Massive machine-type communication (mMTC) is a newly introduced service category in 5G wireless communication systems to support a variety of Internet-of-Things (IoT) applications. In recovering sparsely represented multi-user vectors, compressed sensing-based multi-user detection (CS-MUD) can be used. CS-MUD is a feasible solution to the grant-free uplink non-orthogonal multiple access (NOMA) environments. In CS-MUD, active user detection (AUD) and channel estimation (CE) should be performed before data detection. In this paper, we propose the expectation propagation-based joint AUD and CE (EP-AUD/CE) technique for mMTC networks. The EP algorithm is a Bayesian framework that approximates a computationally intractable probability distribution to an easily tractable distribution. The proposed technique finds a close approximation of the posterior distribution of the sparse channel vector. Using the approximate distribution, AUD and CE are jointly performed. We show by numerical simulations that the proposed technique substantially enhances AUD and CE performances over competing algorithms.

In massive machine-type communication (mMTC) systems, a large number of machine-type devices sporadically transmit small packets with low rates. By exploiting the sporadic activity of machine-type devices, we can cast the detection problem as the compressive sensing-based multi-user detection (CS-MUD). In this paper, we propose a novel CS-MUD algorithm for the active user and symbol detection based on a maximum a posteriori probability (MAP) criterion. By exchanging extrinsic information between active user detector and symbol detector, the proposed algorithm improves the performance of active user detection and the reliability of symbol estimate. Numerical simulations demonstrate that the proposed algorithm achieves outstanding MUD performance.

In this paper, we develop a contention window (CW) control scheme for practical IEEE 802.11 wireless local area networks (WLANs) that have node heterogeneity in terms of the traffic load, transmission rate, and packet size. We introduce activity probability, i.e., the probability that a node contends for medium access opportunities at a given time. We then newly develop a performance analysis model that enables analytic estimation on the contention status including the collision probability, collision time, back-off time, and throughput with comprehensive consideration of node heterogeneity. Based on the newly developed model, we derive the theoretically ideal contention status, and develop a CW control scheme that achieves the ideal contention status in an average sense. We perform extensive NS-3 simulations and real testbed experiments for evaluation of both the proposed performance analysis model and CW control scheme. The results show that the proposed model provides accurate prediction on the contention status, and the proposed CW control scheme achieves considerable throughput improvement compared to the existing schemes which do not comprehensively consider node heterogeneity.

Today, voice over IP (VoIP) service is emerging as a popular and important application in wireless local area networks (WLANs). While rate adaptation (or link adaptation) has been identified as a key factor determining the performance of WLANs, we have observed that most (if not all) rate adaptation algorithms have been developed to improve the throughput of data traffic, not the quality of service (QoS) of VoIP traffic. Accordingly, in this paper, we investigate the characteristics of VoIP traffic and the limitations of state-of-the-art rate adaptation algorithms, and then enhance the QoS of voice over WLAN (VoWLAN) by ameliorating the existing rate adaptation algorithms. Specifically, we design fast decrease to control the transmission rate of retransmissions, and retry scheduling to avoid the deep fading of the wireless channel as well as hidden terminal interference. We comparatively evaluate the QoS of the revised rate adaptation algorithms via ns-3 simulations and MadWiFi implementations in various communication environments, and demonstrate that the proposed schemes improve the R-score performance by up to 80 percent depending on the network scenarios.

Device-to-Device (D2D) communications allow devices to communicate directly without going through infrastructure. It is considered a promising solution to improve communication performance and network capacity of LTE-Advanced system. From the perspective of User Equipment (UE), additional energy consumption is required to support D2D communications. In addition, recent device supports multiple Radio Access Technologies (RATs) with high energy consumption. For an energy efficient D2D communication, the most significant problem is how to detect proximal devices and to establish a D2D link in a timely manner. In this paper, we propose a D2D discovery and link setup procedure, and analyze its performance in terms of energy consumption and delay by utilizing the measurement results with real LTE smartphones. Based on the analysis results, we conclude that there is a trade-off relation between energy consumption and delay performance.

In this paper, we propose an efficient channel feedback algorithm for network MIMO systems that selects the proper number of channels for feedback among the whole channels observed by each user and allocates unequal number of feedback bits to the users. The proposed algorithm considers a limited feedback scenario, in which the channel state information is conveyed from the user to the base station in the form of quantized vector index. Through computer simulations, it is confirmed that the proposed algorithm provides significant performance improvement over previously proposed schemes in terms of the average sum rate.

Voice over IP (VoIP) service is emerging as a popular application in wireless local area networks (WLANs). However, the existing rate adaptation (or link adaptation) schemes for WLANs have been developed to improve the throughput of data traffic, not the quality of service (QoS) of VoIP traffic. In this paper, we investigate the characteristics of VoIP traffic and the limitations of state-of-the-art rate adaptation schemes, and then enhance the QoS of voice over WLANs (VoWLANs) by ameliorating the existing rate adaptation schemes. Specifically, we design fast decrease to control the transmission rate of retransmissions, and retry scheduling to avoid the deep fading of the wireless channel as well as hidden terminal interference. We comparatively evaluate the QoS of the revised rate adaptation schemes via ns-3 simulations and MadWiFi implementations in various communication environments, and demonstrate that the proposed scheme improves the R-score performance by up to 80% depending on the network scenarios.

