With the increasing demand for advanced autonomous driving, the available communication resources may become constrained over different geographic areas. In addition, due to dynamic channel variations and imperfect cell deployments, guaranteeing the required communication resources for data hungry and delay-sensitive applications in autonomous vehicles (AVs), along their entire trips, becomes challenging. To address these issues, the paper investigates the feasibility of a hybrid system-optimum and user-equilibrium AV traffic framework subject to communication constraints, as well as its performance gain. Within such a framework, the paper introduces the problems of communication-constrained routing (CCR) and traffic control (CCTC) in the context of infrastructure-assisted autonomous driving and presents respective solutions. For CCR, an efficient two-layered routing scheme is proposed which can provide optimal trip duration. Simulation results show that the routing scheme achieves a good balance between longer duration of communication coverage and acceptable source-to-destination travel time. For CCTC, it is shown that there exists an optimal AV speed on each road segment, as well as an optimal inter-AV distance and an optimal number of AVs in each cell, to maximize the road-network AV throughput within a single cell. Moreover, spectrum allocation is used to achieve Pareto-optimal road-network throughput across cells, and a new key performance index (KPI) is defined to evaluate the traffic control capability of cellular systems. Simulation results validate the improvement of AV throughput via the proposed CCTC solution.

In cellular communications, deploying a larger number of antennas at the base station, also called massive multiple-input multiple-output (MIMO), can offer a significant improvement in system throughput. In this paper, we exploit the spatial fading correlations in massive MIMO to reduce the downlink training and the corresponding feedback overhead in frequency division duplexing systems. We first study the user clustering, where the users with similar spatial channel correlations are clustered together. In the study, we provide the optimal metric and prove the convergence of the user clustering. Then, we propose an efficient eigenspace training and precoding (EETP) framework, where two different prebeamforming matrices are designed to minimize the channel estimation error and to manage the inter-user interference, respectively. In the results, we show that the channel estimation error for EETP decreases monotonically when either the number of prebeamforming vectors or the number of clusters increases. The spectral efficiency of the new algorithms is evaluated extensively with different user distributions, errors in channel correlations, different numbers of clusters, and different coherence block lengths, as well as with dynamic user scheduling for a large number of users. The new EETP not only achieves significant savings in the downlink training and the corresponding feedback, but also offers significantly higher system throughput compared with the existing schemes in the literature.

By applying a rate-distortion approach, we investigate the relationship between the amount of overhead and the system performance from an information-theoretic point of view. Though the purpose of rate-distortion theory is to find a lower bound on lossy source coding problems, the concepts of distortion and rate can be extended to communication performance measures and the quality of feedback overhead, respectively. In this article, we study the overhead-performance tradeoff for a downlink MU-MISO channel system with limited feedback. The required feedback bits for characterizing the channel state information is represented as a function of tolerable rate loss. The proposed method to derive this tradeoff is valid for any number of transmit antennas and users, and can help in designing practical systems where the impact of channel feedback overhead cannot be neglected. Comparing the obtained rate-distortion curves with vector quantization, it can be concluded that, for each user, by feeding back different numbers of bits for different channel power gains, the optimal performance can be achieved.

Licensed-Assisted Access (LAA) is a promising technology to address the issues of ever-increasing traffic demands in cellular systems. However, the efficient coexistence of Wi-Fi and LAA in the same unlicensed spectrum raises many challenges. Most of the studies addressing this coexistence issue focus on a coexistent LAA and Wi-Fi network sharing a single carrier. In this paper, we investigate a coexistent network with multiple available carriers, which is the case for practical deployments. Specifically, we develop an efficient adaptive energy detection algorithm to avoid frequent collisions in coexistent Wi-Fi and LAA networks. moreover, a carrier selection algorithm, based on the LAA energy detection threshold, is proposed to further improve the coexistence performance. Simulation results validate the effectiveness of the proposed adaptive energy detection and carrier selection schemes.

In today's WLANs, scheduling of packet transmissions solely relies on the collision and success a station may experience. To better support traffic differentiation in dense WLANs, in this paper, we propose a distributed reservation mechanism for the Carrier Sense Multiple Access Extended Collision Avoidance (CSMA/ECA) MAC protocol, termed CSMA/ECA-DR, based on which stations can collaboratively achieve higher network performance. In addition, proper Contention Window (CW) will be chosen based on the instantaneously estimated number of active contenders in the network. Simulation results from dense scenarios with traffic differentiation demonstrate that CSMA/ECA-DR can greatly improve the efficiency of WLANs for traffic differentiation even with large numbers of contenders.

In cellular systems, deploying a large number of antennas at the base station (BS), also called massive multiple-input multiple-output (MIMO), can offer a huge improvement in system throughput. This performance gain achieved through coherent beamforming highly depends on having accurate channel state information (CSI) at the BS. Initially, massive MIMO was considered promising only for time division duplexing (TDD) systems because the downlink training overhead in frequency division duplexing (FDD) systems could be large. To reduce the overhead in FDD systems, we propose efficient eigenspace training and precoding. In the downlink training, the prebeamforming matrix is designed based on the spatial fading correlation to probe the channel and minimize the mean squared error (MSE) in channel estimation. In data transmission, the precoding matrix is designed based on instantaneous CSI to manage the inter-user interference. Our algorithms achieve significant savings in the downlink training because the overhead is associated with the dimension of prebeamforming matrix and no longer limited by the number of BS antennas. Compared with joint spatial division and multiplexing, our algorithms offer much lower MSE and, under some conditions, higher sum rates.

In this study, the authors propose a non‐parametric algorithm to implement the estimation of transmit‐antenna number, which is a prerequisite for blind interception process of multiple‐input multiple‐output orthogonal frequency division multiplexing signals in frequency selective fading. Specifically, a series of test statistics are constructed by exploiting the eigenvalues of the sample covariance matrices from each subcarrier, followed by a combination of these test statistics. As a consequence, the number of transmit antennas can be determined after a serial binary hypothesis testing. The theoretical analysis and simulation results verify the rapid convergence and high reliability of the proposed algorithm at a relatively low signal‐to‐noise ratio.

