interface for B3G.

In the existing V-BLAST system with adaptive modulation, spectral efficiency decreases dramatically at lower SNR due to the invalid subchannnel which does not meet the target BER. To resolve this problem, a power reallocation algorithm is proposed in this paper. The post-detection SNR of the valid subchannel can be improved by reallocating the power quotas of the invalid channel to valid ones. Simulation results show that by means of the proposed method the spectral efficiency increases obviously, especially when the SNR is low.

An improved second-order power control (ISOPC) algorithm for code division multiple access (CDMA) systems is proposed in order to improve the convergence speed in distributed carrier-to-interference ratio balance algorithm. By adopting soft optimum coefficient, the ISOPC algorithm uses current and previous power levels for power updates. Convergence analysis is performed and the condition of ISOPC convergence is provided for feasible systems. The simulation indicates that when soft optimum coefficient equal to 1.2, the ISOPC algorithm has faster convergence speed compared to the second-order power control (SOPC) algorithm.

As a multiple access technology, DS-CDMA operates in an interference limited environment so the system capacity is dominated by multiple access interference (MAI). Since MAI is proportional to the transmit power strengths of other users, the level of transmit power by all participating users requires careful planning and control in order to attain maximum system capacity. The power distribution under a multirate wideband CDMA environment is investigated. It is shown that power control is a function of the spread bandwidth, user data rates, and the user QoS requirements. A power control algorithm, based on a convergence criterion, is proposed. Numerical results which demonstrate the viability of the power control algorithm are presented.