In this paper, a feedback channel selection scheme and a bit allocation scheme are proposed for a network MIMO system with limited feedback. The feedback channel selection is performed by comparing the gain of each channel vector with a certain threshold. In particular, we derive an optimal threshold that maximizes a utility function, which is defined in terms of the expected rate of each user. In addition, we also develop an unequal feedback bit allocation scheme over the selected set of channels to further improve the system performance. It is shown that the joint feedback channel selection and bit allocation can provide significant gain in the average sum rate for a network MIMO system with limited feedback.

In this paper, a generalized MMSE beamforming is proposed for downlink multiple input multiple output (MIMO) systems. Unlike the conventional MMSE beamforming, cellular environments are considered where each user is randomly distributed in a cell and has the different temporal correlation of fading channel. We apply the proposed generalized MMSE beamforming to two important downlink MIMO scenarios, which are multiuser MIMO (MU-MIMO) and multicell MIMO (MC-MIMO). We derive the closed-form expressions of the generalized MMSE beamforming for both scenarios using convex optimization technique. The derived beamforming solution captures the impacts of the random geometry and different temporal correlations of users. Numerical results verify that the generalized MMSE beamforming achieves lower BER and higher average sum-rate than the previous beamforming schemes developed for MU-MIMO and MC-MIMO including the conventional MMSE beamforming.

We propose an interference-aware user selection scheme for uplink multiuser multiple-input multiple-output (MU-MIMO) systems in a multicell environment. The proposed scheme works in a distributed manner. Each mobile station (MS) determines its transmit beamforming vector based on the locally available channel state information (CSI), and informs the associated base station (BS) of the amount of potential interference caused to adjacent cells along with the resulting beamforming vector. Then, the BS selects a set of users to be served simultaneously with consideration of intercell interference. The user selection scheme is devised either to maximize the sum rate or to achieve proportional fairness among users. For each case, we derive an optimal user selection criterion and propose a suboptimal distributed user selection algorithm with low complexity. Simulation results confirm that the proposed scheme offers significant throughput enhancement due to reduction of the intercell interference in a multicell environment.

A novel capacity scaling law for multiple-input multiple-output (MIMO) broadcast channels is derived considering a random user distribution. The random locations cause unequal average SNRs amongst the users, whereas prior work typically assumes that all users have the same average SNR. Most centralized wireless networks, e.g. cellular systems, are more accurately captured by the proposed model. For a large number of users, the average sum capacity is shown to scale like M log K instead of M log log K, where M and K denote the number of transmit antennas and the number of users within a cell. This means that the average sum capacity of cellular MIMO broadcast channels grows significantly faster than previous results indicate.

This letter proposes a statistical cooperative scheme for transmit antenna subset selection in multiple-input multiple-output (MIMO) multicell systems. The proposed scheme is practical in that it only utilizes the statistical information for the adjacent links. However, it provides almost 90 percent of the average mutual information of the full cooperative scheme which requires a large amount of the instantaneous information and joint cooperation between base stations. Furthermore, the proposed scheme shows substantial performance enhancements over the competitive scheme in which the effects of intercell interference generated by the antenna selection are not considered.

We develop a realistic model for multiple-input multiple-output (MIMO) broadcast channels, where each randomly located user's average SNR depends on its distance from the transmitter. With perfect channel state information at the transmitter (CSIT), the average sum capacity is proven to scale for many users like αM/2 log K instead of M log log K, where α, M, and K denote the path loss exponent, the number of transmit antennas, and the number of users in a cell. With only partial CSIT, the sum capacity at high SNR eventually saturates due to interference, and the saturation value scales for large B like MB/M-1, where B denotes the quantization resolution for channel feedback.

In this paper, we propose a joint precoding scheme for both the base station (BS) and relay station (RS) to increase the ergodic capacity of downlink non-regenerative multiple-input multiple-output(MIMO)-relay systems. Different from previous work, we assume only the mean and the covariance matrix are known for the channel state information (CSI) of the RS-mobile station(MS) link. Since the exact solution of this problem is difficult to be found because the RS-MS channel is only partially known, we employ precoding matrices that maximize the upper bound on the ergodic capacity. Based on this approach, the proposed scheme uses an iterative method to determine both BS and RS precoding matrices simultaneously. In numerical results, the proposed joint precoding scheme is shown to outperform the water-filling and amplify-and-forward (WF-AF) scheme in high signal-to-noise ratio (SNR) regions. It is also shown to outperform the spatial multiplexing and RS precoding (SM-RP) scheme in low SNR regions.

Efficient channel feedback methods are becoming more important in limited-feedback multi-user multiple input multiple output (MU-MIMO) systems. We propose a new dynamic channel feedback control (DEFCON) scheme, which efficiently allocates the limited system resources for channel feedback to appropriate users. The main features of the proposed DEFCON scheme are as follows. Firstly, the channel feedback procedure is composed of two stages: rough channel feedback of all users and additional precise channel feedback of the selected users only. Secondly, each selected user is allocated with different amount of resources for additional channel feedback according to its channel status. We mathematically solve the resource allocation problem by proposing a suboptimal method, which utilizes convex optimization technique combined with iterative steepest decent direction search algorithm. Numerical results show that zero-forcing beamforming (ZFBF) with the proposed DEFCON scheme considerably outperforms previous limited-feedback MU-MIMO schemes such as ZFBF with conventional static feedback method and per user unitary beamforming rate control (PU2RC).