This paper investigates joint relay and antenna selection schemes for two-way amplify-and-forward (AF) relaying system, where a multiantenna source exchanges information with a single-antenna source by exploiting a group of single-antenna relays. In view of performance and complexity tradeoffs, two optimal and one suboptimal joint selection schemes are proposed and compared. In contrast to the optimal selection schemes, the suboptimal counterpart is characterized by its low overhead and implementation complexity. The performance of these selection strategies has been investigated by deriving the outage probability approximations in closed form. Moreover, the achievable diversity orders and the corresponding coding gains are derived through high signal-to-noise ratio (SNR) approximations of the outage probability. Our theoretical and simulation results show that the performance gap between the optimal and suboptimal schemes disappears for specific scenarios, which provides useful design guidelines for practical two-way relaying system.

This paper evaluates and compares the performance of single- user (SU) and multi-user (MU) transmission for downlink multiple-input multiple-output (MIMO) channels in terms of system energy efficiency (EE). We introduce power control algorithms to maximize EE. Specifically, to optimize the power allocation, we consider the problem of EE maximization with the satisfaction of the minimum spectral efficiency (SE) gain. Antenna selection is taken into account to further enhance the EE performance for both SU and MU systems. Our results reveal that we should turn off extra antennas at the transmitter, which are originally used for diversity gain but incur large circuit power consumption. Our comparisons between SU and MU differ from conventional comparisons, which focus on SE. Jointly considering EE and SE, we show that SU is more desirable when the transmit power is low, while MU is favored in the case of high transmit power.

The increasing popularity of Internet-capable mobile devices leads to the explosion of mobile data traffic. Although the emerging mobile broadband systems such as 4G LTE promise higher bandwidth and lower latency for video traffic, it is not power efficient to deliver video traffic over cellular networks. This paper studies how peer-to-peer communications via wireless local networks could complement the cellular networks so as to minimize the power consumption of mobile devices. Specifically, we consider the scenario in which a group of nearby mobile devices share the streaming contents, originated from the cellular networks, over the wireless local area networks in a peer-to-peer manner. We focus on investigating how the mobile devices should cooperate to minimize their own power consumption. The problem has been formulated into a linear programming (LP) model, and numerical results show that at least 31% of the power consumption can be saved on mobile devices through the cooperation among them.

Small-cell networks have been proposed to meet the demand of ever growing mobile data traffic. One of the prominent challenges faced by small-cell networks is the lack of sufficient backhaul capacity to connect small-cell base stations (small-BSs) to the core network. We exploit the effective application layer semantics of both spatial and temporal locality to reduce the backhaul traffic. Specifically, we envision that small-BSs are equipped with storage facility to cache contents requested by users. As the cache hit ratio increases, most of the users' requests can be satisfied locally without incurring traffic over the backhaul. To make informed caching decisions, the mobility patterns of users must be carefully considered as users might frequently migrate from one small cell to another. We study the issue of mobility-aware content caching, which is formulated into an optimization problem with the objective to maximize the caching utility. As the problem is NP-complete, we develop a polynomial-time heuristic solution termed MobiCacher with bounded approximation ratio. We conduct trace-based simulations to evaluate the performance of MobiCacher, which show that MobiCacher yields better caching utility than existing solutions.

The impact of overhead on spectral efficiency for different cooperative relaying schemes is quantitatively analyzed in [1], in which half-duplex (HD) relaying is assumed for all cooperative nodes. With the recent development of cancellation methods for full-duplex (FD) relays or radios, FD has become feasible in practical implementations, providing a higher spectral efficiency than HD. In this paper, we model the residual self-loop interference and cross-talk interference caused by imperfect channel estimation, which is an extra overhead for FD compared with that of HD. Then, we analyze the overhead, outage probability and spectral efficiency of three typical cooperative relaying schemes (timer-based best-select, distributed space-time-coded relaying and M-group) operating in FD mode. Finally, numerical results are provided to compare the outage probability and spectral efficiency of HD and FD relaying.

Cooperative communication is an effective technique which provides diversity gains to combat the attenuation of radio signals caused by fading and path loss. By sharing each cooperative node's antenna to form a virtual antenna array, the reliability of the wireless transmission can be significantly improved. On the other hand, cooperation among different nodes may produce a higher level of interference and degrade the overall performance of larger and dynamic networks with multiple concurrent transmissions. For practical environments, it is essential to investigate the tradeoff between cooperative diversity and this additional interference. In this paper, by analyzing and comparing the outage performance of non-cooperative and cooperative strategies, we derive a criterion which determines whether we should implement cooperation or not. Both analytical results and simulations show that a cooperative strategy is preferred for sparse networks. As the wireless network becomes more dense, the diversity gain is eventually eliminated by the excessive amount of interference, which implies that non-cooperative strategies should be used.

To support the heterogeneous demands for the network and mobile user equipment (UE), multiple radio access technologies (RATs), operating with different system configurations and resources, have evolved and now coexist. Recently, with data traffic exponentially increasing, there has been a significant expansion in wireless network infrastructure, raising a justifiable concern over the concomitant drastic increase in energy consumption. Thus, energy-efficient design in multi-RAT networks is becoming increasingly important. In this paper, we consider resource allocation that maximizes the energy efficiency (EE) for orthogonal frequency division multiple access (OFDMA) in multi-RAT networks. To this end, we present the optimal resource allocation problem for parallel transmission utilizing multiple RATs. Since the formulated problem is NP-hard, a modified problem is proposed, for which a near-optimal resource allocation algorithm is developed by exploiting the intrinsic quasiconcavity of the problem. To reduce the computational complexity, we develop a low-complexity suboptimal allocation strategy based on joint iterative subcarrier and waterfilling power allocation over multiple RATs. Simulation results show that the proposed algorithms can achieve a higher EE compared to a conventional spectral-efficiency-based approach and can obtain performance comparable with the optimal solution, but with much less complexity.