Uplink multiple-input multiple-output (MIMO) transmission scheme is developed for time division duplex (TDD) systems in a multicell environment. We propose a precoding scheme that maximizes the total achievable rate and works in the decentralized manner with only locally available channel state information (CSI) at each transmitter. We first establish and solve a decentralized optimization problem for the case of multiple-input single-output (MISO) channels, introducing a new precoding design metric called signal to generated interference plus noise ratio (SGINR). By extending the result to general MIMO channels, we propose an SGINR-based precoding scheme where the number of transmit streams is selected adaptively to the surrounding environments. Simulation results confirm that the proposed precoding scheme offers significant throughput enhancement in multicell environments.

Recently, wireless multimedia services have been growing because of the spread of various wireless applications. Hence, time division duplex (TDD) systems and their crossed-slot interference problems in a cellular environment have been attracting growing interests. Considering large signaling overhead between cells, decentralized time slot allocation (TSA) strategy is suitable for practical implementation. Thus, Fixed-TSA strategy which fixes the same ratio of uplink and downlink time slots in all cells is adopted in commercial WiBro systems (IEEE802.16e Mobile WiMax) to mitigate the crossed-slot interference. However, the Fixed-TSA strategy reduces the flexibility in time slot allocation due to a strong constraint set by a predefined boundary. Moreover, the mathematical derivation of the optimal value for the predefined boundary has not been presented yet. In this paper, we propose two decentralized TSA strategies. The first one is Enhanced Fixed-TSA strategy, which dynamically adapts the predefined boundary of the conventional Fixed-TSA strategy according to traffic conditions. The second one is region-based Decentralized Time Slot Allocation (RED-TSA) strategy, which utilizes partial location information of MSs to reduce crossedslot interference. We fully analyze the proposed TSA strategies in terms of new call blocking probability, average bit error probability, and the overall system throughput. Numerical results show that the proposed RED-TSA strategy provides the highest system throughput by compromising both new call blocking performance and average bit error performance, whereas it requires additional location information and complicated computation. On the contrary, the proposed Enhanced Fixed-TSA strategy requires reasonable computational complexity while it provides almost the same system throughput with that of the proposed RED-TSA strategy. In a nutshell, the proposed Enhanced Fixed- TSA strategy is more appropriate for the practical systems. Moreover, it can be directly applied to the current commercial WiMax/WiBro systems with minimum changes.

Multiple-input multiple-output (MIMO) precoding scheme is developed for time division duplex (TDD) systems in a multicell environment. The proposed scheme is designed to maximize the total achievable rate, and to work in the decentralized manner with only locally available channel state information (CSI) at each transmitter. We first establish and solve a decentralized optimization problem for the case of multiple-input single-output (MISO) channels. We introduce a new precoding design metric called signal to generating interference plus noise ratio (SGINR). Based on the SGINR metric, the MISO precoding scheme is extended to general MIMO channels. Simulation results confirm that proposed precoding scheme offers significant throughput enhancement in multicell environments.

Design strategy for non-regenerative multiple-input multiple-output (MIMO) relay system with partial channel state information (CSI) is developed. We assume that the CSI of the base station-relay link is fully known to the relay, while the CSI of the relay-mobile station link is not known except its channel statistics. Based on this model, weighting matrices to increase ergodic capacity is proposed for downlink and uplink MIMO-relay system, respectively, with the approximation for both the high and low signal-to-noise ratio (SNR) region. We also propose a switching scheme to cover the intermediate SNR region. Numerical results show that the proposed scheme outperforms the conventional one especially when the SNR becomes high.

Multiple-input multiple-output (MIMO) system with multicode transmission can provide high speed data services by transmitting independent parallel substreams from multiple antennas and through multicode channelization. In this paper, we first introduce an iterative two-stage successive interference cancellation (SIC) detection scheme for a multicode MIMO system. The proposed technique cancels the interference signals successively in the space domain followed by the code domain. Next, we develop various transmit power allocation schemes over different data substreams for the proposed detection process to improve error rate performance. The joint transmit power allocation is derived to make the post-detection signal-to-interference-plus-noise ratio (SINR) become the same for all substreams in both the space and code domains. As a computationally efficient scheme, we propose a two-stage power allocation scheme, which allocates the total transmit power to the substreams in the code domain at the first stage, and allocates this code domain power to the substreams in the space domain at the second stage. Furthermore, variable and constant power ratio (PR) schemes are derived to reduce the feedback overhead. In particular, the constant PR scheme utilizes the transmit power ratio determined by the long-term statistical properties of the fading channel amplitudes, and achieves significantly reduced feedback rate. Numerical results show that the proposed transmit power allocation schemes for the two-stage SIC significantly outperform the equal power allocation scheme.

In a wireless multiple-input multiple-output (MIMO) system, transmit antenna selection is an effective means of achieving good performance with low complexity. We consider spatial multiplexing with linear receivers, and equal power and equal rate allocation over different selected transmit antennas in order to reduce feedback overhead. Under these constraints, we address the problem of statistical multimode transmit antenna subset selection to improve the capacity of spatially correlated MIMO fading channels. In particular, we first derive an analytical closed-form expression for the expectation of the lower bound on the capacity using the smallest eigenvalue distribution of a Wishart matrix. Then, we propose a transmit antenna subset selection criterion of maximizing this average lower-bound capacity.

This paper discusses transmit antenna subset selection schemes for downlink closed-loop multiple-input multiple-output (MIMO) systems in multicell environments. A simplified cooperative scheme is proposed to increase the sum of the mutual information of the links in all cells. The proposed simplified-cooperative scheme is realistic in that it only utilizes a limited amount of information from adjacent links, while the conventional cooperative scheme requires a large amount of information such as channel state information and MIMO transmit configurations of adjacent links. In spite of the use of a limited amount of information, the proposed scheme provides almost 90 percent of the average mutual information provided by the cooperative scheme. Furthermore, the proposed scheme shows significant enhancements over the competitive scheme which does not consider the effects of antenna subset selection on the performance of adjacent links.

An efficient uplink user selection algorithm for distributed antenna systems (DAS) is proposed in this paper. Previous user selection algorithms in multiple-input-multiple-output (MIMO) systems are designed for conventional MIMO systems where antennas are located together. Since DAS can be thought as a variation of MIMO systems, the user selection algorithms which are originally designed for conventional MIMO systems can also be applied to DAS. However, the channel characteristics of DAS are different from those of conventional MIMO systems. Thus there should be specific user selection algorithm for DAS environments which utilizes the channel characteristics of DAS. This paper proposes an efficient uplink user selection algorithm specified for DAS. Simulation results show that the proposed algorithm gives near optimal performance which reaches up to 98.5% of the optimal spectral efficiency. Furthermore, the complexity of the proposed algorithm is also examined and the results show that it is easy to implement in practical systems.

In this paper, we propose transmit power adaptation scheme for downlink multiple relay orthogonal frequency division multiplexing (MR-OFDM) system to maximize total data rate with fixed bit error rate (BER) and sum power constraint. At first, by allocating the same transmit power at each subcarrier, the proposed scheme only has to select the best subcarriers to each relay station (RS) for both base station to RS link and RS to mobile station link. For efficient subcarrier allocation, the proposed scheme employs an iterative method utilizing Hungarian algorithm. In numerical results, data rate of MR- OFDM based on the proposed scheme is compared to that of the single relay OFDM (SR-OFDM) which does not obtain multiple relay diversity gain. It is also shown that the performance loss of the proposed scheme is negligible compared to the optimal scheme, while its computational complexity is reduced considerably.

This paper proposes the decentralized time slot allocation (DTSA) algorithm in dynamic TDD (DTDD) system to support traffic asymmetry. In D-TDD systems, additional interferences are introduced when one cell is uplink (downlink) and the other cell is downlink (uplink). These kinds of interferences result in severe performance degradation of SIR, which occurs especially to users in cell edge than users in cell center. To reduce additional interferences, DTSA algorithm is applied to D-TDD system. This algorithm assumes decentralized cellular network, so, it operates independently in each cell and has lower complexity than the other systems. Using DTSA algorithm, the outage probability of uplink and downlink at 0dB are decreased about 4.3% and 8.5% compared to conventional D-TDD system, respectively. Also DTSA algorithm increases the system capacity compared to S-TDD system and conventional D-TDD system. Simulation results show that D-TDD system with DTSA algorithm can support more asymmetric traffic than other systems, which increase system capacity.

Distributed wireless communication system (DWCS) is one of the candidate architectures for future mobile communication system. In this paper, based on the TDD DWCS, a new cell structure is proposed with universal frequency reuse. In the proposed structure, the cells are divided into 2 groups, and when one group is in the uplink period, the other transmits the downlink data. All the base station share the whole frequency spectrum. By use of the advantage of the distributed processors, the inter-cell interference in the uplink of a cell can be perfectly removed. As to the downlink, the inter-cell interference can also be avoided by proper resource allocation among mobile users. With universal frequency reuse and without the inter-cell interference, simulation results show that DWCS with the proposed structure has an extremely high capacity

Multiple-input multiple-output (MIMO) system with multicode transmission can provide high speed data services by transmitting independent parallel substreams from multiple antennas and through multicode channelization. In this paper, we first introduce an iterative two-stage successive interference cancellation (SIC) detection scheme for a multicode MIMO system. Next, we derive various transmit power allocation schemes over different data substreams for the proposed detection process to improve error rate performance. The power allocation is performed to make the signal-to-interference-plus-noise ratio (SINR) become balanced for all data substreams. Numerical results show that the proposed transmit power allocation schemes for the two-stage SIC significantly outperform the equal power allocation scheme.

Multiple transmit (Tx) and multiple receive (Rx) antennas systems, referred to as multiple-input multiple-output (MIMO) systems, have been proposed to achieve higher data rates in wireless communication systems. In this paper, we investigate joint design of transmitter and receiver for the MIMO system when the channel information is available at both transmitter and receiver. We discuss the problem concerning the design of Tx and Rx filters with the aim of minimizing the bit-error probability (BEP). We derive the optimum Tx and Rx filters when the number of data symbols is two. For a general number of data symbols, we derive a Tx and Rx filters design criterion referred to as equal signal-to-noise plus interference ratio (SNIR) criterion, and propose Tx and Rx filters based on this equal SNIR criterion. The performance of the proposed filter is compared with that of the conventional minimum mean-squared error (MMSE) filter. Performance analysis shows that the proposed filter provides a significant improvement over the MMSE filter in BEP and spectral efficiency.