In previous work on multiple-input multiple-output (MIMO) interference channels, it has usually been assumed that the transmit power at each transmitter is fixed. Power is equally allocated among different streams, and MIMO beamforming is applied only to mitigate interference and improve the system performance. In this paper, we design the transmit powers, power allocation among streams, and beamforming matrices jointly for each transmit-receive pair (link) to maximize the energy efficiency in a MIMO interference channel. Centralized and decentralized energy-efficient beamforming algorithms are developed based on global and local channel state information (CSI) at each transmitter, respectively. A distributed beamforming algorithm that combines minimum mean squared error (MMSE) and two power allocation algorithms is also developed; this algorithm only requires the information of the desired link. The decentralized and distributed schemes can be combined with scheduling to achieve good performance when the interference among links cannot be canceled. Simulation results show that the proposed algorithms can achieve good performance close to the upper bound, and the decentralized algorithm can perform as well as the centralized scheme. The distributed algorithm is suboptimal, but requires much less signaling. In addition, we show that the decentralized and distributed schemes result in a fairer allocation than the centralized approach.

Recently, due to the growing concern over the ever-increasing energy consumption, much attention has been given to the energy-efficient design of wireless communication systems. Various strategies for improving the energy efficiency, which is defined as the transmitted bits per total energy consumed (in bits/Joule), have been proposed. In this paper, we analyze the energy efficiency of clustered cooperative beamforming where distributed relay nodes construct a virtual Multiple-Input Single-Output beamforming system with a maximum transmit power constraint, and compare its performance with that of direct communications. All the energy consumption overheads incurred are taken into account in the energy efficiency analysis assuming a constant circuit-power consumption model. We then study the number of relay nodes that maximizes the efficiency for a given outage constraint. In addition, we propose a mode switching algorithm which optimally chooses between using cooperative beamforming or direct communications for transmission. We show that cooperative beamforming significantly outperforms direct communications in energy efficiency, as well as in spectral efficiency, even though a much larger amount of circuit power is consumed.

It is well known that cooperative relaying has the potential to improve the performance of wireless communication. However, compared with point-to-point communication, cooperative relaying requires much more overhead to implement. In this paper, we quantitatively analyze the overhead in implementing cooperative relaying, in particular, the overhead to acquire essential channel state information, to select the relay(s), and to provide the required coordination. Factoring in the impact of overhead, the spectral efficiency of cooperative relaying schemes is then investigated. A comparison of three cooperative relaying schemes (Timer-Based Best-Select, Distributed Space-Time-Coded, and M-group) is presented. Numerical results are provided to illustrate the impact of overhead on the spectral efficiency. Finally, based on the analysis and simulations, we provide guidelines for determining the appropriate cooperative relaying scheme for specific scenarios.

In this paper, for an energy-constrained two-stage selective decode-and-forward (DF) relaying network which employs a Best-Select form of cooperation, we focus on maximizing the minimum node lifetime when only the mean channel state information (CSI) toward the destination is available at each relay node. If only the mean CSI can be exploited, an optimal relay selection strategy for lifetime maximization in an energy-constrained network is not readily available. In this paper, a simple but effective relay-selection strategy is proposed by exploiting only the mean CSI and the residual energy information (REI) at relay nodes. For any given transmit power at each node, the proposed strategy can maximize the minimum node lifetime while achieving fairly stable end-to-end outage performance. A decentralized implementation with minimal overhead is also designed, so that only the local mean CSI and REI is required at each relay node. Both asymptotic performance analyses and realistic simulations verify the advantages of the proposed relay-selection strategy.

In this paper, we propose a novel linear network model to characterize the randomness in the node locations. Unlike most previous work which usually assumes that the nodes are uniformly distributed on a line, we consider a linear network with randomly located nodes. We model the randomness in distance and show that the proposed model is a reasonable approximation. We evaluate a performance bound on spectral efficiency, and then calculate the optimum number of hops. Simulation results verify the model.

Energy efficiency is a critical issue in wireless communication networks since most of the communication devices have limited energy capacity. Various strategies for maximizing energy efficiency have been proposed. In this paper, we analyze the energy efficiency of cooperative beamforming where the source and relay nodes construct a virtual Multiple-Input Single-Output (MISO) beamforming system. In the analysis, we consider all the energy consumption overheads incurred in forming a virtual MISO system. We show that cooperative beamforming achieves not only a higher energy efficiency, but also better spectral efficiency for large transmission distances.

It is well known that cooperative relaying has the potential to improve the performance of end-to-end wireless communications. However, compared to point-to-point communications, implementing cooperative relaying requires much more overhead. For example, more complicated coordination and transmission schemes are required. In this paper, the spectral efficiency of cooperative relaying is investigated, taking into account the overhead to acquire essential channel state information, as well as that required to select the relay(s). A comparison of three cooperative relaying schemes (Timer-based Best-Select relaying, Dis-STBC All-Select relaying, and M-group All-Select relaying) is presented. Numerical results are further provided to illustrate the impact of the overhead on the spectral efficiency.

In the previous works on multiple-input multiple-output (MIMO) interference channels, it is usually assumed that the transmit power is fixed and only MIMO beamforming is applied to mitigate interference. In this paper, we design the transmit powers and beamforming vectors jointly for each transmit-receive pairs (links) to maximize the energy efficiency in MIMO interference channels. Centralized and decentralized energy-efficient beamforming algorithms are developed based on global and local channel state information (CSI) at each transmitter, respectively. The decentralized scheme can be combined with scheduling to achieve good performance when the interference among links cannot be canceled. Simulation results show that the proposed algorithms can achieve good performance, close to the upper bound, and the decentralized algorithm can perform as well as the centralized scheme.

In a cellular-like system, the multiple-input multiple-output (MIMO) mode used by a base station (BS) can be adaptively varied among several candidate MIMO modes, including space-time block coding (STBC), open-loop spatial multiplexing (OLSM), closed-loop spatial multiplexing (CLSM), MIMO beamforming (MBF), and maximal ratio transmission (MRT). In practice, during any one given period of time, when the desired BS performs downlink transmission with a specific MIMO mode, each co-channel interfering BS will use a MIMO mode the same as or different from that used in the desired link and any two co-channel interfering BSs may use different MIMO modes, so that the interference scenario becomes quite complicated. In this paper, considering realistic propagation conditions including both path-loss and Rayleigh fading and considering the existence of unequal-power interferers, the statistical distributions of the post-processing signal-to-interference ratios (SIRs) for either MBF or STBC downlink transmissions are analyzed when co-channel interfering BSs employ arbitrary MIMO modes. In particular, closed-form expressions for the probability density function (PDF) of the interference terms in the post-processing SIRs are derived. Further, the statistical distributions of the post-processing SIRs are analyzed. Simulation results verify the validity of the theoretical analyses.