In this paper, resource allocation for heterogeneous services is studied in multiuser orthogonal frequency division multiplexing (OFDM) systems. We propose a resource allocation algorithm, which is designed to improve the system throughput while satisfying the quality of service (QoS) requirements of both the real-time and nonreal-time services. In the proposed algorithm, the resources, composed of subcarriers and transmit power, are adaptively allocated to the users based on their service types and channel states over two sequential steps: (1) resource allocation for the real-time users that minimizes the resource usage required to satisfy the data rate requirement, and (2) resource allocation for the nonreal-time users that maximizes the system throughput using the remaining resource. The performance of the proposed resource allocation algorithm is evaluated in a frequency-selective fading channel, and compared with that of a simple resource allocation algorithm. Numerical results show that the proposed algorithm provides a significant throughput gain over the conventional algorithm.

We propose a quality of service (QoS) management framework and a packet scheduling scheme over a shared wireless link for heterogeneous classes of traffic. The QoS management framework is developed based on the characterization of different QoS requirements of heterogeneous traffic classes. The framework comprises two subsequent packet scheduling modules and load control modules. Based on the QoS management framework, we propose an effective packet scheduling scheme that accounts for the wireless channel, status of packet queues, the degree of QoS satisfaction, and fairness among sessions. Simulation results show that the proposed packet scheduling scheme provides better performance than the previously proposed schemes in light or moderate system load, and that it provides fair sharing of QoS degradation in overload.

Recently, multiple-input multiple-output (MIMO) systems have shown the potential to achieve high speed data transmissions by improving spectral efficiency over wireless fading channels. In multiuser MIMO environment, the radio base station (Node-B) supports mobile user equipments (UEs) with different quality-of-service (QoS) requirements, where the adopted scheduling scheme controls the selection of UEs for service provision and has an impact on the system performance. In this paper, two basic scheduling rules are described, which are combined with and without successive interference cancellation (SIC) receivers, respectively. We then propose the hybrid scheduler which outperforms those two rules in terms of system capacity. Simulation results show that the performances of conventional scheduling schemes heavily depend on the number of UEs, to which the proposed scheme is robust and achieves the improved capacity gain.

Abstract — Resource management in wireless networks is an important issue because of the scarceness of communication resource. So optimization schemes for efficient resource usage have been extensively researched. Throughput maximization under the condition of limited total power or power minimization under the condition of minimum bit rate are usually considered objective in many papers. These researches focused on the power allocation, but they did not consider time slot allocation which can be also used to improve the efficiency. In this paper, we introduce a time slot allocation problem for multi-user wireless networks to achieve minimum rate guarantee under the condition of fixed total power. We solve the allocation problem by using dynamic programming method, which has the complexity of where is the number of users. This complexity is lower than the non-polynomial complexity of exhaustive searching for the optimal solution. The performance of our proposed algorithm is compared with that of the waterfilling algorithm. An advantage of using the time slot optimization approach is that it can support high mobility users with QoS requirements. From simulation results, we demonstrate that our proposed algorithm shows better performance than a conventional scheme that does not use any time slot allocation. I.

For a closed-loop transmit (Tx) diversity, the Tx weights are calculated at a receiver, and fed back to a transmitter. As the number of Tx antennas increases, the potential gain of closed-loop Tx diversity may be significant. However, the amount of feedback information, which is the number of Tx weights that should be fed back, linearly increases, and the performance improvement of a closed-loop Tx diversity system may not be as significant as expected due to delay in the feedback process. Thus, an efficient Tx weight representation, which can reduce the amount of feedback information, is needed. In this letter, a Tx weight vector representation is presented, and its performance is analyzed. Analysis shows that this weight vector representation, referred to as basis selection, significantly reduces the amount of feedback information with little performance degradation.

In this letter, we present a modification of Vertical Bell Laboratories Layered Space-Time (V-BLAST), and propose an effective transmit power allocation (TPA) scheme for the modified system. The proposed TPA scheme minimizes the uncoded bit-error rate (BER) averaged over all detection stages, and requires small feedback overhead. Simulation results show that the modified V-BLAST system with the proposed TPA scheme provides a significant reduction in the uncoded BER compared with the conventional V-BLAST system. When the minimum mean-square error nulling is adopted, the modified V-BLAST system is found to achieve the uncoded BER performance comparable to that of the maximum-likelihood detection for the conventional V-BLAST architecture.

In a multicarrier system, transmit power allocation over different subchannels is an effective means of improving the performance. We develop the optimal transmit power allocation scheme to improve bit-error rate (BER) performance in a multicarrier system with diversity reception. A simple suboptimal scheme is also derived from the optimal one, and an asymptotic case referred to as the equal-signal-to-noise ratio scheme is discussed. Numerical results show that the optimal and suboptimal power allocation schemes significantly outperform the equal power allocation scheme. The effects of the modulation level, the number of receiving antennas, and the number of subchannels on the BER performance are also investigated.

In this paper, a new direction-of-arrival (DOA) tracking algorithm, direction lock loop (DiLL), is proposed for wireless direct-sequence code-division multiple-access systems. It has a similar concept to the delay lock loop that is used for timing synchronization. It may track the DOA of sources by iterations. Its computational requirements are NK+O(K) for coherent DiLL and 2NK+O(K) for noncoherent DiLL, where N is the number of antenna elements and K is the number of signal sources, which is less than that of the projection approximation subspace tracking with deflation algorithm. The DOA tracking accuracy is demonstrated by analysis and computer simulations.

In this paper, we propose a computationally efficient algorithm for transmit power and bit allocations in wireless OFDM systems aiming at maximization of data rate under the constraint of total transmit power and bit error rate. Although the proposed algorithm is based on the water-filling approach, the computational complexity is only O(k/spl times/M), where k denotes the number of iterations required to meet the constraints and M is the number of subchannels. By computer simulations, we verify that 10 iterations are sufficient to satisfy the constraints, and the decrease of data rate for the proposed algorithm is shown to be less than one percent relative to the optimal method at the average SNR above 4 dB. The proposed algorithm is appropriate for OFDM systems in a time varying wireless channel for its low computational complexity and good performance.

In this paper, we develop a transmit power adaptation method that maximizes the total data rate of multiuser orthogonal frequency division multiplexing (OFDM) systems in a downlink transmission. We generally formulate the data rate maximization problem by allowing that a subcarrier could be shared by multiple users. The transmit power adaptation scheme is derived by solving the maximization problem via two steps: subcarrier assignment for users and power allocation for subcarriers. We have found that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user with the best channel gain for that subcarrier and the transmit power is distributed over the subcarriers by the water-filling policy. In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method, we also propose a simple method where users with the best channel gain for each subcarrier are selected and then the transmit power is equally distributed among the subcarriers. Results show that the total data rate for the proposed transmit power adaptation methods significantly increases with the number of users owing to the multiuser diversity effects and is greater than that for the conventional frequency-division multiple access (FDMA)-like transmit power adaptation schemes. Furthermore, we have found that the total data rate of the multiuser OFDM system with the proposed transmit power adaptation methods becomes even higher than the capacity of the AWGN channel when the number of users is large enough.

The use of differentially coherent combining is proposed to improve the performance of a double-dwell acquisition system by increasing the reliability of a decision in the verification stage. The detection and mean acquisition time performance of the acquisition scheme with the proposed combining scheme is analyzed in frequency-selective Rayleigh fading channels, and compared with that of two previously published double-dwell acquisition schemes based on long correlation intervals and noncoherent combining. It is shown that the proposed acquisition scheme outperforms the previous ones, and that the performance improvement increases as the frequency offset increases.

A generalized code acquisition scheme for direct-sequence code-division multiple-access systems with multiple antennas is proposed over frequency-selective fading channels. The proposed scheme is developed on the framework of a generalized configuration of multiple antennas and correlators. The nonconsecutive search method is generalized and extended to multiple antenna systems to exploit multipath signals in improving acquisition performance over frequency-selective fading channels. The proposed scheme also adopts a hybrid decision strategy to make effective decisions on acquisition. The mean acquisition time performance of the proposed acquisition scheme is analyzed and evaluated in frequency-selective Rayleigh-fading channels with general multipath delay profiles and spatial-fading correlations. The effects of nonconsecutive search on mean acquisition time are investigated for various channel environments, and the optimal choice of decision strategy is discussed. Furthermore, effects of various configurations of multiple antennas and correlators, decision thresholds, and correlation interval on the performance are also investigated.

A generalized acquisition scheme is proposed for direct sequence code-division multiple-access systems with multiple antennas. The proposed scheme employs grouping of multiple antennas as a means of a tradeoff between two important factors determining the mean acquisition time, combining gain and search time. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh-fading channels with consideration of spatial correlations. Numerical results show that the use of the largest number of antenna groups is preferable to reducing the mean acquisition time at low signal-to-interference ratio (SIR) values. At high SIR values, on the contrary, the mean acquisition time is found to increase in proportion to the number of antenna groups. In a typical environment, the presence of spatial correlation is shown to increase or decrease the mean acquisition time within 50% compared with the uncorrelated fading case.

For a closed-loop transmit (Tx) diversity, the Tx weights are calculated at a receiver, and fed back to a transmitter. As the number of Tx antennas increases, the potential gain of closed-loop Tx diversity may be significant. However, the amount of feedback information linearly increases, and the performance improvement of a closed-loop Tx diversity system may not be as significant as expected due to delay in feedback process. Thus, an efficient Tx weight representation, which can reduce the amount of feedback information, is needed. A Tx weight vector representation is presented, and its performance is analyzed.

A code acquisition scheme is proposed for DS/CDMA systems with multiple antennas. The proposed scheme employs a new search scheme, referred to as sparse search, to utilize multipath signals in reducing mean acquisition time over frequency-selective fading channels. The mean acquisition time performance of the proposed scheme is analyzed and evaluated in frequency-selective Rayleigh fading channels. The performance of the proposed scheme is compared with that of the conventional one. Numerical results show that the proposed acquisition scheme outperforms the conventional one, and that the performance improvement becomes greater as the number of resolvable paths increases. The effects of configuration of multiple antennas and receiver complexity on mean acquisition time are also investigated.

The effective use of multiple antennas is investigated for code acquisition. A generalized acquisition scheme is proposed for DS/CDMA systems with multiple antennas. The proposed scheme provides means of a trade-off between two important factors determining the mean acquisition time, which are referred to as combining gain and search time, respectively. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh fading channels. In the analysis, the effects of spatial fading correlations on acquisition performance are considered. The mean acquisition time performance is evaluated for various environments, and the configuration of multiple antennas is investigated to reduce the mean acquisition time. The effects of spatial correlation on mean acquisition time are also investigated. The mean acquisition time is found to decrease significantly as the number of antenna elements increases. It is found that various parameters such as the number of antenna elements, operating signal-to-interference ratio (SIR), and the number of resolvable paths should be considered in determining configuration of multiple antennas to reduce mean acquisition time.