An orthogonal frequency division multiplexing system suffers performance degradation when the length of the cyclic prefix is less than the channel impulse response. The root cause of this degradation is the inter-carrier interference (ICI) and inter-symbol interference (ISI) introduced by the excessive multipath delay. Generally, multiple-input multiple-output (MIMO) beamforming is helpful in mitigating such interference because it can spatially suppress some of the multipath. However, the effectiveness of this suppression is very limited. In this paper, we propose an ICI/ISI-aware beamforming algorithm which explicitly takes into account the multipath characteristic of the channel. Optimal steering vectors are derived to maximize the signal-to-interference-plus-noise ratio. This technique not only achieves the beamforming benefit, but also significantly mitigates the ICI and ISI. We show, via simulations, that the proposed algorithm can dramatically reduce the block error rate, permitting good performance for channel delay profiles that would break conventional links. This is vitally important for the extension of indoor wireless LAN designs to outdoor uses.

In this paper, we evaluate the impact of a mix of co-channel MIMO interferers on the performance of Alamouti Coding in the downlink of a cellular-like system. In particular, taking into account the effects of both path-loss and Rayleigh fading, we derive closed-form expressions for the probability density functions of the resulting signal-to-interference ratios. Simulation results verify the validity of the analyses.

It has been shown that cooperative communication can significantly improve performance by utilizing distributed relay nodes. In most cases, cooperative communication systems have been designed and evaluated assuming perfect channel state information (CSI). Due to the feedback delay of the estimated CSI, a level of uncertainty in the channel estimates is inevitable, and systems designed for perfect CSI might not be optimal. In this paper we propose and evaluate several robust relay selection schemes for a best-select, amplify-and-forward cooperative communication system. Simulation results show that the proposed schemes can improve the average received SNR by 1.5 dB when the channel estimates are highly unreliable.

Cooperative transmissions combat various fading effects in wireless communications by employing multiple antennas from different nodes to achieve spatial diversity. Virtual Multiple-Input Single-Output (MISO) is one instance of cooperative transmissions capable of achieving higher receiving SNR, which can either extend the transmission range or increase the transmission rate. While the physical layer performance of Virtual MISO has been well studied, this paper describes the Cooperative Source Routing (CSR) protocol to convert physical layer gain into network level performance improvement. With both route request and route reply control packets being transmitted cooperatively, CSR can explore routes with high cooperative diversity. We demonstrate CSR's performance through simulation and compare CSR with other protocols.

Hierarchical femtocell architectures have become popular recently because of their potential to provide increased coverage and capacity in cellular systems. However, introduction of femtocells might reduce the overlay macrocell system performance due to increased interference caused to macrocel-lular users. With MIMO precoding, the Least Interference (LI) technique can be employed at the femtocellular base stations (FBSs) to minimize the interference to the macrocellular user. This approach maximizes the macrocellular throughput, but also results in significant reduction in femtocellular throughput. As is well-known, a Precoding Matrix Index (PMI) approach to beamforming can maximize the signal power at a desired receiver, with minimal feedback. In this paper, we apply mode adaptation between LI and PMI at the FBSs in order to increase both the macrocellular and femtocellular throughputs. We show that allowing for mode adaptation at each FBS improves the system performance when compared with using the same mode across the system, and a simple binary choice at each FBS can nearly achieve the optimum mode-adaptation performance. Analysis and simulation results in a multicell environment are presented to illustrate the improvement in system performance with the proposed techniques.

Geographic routing is a scalable routing scheme for wireless networks, where nodes make local routing decisions using position information. Cooperative transmissions utilize spatial diversity to combat fading in wireless channels. In this paper, we incorporate cooperative transmissions into geographic routing, and propose the Location-Aware Cooperative Routing (LACR). In LACR, a node that receives Route Request (RREQ) makes an individual decision on whether and how to rebroadcast RREQ based on its position and capability to cooperate. A theoretical analysis of the impact of cooperative transmissions on the transmission range extension is presented to guide the measurement of the potential performance of each node. Through simulation, we show that LACR performs better in terms of higher probability to find a route and higher throughput in comparison to Single-Input-Single-Output (SISO) based routing protocol.

In this paper, the impact of co-channel MIMO interference on MIMO Beamforming (MBF) in the desired link is investigated. The exact closed-form expression of the probability density function (pdf ) of the signal-to-interference ratio when the desired user has one antenna is derived. Due to the unavailability of the closed-form pdf expression for arbitrary (N,M) (where N and M are the number of transmit and receive antennas, respectively), a simple approximation for an arbitrary number of antennas is proposed and used to analyze the impact of various MIMO modes used by the co-channel base stations. Simulation results verify the validity of the analysis and the impact of cochannel MIMO modes on the desired MBF receiver is discussed.

In this paper, focusing on a multi-source, two-stage selective decode-and-forward (DF) relaying network in which the multiple sources have a common destination, we propose an effective cooperative relaying transmission scheme via exploiting decentralized distributed space-time block coding (Dis-STBC). This scheme is a unified multi-source cooperative transmission scheme, regardless of the number of the potential relays which can recover the messages from all the active sources. In particular, even if most or all of potential relays can only recover the messages from a subset of all the active sources, the proposed scheme can work well without introducing additional overhead and increasing processing complexity. Because the achieved diversity order of decentralized Dis-STBC is a random variable, based on the proposed scheme, the multi-source diversity benefit is analyzed by deriving the probability distribution of the achieved diversity order. Finally, the end-to-end outage performance is evaluated under realistic environment.

In a cellular system, when multiple-input multiple-output (MIMO) techniques are employed, the interference scenario is quite complicated. In this paper, considering realistic propagation conditions including both path loss and Rayleigh fading, and considering the existence of unequal-power interferers, we analyze the statistical distributions of the post-processing SIRs for downlink space-time block coding (STBC) transmission with different co-channel MIMO interferers. Simulation results verify the validity of the theoretical analyses.