Some blind beamformers require the direction of the desired signal. The performance of such type beam-formers can be severely degraded by a small directional error. We propose a blind beamforming scheme robust to directional error by employing a simple steering vector estimator. The performance of the proposed beamformer can be further improved by re-initializing the weights of the beamformer.

A multicarrier system, which is under consideration for high data rate transmission in the future, offers robustness to multipath fading in a wireless channel. We develop the optimal transmit power adaptation method to maximize the channel capacity of a multicarrier system, considering both frequency and time domains. The optimal transmit power adaptation policy with average transmit power constraint is found to be "water-pouring" in the frequency-time domain. We also show that the channel capacity with the frequency-time domain transmit power adaptation is greater than that with only the frequency domain transmit power adaptation.

Vertical Bell Laboratories Layered Space-Time (V-BLAST) is a promising system that realizes the enormous capacity of multiple-input multiple-output (MIMO) communications. We present an extension of V-BLAST, and propose an effective transmit power allocation scheme for the extended system. The proposed transmit power allocation scheme minimizes the bit error rate (BER) averaged over all detection stages, and requires small feedback overhead from the receiver to the transmitter. Simulation results show that the extended V-BLAST system with the proposed transmit power allocation scheme provides a significant reduction in the BER compared to the conventional V-BLAST system. When the minimum mean square error (MMSE) nulling is adopted, the extended V-BLAST system is found to achieve the BER performance comparable to that of the maximum likelihood (ML) detection for the conventional V-BLAST architecture.

In a multicarrier system, transmit power allocation over different subchannels is an effective means of improving the performance. In this paper, the optimal transmit power allocation scheme is developed to improve bit error rate (BER) performance in a multicarrier system with diversity reception. A simple suboptimal scheme is also derived from the optimal one, and an asymptotic case referred to as the equal SNR scheme is discussed. Numerical results show that the optimal and suboptimal power allocation schemes significantly outperform the equal power allocation scheme. The effects of the modulation level, the number of receiving antennas, and the number of subchannels on the BER performance are also investigated.

There have been various receiver-based techniques for performance improvement of wireless communications systems. However, receiver-based techniques have a restricted application in the down-link because they increase computational complexity of the mobile station (MS). In this paper, a transmitter-based processing scheme for the down-link direct-sequence code division multiple access (DS/CDMA) systems with multiple transmit antennas is proposed. The proposed scheme uses a pre-RAKE method to exploit diversity gain and pre-decorrelating method to suppress multiple access interference (MAI) and multiple path interference (MPI). To make the total transmit power the same as that without pre-decorrelating processing, we present three power normalization methods and compare the performance of those with one another.

The use of differentially coherent combining is proposed to improve the performance of a double-dwell acquisition system. The differentially coherent combining is employed to increase the reliability of a decision in the verification stage. The mean acquisition time performance of the acquisition scheme with the proposed combining scheme is analyzed in a Rayleigh fading channel and compared with that of a conventional double-dwell acquisition scheme with noncoherent combining. It is shown that the proposed acquisition scheme outperforms the conventional one, and that the performance improvement increases as the frequency offset increases.

To reduce the equalizer training time with robustness to the timing phase error in fixed wireless channels, we propose a fractionally-spaced modified DFE (FS-MDFE) scheme. The proposed FS-MDFE is an extension of the previously proposed symbol-spaced modified DFE (SS-MDFE) (see Ariyavisitakul, S. and Greenstein, L.J., IEEE J. Select. Areas Commun., vol.15, no.1, p.5-15, 1997). The performance of the FS-MDFE is analytically evaluated in terms of the mean squared error. The optimum MMSE solution is derived for initialization of the feedforward filter of the MDFE. We also propose a simplified initialization method to reduce the implementation complexity.

When a multi-level quadrature amplitude modulation (QAM) signal is transmitted over the wireless channel, transmit power needs to be controlled to avoid severe performance degradation due to the fading. Most studies on the transmit power control (TPC) were considered in the flat fading channel. In this paper, we propose a power control scheme in the frequency selective fading channel, where the transmit power is controlled based on the mean-squared error (MSE) of the equalizer output. The performance of the proposed scheme is analytically evaluated and verified by computer simulation in the fixed wireless channel with low Doppler frequency.

A fractionally-spaced frequency domain equalizer (FS-FDE) is needed to transform a received signal into the frequency domain. We consider two fast Fourier transform (FFT) schemes, one with a single 2N-point FFT and one with two N-point FFTs, for the transform of oversampled N symbol data in the FS-FDE receiver. It is shown that the two receiver schemes are equivalent in terms of the characteristics of the FFT output, per-bin SNR after maximum-ratio combining, computational complexity and the residual out-of-band noise rejection property. These properties are verified by computer simulation in additive white Gaussian noise (AWGN) and typical multipath channel.