In this paper, packet scheduling in an orthogonal frequency division multiplex access downlink is investigated based on cross-layer design and optimization. We first develop max-delay-utility scheduling with the help of channel and queue state information to exploit multiuser diversity and guarantee quality of service. The stability property of a scheduling policy is characterized by the stability region, which is the largest region on arrival rates for which the queueing system can be stabilized by the scheduling policy. It is shown in this paper that under very loose conditions, the max-delay-utility scheduling has the maximum stability region. In environments with insufficient scattering or strong line-of-sight components, delay transmit diversity can increase the fluctuation in the frequency domain to improve the performance. The simulation results show that the max-delay-utility scheduling with a frugality constraint is highly advantageous to data transmission with a low latency requirement over shared multiple channels, and that joint scheduling and power allocation can substantially boost the performance.

In ad-hoc networks, cooperative diversity is especially beneficial where the use of multiple antennas may be impractical. There has been a lot of work on improving the peer-to-peer link quality by using advanced coding or power and rate allocation between a single source node and its relays. However, how to fairly and efficiently allocate resources among multiple users and their relays is still unknown. In this paper, a multiuser cooperative protocol is proposed, where a power reward is adopted by each node to evaluate the power contributed to and by others. It will be shown that the proposed fair cooperative protocol (FAP) can significantly improve the fairness performance compared to full cooperation. It is further demonstrated that in energy-constrained cooperative ad-hoc networks, fairness can actually bring significant throughput gains. The tradeoff between fairness and throughput is analyzed and two price-aware protocols, FAP-R and FAP-S, will be further proposed to improve fairness. Simulation results will validate our analysis and show that compared to the direct transmission (i.e., without cooperation) and the full cooperation, our proposed FAP, FAP-R and FAP-S can achieve much better fairness performance along with substantial throughput gains.

The fading characteristics and broadcast nature of wireless channels are usually not fully considered in the design of routing protocols for wireless networks. In this paper, we combine routing and cooperative diversity, with the consideration of a realistic channel model. We focus on a multihop network with multiple relays at each hop, and three routing strategies are designed to achieve the full diversity gain provided by cooperation among the relays. In particular, an optimal routing strategy is proposed to minimize the end-to-end outage, which requires the channel information of all the links and serves as a performance bound. An ad-hoc routing strategy is then proposed based on a hop-by-hop relay selection, which can be easily implemented in a distributed way. As expected, ad-hoc routing performs worse than optimal routing, especially with a large number of hops. To achieve a good complexity-performance tradeoff, an N-hop routing strategy is further proposed, where a joint optimization is performed every N hops. Simulation results are provided which verify the outage analyses of the proposed routing strategies.

In this paper, we propose two ad-hoc, yet efficient, power allocation strategies for a decentralized distributed space-time block coding (Dis-STBC) system where knowledge about the channel state information (CSI) is not available at the transmitter(s). The first is an open-loop strategy which requires no control signaling; the second is a feedback-assisted strategy which requires some control signaling, but which can achieve better power-efficiency. Focusing on a particular decentralized Dis-STBC scheme (m-group), the asymptotic outage probability is derived and the power-efficiency advantages of the proposed strategies over a uniform-power strategy are illustrated by evaluating the outage and link failure probabilities.

When MIMO techniques are used in a cellular system, the interference environment can become quite complicated. In this paper, we evaluate the impact of co-channel MIMO interference on the performance of space-time block coding in the downlink of a cellular-like system. In particular, we derive closed-form expressions for the probability density functions of the resulting signal-to-interference ratio with and various MIMO modes used by the co-channel base stations. Simulation results verify the validity of the analysis.

In this paper, we investigate two selective relaying schemes in cooperative OFDM systems. Selective OFDMA relaying, where the relay selection is performed in a per-subcarrier manner, and selective OFDM relaying, where one best relay among the L potential relays is selected to relay the entire OFDM block, are compared in a two-hop random network. The outage performance of equal bit allocation (EBA), where each subchannel has the same number of bits, and bit loading (BL), where bits are adaptively allocated to each subchannel, are analyzed and compared for these two approaches. The outage analysis clearly shows that a significant performance gain can be achieved by selective OFDMA relaying, whether EBA or BL is employed, compared with selective OFDM relaying. The performance gain remains the same for different relay locations. With EBA, the performance gain increases with an increase in L and N, the number of independent subchannels. For BL, the performance gain also increases with an increase in R, the average number of bits per subchannel, in addition to L and N. Centralized and decentralized implementation issues are also considered. For EBA, selective OFDMA relaying scheme is preferred because of its superior performance and simple decentralized implementation. For BL, selective OFDMA relaying scheme is a good choice for centralized systems and selective OFDM relaying is more suitable for decentralized systems at the expense of a loss in performance.

In this paper, focusing on multihop decode-and- forward cooperative networks, a hop-by-hop routing strategy is designed to optimize the end-to-end outage performance when only knowledge about the mean channel state information (CSI) is available. In particular, by only utilizing the mean CSI at the current hop, a novel optimization metric is devised to select the best relay on a per-hop basis. The outage analysis illustrates the advantage of the proposed hop-by-hop routing strategy. In addition, a decentralized implementation scheme with minimal overhead is presented. Under realistic propagation conditions, including the effects of path loss and Rayleigh fading, the end- to-end outage performance is evaluated. Simulation results show that, when only the mean CSI can be exploited, the proposed hop-by-hop routing strategy can provide a good tradeoff between performance and overhead.

The 17 papers in this special issue focus on MIMO systems and applications: field experience, practical aspects, limitations and challenges. The papers are summarized here.

There has been growing interest in the integration of multihop (or relaying) capability into conventional wireless networks. In this paper, we propose an OFDM-based selective relaying scheme, where the relay selection at each hop is performed on a per-subcarrier basis and joint selection is adopted at the last two hops. The outage analysis clearly shows that full spatial diversity gain can be achieved with this proposed selective OFDMA relaying. In contrast, no diversity gain can be obtained if the entire OFDM block chooses the same relay with the highest combined SNR. It is also demonstrated that with coding among the subcarriers, superior performance can be achieved by selective OFDMA relaying with only symbol detection at each relay. This is highly attractive as the processing complexity and decoding delay incurred are very small.

The fading characteristics and broadcast nature of wireless channels are usually not fully considered in the design of routing protocols for wireless networks. In this paper, we address the routing issue from a link layer point of view. We focus on a multihop network with multiple relays at each hop and three routing strategies are designed to achieve the full diversity gain provided by the cooperation among relays. In particular, an optimal routing strategy is proposed to minimize the end-to-end outage, which requires the channel information of all the links and serves as a performance bound. An ad-hoc routing strategy is then proposed based on a hop-by-hop relay selection, which can be easily implemented in a distributed way. The outage analysis shows that the performance gap between these two routing strategies increases with the number of hops. To achieve a good complexity-performance tradeoff, a N-hop routing strategy is further proposed, where a joint optimization is performed every N hops. Simulation results are presented which verify the analysis.

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity

In the above titled paper (ibid., vol. 55, no. 5, pp. 941-952, May 07), a change to equation (7), that was requested by the author, was omitted in the final version. The paragraph leading up to, and including the equation, is repeated here.

We study power allocation for the decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly distributed node locations, we aim to find the power allocation that minimizes the outage probability under a short-term power constraint, wherein the total power for all nodes is less than a prescribed value during each two-stage transmission. Due to the computational and implementation complexity of the optimal solution, we derived a simple near-optimal solution. In this near-optimal scheme, a fixed fraction of the total power is allocated to the source node in stage I. In stage II, the remaining power is split equally among a set of selected nodes if the selected set is not empty, and otherwise is allocated to the source node. A node is selected if it can decode the message from the source and its mean channel gain to the destination is above a threshold. In this scheme, each node only needs to know its own mean channel gain to the destination and the number of selected nodes. Simulation results show that the proposed scheme achieves an outage probability close to that for the optimal scheme obtained by numerical search, and achieves significant performance gain over other schemes in the literature

In opportunistic SDMA, the base station forms several random beams simultaneously and relies on fast but limited feedback for scheduling users on the beams. In this paper, opportunistic SDMA is applied to an OFDMA downlink by having independent beamformers run in parallel on different frequency bands (SD-OFDMA). A method for improving the performance of completely random beamforming is proposed and evaluated. It exploits the temporal channel correlation and the partial channel state information available at the transmitter to keep "successful" beams over longer periods of time than the default block duration. Extensive simulation results show a significant improvement in cell throughput compared to a completely random scheme. The results also show that in order to gain more from the multiple simultaneous beams than what is lost in additional interference, the system load must be high.

In this paper, we consider the unclaimed sub-carriers that no user feeds back in an opportunistic OFDMA system with a per sub-carrier power constraint. Unclaimed sub- carriers appear in OFDMA systems with reduced feedback, where the users concentrate the feedback on the sub-carriers with good channel quality. Here we propose and evaluate two ways to use these sub-carriers to improve the system performance. One approach is to transmit pilots symbols on the unclaimed sub- carriers. A second approach is to schedule users based on their feedback about adjacent sub-carriers. Simulation results show that for low to moderate channel RMS delay spread, it is more advantageous to transmit data on unclaimed sub-carriers, even at a low rate, than to use them for additional pilot symbols. For high delay spreads however, transmitting pilots on the unclaimed sub-carriers gives higher system throughput.

Adaptive modulation has been shown to have significant benefits for high-speed wireless data transmission when orthogonal frequency division multiplexing (OFDM) is employed. However, accurate channel state information (CSI) is required at the transmitter to achieve the benefits. Imperfect CSI arises from noisy channel estimates, which may also be outdated due to a delay in getting the CSI to the transmitter. In this paper, we study adaptive OFDM with imperfect CSI for the uncoded variable bit rate case, where a target bit error rate is set. A loading algorithm based on the statistics of the real channel is proposed. Performance results in terms of the average spectral efficiency are provided for adaptive OFDM systems when there is noisy channel estimation or CSI delay. The use of multiple estimates is then proposed to improve the performance. It is shown that multiple estimates from different frequencies or times can enhance the performance significantly, which enables the system to tolerate larger errors in channel estimation or longer delay in CSI

We study power allocation for the space-time-coded decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly random node locations, a near-optimal power allocation that minimizes the outage probability is derived under a short-term power constraint, wherein the total power is fixed for each two-stage transmission. This near-optimal scheme allocates one half of the total power to the source node and splits the remaining half equally among selected relay nodes; a node is selected for relay if it is able to decode the signal from the source and its mean channel gain to the destination is above a threshold. Numerical results show that this scheme significantly outperforms the constant-power scheme, wherein all nodes use the same power at all times, and the best-select scheme, which employs one relay node with the largest mean relay-destination gain

This paper investigates multiuser downlink data scheduling with quality-of-service (QoS) provisioning over multiple shared fading channels, which, for a network point of view, provides line flexibility and granularity for resource allocation. A user-centric metric-a utility function with respect to mean waiting time-which is able to maintain fairness among users while providing delay QoS to individual users is used. This paper proposes scheduling algorithms that are aware of both channel and queue state information to achieve the maximum aggregate utility in the network. Simulation results confirm the significant performance and stability improvement provided by the utility-based scheduling scheme balancing multiuser diversity and queueing delay.

UWB systems based on impulse radio have the potential to provide very high data rates over short distances. In this paper, a new pulse shape is presented that satisfies the FCC spectral mask. Using this pulse, the link budget is calculated to quantify the relationship between data rate and distance. It is shown that UWB can be a good candidate for reliably transmitting 100 Mbps over distances at about 10 meters.

We explore the feasibility of designing an outdoor cellular network based on the IEEE 802.11 specification. Since the standard is intended for wireless local-area networks (WLAN), there are many technical challenges when applying the air interface to the outdoor environment. We study how the 802.11 medium access control (MAC) protocol can be applied and how it performs in the outdoor network. By exploiting the fact that timeout intervals are not explicitly specified, without modifying the standard, we propose a new timing structure for the distribution coordination function (DCF) and the handshake of request-to-send (RTS) and clear-to-send (CTS) to handle increased signal propagation delay in the outdoor network. We find that the DCF and RTS/CTS protocols as specified in the standard continue to work properly for a link distance up to 6 km. Our analysis reveals that the DCF performance degrades slightly in the 802.11 network with a cell size of 6 km when compared with the 600 m WLAN. Thus, as far as the MAC protocol is concerned, the 802.11 outdoor, cellular network with 6 km cell size is feasible.