Slot synchronization is a critical step for fast and reliable cell search in intercell asynchronous direct sequence-code division multiple access systems. To increase reliability, observations over a number of slots may be combined. In this paper, combining schemes of multiple observations are studied for slot synchronization. The optimal combining rule is determined based on detection theory. It is found that two known combining schemes correspond to special cases of the optimal combining. These schemes may not work well in typical environments, since the schemes are optimized for specific environments. To improve slot synchronization performance in typical environments, a new combining scheme is proposed. The performance of the proposed combining scheme as well as other combining schemes is analyzed for Rayleigh fading channels with frequency offset. Numerical analysis shows that the proposed combining scheme significantly outperforms other combining schemes in typical environments.

We investigate the performance of coherent direct-sequence code division multiple access (DS/CDMA) systems with antenna diversity in Rayleigh fading channels. For a DS/CDMA system operated in fading channels, the multiple access interference (MAI) is affected by fading. We determine the distribution of multiple access interference (MAI) in Rayleigh fading channels. Two simple approximation methods for MAI distribution are proposed, and compared with the exact MAI distribution. Using the exact and approximated MAI distributions, we evaluate the bit error probability (BEP) expression for coherent DS/CDMA systems with antenna diversity. Moreover, the BEP accuracy of the standard Gaussian approximation is investigated as the number of users, processing gain, and the number of antennas are varied.

The performance of frequency-hopping spread-spectrum systems employing noncoherent reception and transmission diversity is analyzed for frequency-selective Rayleigh fading channels. Two different types of transmission diversity systems, a fast frequency-hopping (FFH) system and a multicarrier frequency-hopping (MCFH) system, are investigated. In order to combine received signals from transmit diversity channels, the optimum diversity combining rule based on the maximum-likelihood criterion is developed. Probability of error equations are derived, and utilized to evaluate the performance of the two systems. The MCFH systems are found to outperform FFH systems when the channel delay spread is severe, while FFH systems are superior to MCFH systems when a channel varies rapidly. Furthermore, it is found that performance enhancement due to an increase of diversity order is more significant for MCFH systems than for FFH systems in frequency-selective fading channels. The effect of frequency-selective fading is also investigated in determining optimum frequency deviations of binary frequency-shift keying signals.

A novel acquisition scheme that utilizes multipaths to improve the acquisition performance is proposed for frequency-selective fading channels. The proposed acquisition scheme employs nonconsecutive search and joint triple-cell detection. The performance is analyzed in frequency-selective Rayleigh fading channels. Equations for the probabilities of detection and false alarm are derived, and an expression for the mean acquisition time is developed. The mean acquisition time performance of the proposed and conventional acquisition schemes is evaluated and compared. It is found that the proposed acquisition scheme significantly outperforms the conventional one. The effects of various channel parameters such as the number of resolvable paths, the shape of the multipath intensity profile (MIP) and the signal-to-interference ratio (SIR) on acquisition performance are also investigated.

Slot combining is a method to achieve fast code acquisition in inter-cell asynchronous DS/SS systems by employing multiple observations. The optimal slot combining is difficult to implement, since it requires information on channel and frequency offset. Two practical combining schemes, a noncoherent combining and a coherent combining, are found to be special cases of the optimal combining. In this paper, a novel differentially coherent combining scheme is proposed. This combining scheme may be viewed as a combination of differential detection and coherent combining techniques. The performances of the differentially coherent combining as well as the optimal combining, noncoherent combining, and coherent combining are analyzed for Rayleigh fading environments with frequency offset. It is found that the differentially coherent combining scheme significantly outperforms the noncoherent combining and coherent combining schemes for a typical range of Doppler spread and frequency offset.

A novel acquisition scheme that utilizes multipaths to improve the acquisition performance is proposed for frequency-selective fading channels. The proposed acquisition scheme employs nonconsecutive search with joint triple-cell detection. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh fading channels, and compared with that of the conventional one. The proposed acquisition scheme is found to significantly outperform the conventional one. The effects of various channel parameters on the acquisition performance are also investigated.

Performance analysis schemes for wide-band M-ary frequency-shift keying systems with a limiter-discriminator-integrator receiver are presented along with analysis and simulation results for 4-ary and 8-ary systems. The probability distribution of clicks is required for performance analysis and is determined for Rayleigh fading channels.

A digital baseband receiver called zero-intermediate frequency zero-crossing demodulator (ZIFZCD) was developed for digital FM signal detection. ZIFZCD is applicable to many worldwide mobile and personal communications systems. In addition, ZIFZCD offers lower power consumption and simpler implementation, compared to the conventional analog implementation [e.g., a limiter-discriminator integrator and dump (LDI)] and the conventional digital implementation [e.g., the cross-differentiate-multiply demodulator (CDM)]. This paper introduces the ZIFZCD and reports the bit-error rate (BER) of the ZIFZCD under both static and fading environments. The analyzed and simulated BER results show that the ZIFZCD is comparable to the conventional CDM for narrowband digital FM with a modulation index of 0.5, and the ZIFZCD is significantly better than the CDM for wideband digital FM with a modulation index larger than 1.5.

Rapid converging adaptive interference suppression algorithms for direct sequence code-division multiple-access (DS/CDMA) systems are presented after showing the limitations of conventional adaptive algorithms. The rapid converging algorithms are based on an orthogonal transformation preprocessing, and are adaptive implementations of the linear minimum mean square error (MMSE) receiver for interference suppression. These algorithms do not require a priori knowledge on interfering signal parameters such as spreading sequences and relative signal power levels.