A major impediment to high-bit-rate wireless transmission is multipath delay spread. Orthogonal frequency division multiplexing (OFDM) addresses this impairment, transmitting several parallel lower bit rate streams. Independent transmit and receive oscillators create frequency offset, generating interchannel interference and degrading performance. Several frequency offset estimation techniques have been proposed but varied test conditions make comparison difficult. Here, we compare these schemes under identical conditions and propose three new offset estimation techniques and include them in our comparison.

We investigate the performance of adaptive modulation in OFDM under a constant-bit-rate requirement When the transmitter knows the channel state information perfectly, a large gain over a non-adaptive system is possible. However, channel estimation errors and delay in feedback may cause channel mismatch, which diminishes the gain of adaptive modulation. The mechanism of how channel mismatch leads to performance degradation is also investigated. Based on the insights we obtain, two approaches are proposed to mitigate the adverse effect: a coding technique assisted by differential channel information, and a robust loading scheme.

Adaptive modulation has been shown to have significant benefits for high-speed wireless data transmission when orthogonal frequency division multiplexing (OFDM) is employed. However, accurate channel state information (CSI) is required at the transmitter to achieve the benefits. Imperfect CSI arises from noisy channel estimates, which may also be outdated due to a delay in getting the CSI to the transmitter. We study adaptive OFDM with both perfect and imperfect CSI for the variable bit rate case, where a target bit error rate is set. Performance results are provided for adaptive OFDM with imperfect CSI. The use of multiple estimates is shown to mitigate the effect of CSI delay. In addition, the fundamental error mechanisms which result in performance degradation are studied. Robust approaches are then proposed that are less sensitive to CSI errors.

It has been shown that link adaptation and power control can improve the performance of our future wireless packet networks. Realizing the expected performance gain of these techniques requires accurate prediction of future interference power. We propose a new method based on Kalman filtering for interference estimation. The new method is devised by observing: (a) it is possible to identify fairly accurately the number of active co-channel interferers in the cellular networks and (b) interference power is positively correlated with the number of active interferers. The new technique uses a two-dimensional Kalman filter to exploit that correlation to enhance the prediction accuracy. Using a cellular network with 1/3 frequency reuse and partial traffic loading, the performance of the new method is compared with a simplified method using a one-dimensional Kalman filter where the number of active interferers is not considered. Further, the new method is compared with the traditional exponential filtering. Since the proposed method and the simplified method track interference and measurement errors separately, their predictions represent closely the best estimation by exponential filtering with the optimal parameter. In addition, for a typical network environment, the two-dimensional method yields the lowest prediction errors for a wide range of parameters, and provides a 0.5 dB improvement for the 90th percentile estimation error over the simplified method due to exploitation of the positive correlation between interference and the number of active interferers.

OFDM (orthogonal frequency division multiplexing) is an effective technique for achieving reliable high-bit-rate transmission over wireless channels. Generally, for wireless applications, a guard interval, most often in the form of a cyclic extension of the OFDM data symbol, is inserted between OFDM blocks to minimize interblock interference. We quantify the penalties incurred when a zero-padded guard interval is used instead of a cyclic extension. Specifically, the resulting interchannel interference is computed and the degradation in the bit error rate is quantified.

In this article, tight and universal bounds have been derived for the interchannel interference (ICI) of an orthogonal frequency-division multiplexing (OFDM) signal resulting from Doppler spread. The universal bound depends only on the product of the maximum Doppler frequency and the OFDM symbol duration. The tight bound also depends on the variance of the Doppler spectrum. Compared with the exact ICI expressions derived by other researchers, these bounds are easier to evaluate and can provide useful insight.

OFDM is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAP) of a multicarrier signal has limited its application. Previously, we described a technique, based on partial transmit sequences, that provides improved PAP statistics with little loss in efficiency, and which is more easily implemented than the standard partial transmit sequence approach. One of the remaining challenges is how to transmit knowledge about the combining sequence to the receiver in a reliable fashion. Here, we describe a technique, using no overhead, for embedding this information within the transmitted data and reliably recovering it at the receiver.

In this paper, tight and universal bounds are derived for the interchannel interference (ICI) of an OFDM signal resulting from Doppler spread. The universal bound depends only on the product of the maximum Doppler frequency and the OFDM symbol duration. The tight bound also depends on the variance of the Doppler spectrum. Compared with exact ICI expressions derived by other researchers, these bounds are easier to evaluate and can more easily provide useful insight.

Orthogonal frequency division multiplexing (OFDM) is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAP) of a multicarrier signal has limited its application. Two promising techniques for improving the statistics of the PAP of an OFDM signal have previously been proposed: the selective mapping and partial transmit sequence approaches. Here, we summarize these techniques and present suboptimal strategies for combining partial transmit sequences that achieve similar performance but with reduced complexity.

Orthogonal frequency division multiplexing (OFDM) modulation is a promising technique for achieving the high-bit-rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3 dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator to channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

Orthogonal frequency-division multiplexing (OFDM) modulation is a promising technique for achieving the high bit rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3-dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MMSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator-to-channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

An elegant means by which high-speed burst wireless transmission can be accomplished with small amounts of overhead is through a novel technique referred to as clustered-OFDM. By using OFDM modulation with a long symbol interval, this scheme overcomes the complex and costly equalization requirements associated with single carrier systems. Moreover, the need for highly linear power amplifiers typically required in OFDM systems is alleviated through the use of multiple transmit antennas combined with nonlinear coding. The clustering technique also leads to a natural implementation of transmit diversity. This paper reports on the implementation of a clustered-OFDM transmitter which was constructed using off-the-shelf components in a relatively short period of time. Despite the limitations associated with the prototyped system, it can deliver 7.5 Mbps and it is expected that this data rate could be easily tripled with existing technology. In addition to providing an introduction to clustered-OFDM, we describe the various architectural trade-offs which were made in order to reduce the hardware complexity of the boards as well as some experimental results showing the operation of the transmitter.

We determine the call blocking performance of channel-allocation algorithms where every channel is available for use in every cell and where decisions are made by mobiles/portables based only on local observations. Using a novel Erlang-B approximation method, together with simulation, we demonstrate that even the simplest algorithm, the timid, compares favorably with impractical, centrally administered fixed channel allocation. Our results suggest that an aggressive algorithm, that is, one requiring call reconfigurations, could provide a substantially reduced blocking probability. We also present some algorithms which take major steps toward achieving the excellent blocking performance of the hypothetical aggressive algorithm but having the stability of the timid algorithm.>

One important quantity in assessing the viability of local, autonomous, dynamic channel allocation for microcellular systems is user capacity, defined as the average number of users per channel per cell. Here, we determine the capacity for infinite linear and planar arrays of microcells using a very idealized environment. In particular, propagation and interference considerations are simply represented by the constraint that, if a channel is used in a given cell, it cannot be used in R-consecutive rings of cells around that cell. We investigate the elementary case where there is only a single channel available for use in the system. Using this representation, we compute the best and worst user capacities as well as the capacity achieved by random channel placement. While the environment under which these capacities are derived is highly idealized, the results are useful in two important ways. First, the best capacity and the random channel placement capacity we find here for single-channel, self-organized access are fundamental for computing the traffic characteristics of important multichannel dynamic channel allocation algorithms. Second, the random channel placement capacity is close enough to the best that can be achieved to suggest that local, autonomously implemented, dynamic channel allocation loses little capacity when compared with centrally administered fixed channel allocation.>

The question of whether multilevel signaling can improve the spectral efficiency of optical frequency division multiplexing (FDM) systems is investigated. Multilevel amplitude-shift keying (ASK) and a receiver consisting of an optical channel selection filter (specifically, a single-stage Fabry-Perot), a pin diode for direct detection, and an integrator are assumed. It is also assumed that additive noise is negligible and that the major impairments are adjacent channel interference and intersymbol interference. The analysis shows that, for such a receiver, there is no advantage to going beyond binary signaling. It is also shown that this finding holds up when additive and phase noises and alternative approaches to filtering are considered.>

The general principles, implementation, and performance of a 1-Mb/s time-division-multiple-access (TDMA), slow frequency hopping and coding, 1.5-GHz radio communications system for a medium-sized office building are discussed. TDMA is provided for service flexibility and slow frequency hopping and coding for immunity against multipath fading and interference. Measurements show that, with a hallway-mounted distributed antenna system installed on one floor of the building and with the mobile unit transmitting only 1 mW of peak RF power from anywhere on that floor, only a single 384-b frame out of a total of 200000 transmitted frames suffered an unrecoverable error. This kind of performance is comparable to that of wired data modems. Similar performance was obtained from a central antenna covering the same floor. However, the required transmitted power in this case was 100 mW. Implementation issues, such as spectrum allocation, cellular subdivisions, and spectrum efficiency, are also discussed.>

The benefits of coding for an optical communication system that employs binary on-off keying and heterodyne detection are quantified. The system is impaired by laser phase noise as well as by additive white Gaussian noise (AWGN). A receiver structure especially designed to mitigate the effects of phase noise in the presence of AWGN is assumed. This special receiver structure requires a wider-band front-end IF filter than would be required for a phase-noise-free signal. The results, computed for several different coding schemes, indicate that the benefits of coding are large and the costs are small. For a linewidth-to-bit-rate ratio ( beta T) of 0.64 (for example, 45 Mb/s and 29 MHz linewidth), a half-rate binary code that can correct 3 bit errors provides a 50% reduction in the required IF filter bandwidth (and, therefore, the required IF) and about 5 dB of reduction in required laser power. The benefits of coding are greatest under high- beta T conditions, corresponding to low bit rates where coders and decoders are most practical to implement.>

The authors describe and analyze a flexible, robust, multichannel time-division-multiple-access (TDMA) radio communications system that is ideally suited for portable as well as mobile indoor use. It uses noncoherent signaling and incorporates slow cyclical frequency hopping and burst-error-correcting coding to provide low-error communications at aggregate rates of several megabits per second without the need for antenna diversity or adaptive equalization. The system can be implemented in a small building using a centralized architecture or in a large building using a distributed antenna system. The TDMA protocol allows the various users the flexibility of choosing their own desired rates of data communications. Moreover, the capability of using different levels of coding gives the users the option of selecting their own error performance. The ability to combat interference is one of the salient features of this system.>

A local fiber bus system is described wherein many users sequentially phase-modulate the output of a single light source. Since the modulated signal has a constant envelope, optical amplifiers can be used to boost the signal power along the bus to a level sufficient for reliable detection. Also, since a single laser source is used, a higher quality device (for example, with small linewidth) is more readily affordable. This system is analyzed to determine the limits of performance due to noise. The analysis indicates that many wideband signals could be accommodated if noise sources only, rather than signal distortions caused by nonideal optical devices, were the limiting factor. For example, with optical amplifiers distributed along the bus, this system can support 1000 10-MHz users.>

This paper discusses the analysis and simulation of a technique for combating the effects of multipath propagation and cochannel interference on a narrow-band digital mobile channel. This system uses the discrete Fourier transform to orthogonally frequency multiplex many narrow subchannels, each signaling at a very low rate, into one high-rate channel. When this technique is used with pilot-based correction, the effects of flat Rayleigh fading can be reduced significantly. An improvement in signal-to-interference ratio of 6 dB can be obtained over the bursty Rayleigh channel. In addition, with each subchannel signaling at a low rate, this technique can provide added protection against delay spread. To enhance the behavior of the technique in a heavily frequency-selective environment, interpolated pilots are used. A frequency offset reference scheme is employed for the pilots to improve protection against cochannel interference.

Matched and Wiener filters are considered for two-dimensional signal processing applications when the a priori information about signal and noise characteristics are not completely specified. This work is an extension of previous work on robust filters for one-dimensional processing. The approach is to design filters which are saddle-point or mini-max solutions for the criterion functional (mean-squared-error or signal-to-noise ratio) over the classes of allowable signal shapes and signal and noise spectral densities.