We propose and study a class of transmit beamforming techniques for systems with multiple transmit and multiple receive antennas with a per-antenna transmit power constraint. The per-antenna transmit power constraint is more realistic than the widely used total (across all transmit antennas) power constraint, since in practice each transmit antenna is driven by a separate power amplifier with a maximum power rating. Under the per-antenna power constraint, from an implementation perspective, it becomes desirable to vary only the phases (as opposed to both power and phase variation) of the signals departing from the transmit antennas. We name this class of techniques generalized co-phasing and formulate an optimization problem to calculate the transmit antenna phases. Furthermore, we propose five heuristic algorithms to solve the optimization problem. All the proposed algorithms except one are optimal for the case of two transmit antennas and an arbitrary number of receive antennas. For an arbitrary number of transmit and receive antennas, simulations indicate that the proposed algorithms perform very close to the optimal solution calculated through an exhaustive search of all possible transmit phases.

This paper provides an exact diversity gain analysis for transmit antenna selection in MIMO-OFDM systems with linear receivers, a topic scarcely addressed in prior literature. In a frequency-selective fading (as opposed to a frequency-flat fading) channel one of either two methods of antenna selection may be applied. We name these methods bulk selection and per-tone selection. From an implementation perspective each method presents a different complexity-performance tradeoff. Through rigorous mathematical derivation, we demonstrate that both methods realize the same diversity order. Moreover, the coding gain realized by the two selection methods is also analyzed and compared through simulation. Overall, this paper confirms the advantage of antenna selection technology in MIMO-OFDM systems, and provides comprehensive guidelines for antenna selection strategies in realistic scenarios, from both the performance and complexity perspectives.

For practical multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, it is important to develop transmit diversity schemes that are transparent to a receiver in that the receiver does not need to perform additional processing. In this paper, we examine one of the most widely-used receiver-transparent transmit diversity schemes - the cyclic delay diversity (CDD) scheme. It is shown that the CDD scheme effectively increases the delay spread of the channel and that the performance degrades under non-ideal channel estimation. As a remedy to the performance loss due to non-ideal channel estimation, a cluster-based transmit diversity scheme is proposed, which garners almost all the benefits of the CDD scheme without degrading the performance of a cluster- based channel estimation. The simulation results are provided in the realistic setting of IEEE 802.16e WiMAX systems.

Routing data through a wireless network is made challenging by impairments in the wireless medium, such as fading. Nevertheless, wireless routing if implemented, will result in tremendous cost-savings in next generation wireless networks. This paper examines centralized and decentralized approaches for wireless routing from a PHY/MAC perspective. We show that decentralized routing strategies are capable of realizing full spatial diversity gain if an appropriate level of channel knowledge is available at the relay terminals. Furthermore, we propose a new carrier sensing random access based routing technique designed to avoid collision and conserve network power. A numerical example is provided to demonstrate that the technique is capable of realizing full spatial diversity gain.

In this paper, we propose a probabilistic technique to achieve joint channel estimation and symbol detection for orthogonal space time block coding (OSTBC) systems. The key result is that a multiple-input single-output system using OSTBC can be translated to an equivalent single-input multiple-output system using the implicit redundancy in OSTBC. This allows some probabilistic methods to be applicable at each output branch. Since each output branch is excited by the same input, redundant information can be exchanged between the multiple branches to improve the detection performance at individual branches. The proposed technique has the potential to be concatenated with a channel decoder and form a turbo receiver. The performance of the proposed algorithm is demonstrated through simulation results and compared with an existing scheme.

The use of multiple antennas at both ends of a wireless link, popularly known as multiple-input multiple-output (MIMO) wireless, has been shown to offer significant improvements in spectral efficiency and link reliability through spatial multiplexing and space-time coding, respectively. This paper demonstrates that similar performance gains can be obtained in wireless relay networks employing terminals with MIMO capability. We consider a setup where a designated source terminal communicates with a designated destination terminal, both equipped with M antennas, assisted by K single-antenna or multiple-antenna relay terminals using a half-duplex protocol. Assuming perfect channel state information (CSI) at the destination and the relay terminals and no CSI at the source, we show that the corresponding network capacity scales as C = (M/2) log(K) + O(1) for fixed M, arbitrary (but fixed) number of (transmit and receive) antennas N at each of the relay terminals, and K rarr infin. We propose a protocol that assigns each relay terminal to one of the multiplexed data streams forwarded in a "doubly coherent" fashion (through matched filtering) to the destination terminal. It is shown that this protocol achieves the cut-set upper bound on network capacity for fixed M and K rarr infin (up to an O(1)-term) by employing independent stream decoding at the destination terminal. Our protocol performs inter-stream interference cancellation in a completely decentralized fashion, thereby orthogonalizing the effective MIMO channel between source and destination terminals. Finally, we discuss the case where the relay terminals do not have CSI and show that simple amplify-and-forward relaying, asymptotically in K, for fixed M and fixed N ges 1, turns the relay network into a point-to-point MIMO link with high-SNR capacity C = (M/2) log(SNR) + O(1), demonstrating that the use of relays as active scatterers can recover spatial multiplexing gain in poor scattering environments

We study fading interference relay networks where M single-antenna source-destination terminal pairs communicate through a set of K relays using half-duplex two-hop relaying. Two specific protocols are considered, P1 introduced in H. Bolcskei, et al. (2004), H. Bolcskei and R.U. Nabar (2004) and P2 introduced in A.F. Dana and B. Hassibi (2003). P1 relies on the idea of relay partitioning and requires each relay terminal to know one backward and one forward fading coefficient only. P2 requires each relay terminal to know all M backward and M forward fading coefficients and does not need relay partitioning. We prove that in the large-M limit the minimum rate of growth of K for P1 to achieve a strictly positive per source-destination terminal pair capacity is K infin M3whereas in P2 it is K infin M2. The protocols P1 and P2 are thus found to trade off the number of relay terminals for channel state information (CSI) at the relays; more CSI at the relays reduces the total number of relays needed to achieve a strictly positive per source-destination terminal pair capacity in the large-M limit

In this paper, we examine the basic building block of cooperative diversity systems, a simple fading relay channel where the source, destination and relay terminals are each equipped with single antenna transceivers. We consider three different TDMA-based cooperative protocols that vary the degree of broadcasting and receive collision. For each protocol, the relay terminal simply amplifies-and-forwards the signal received from the source terminal to the destination. We study the ergodic and outage capacity behavior of each of the protocols assuming Gaussian codebooks and show that full spatial diversity (second-order in this case) is achieved by certain protocols provided that appropriate power control is employed. Finally, we establish the superiority (both from a capacity as well as a diversity point-of-view) of a new protocol proposed in this paper.

This paper considers wireless networks where L single-antenna source-destination terminal pairs communicate concurrently through a common set of K single-antenna relay terminals using one-hop relaying. It is shown that asymptotically in K, the sum capacity of this network scales as C=(L/2) log (K)+O(1) and can be achieved without cooperation between any of the terminals.

Cooperative diversity is a transmission technique, where multiple terminals pool their resources to form a virtual antenna array that realizes spatial diversity gain in a distributed fashion. In this paper, we examine the basic building block of cooperative diversity systems, a simple fading relay channel where the source, destination, and relay terminals are each equipped with single antenna transceivers. We consider three different time-division multiple-access-based cooperative protocols that vary the degree of broadcasting and receive collision. The relay terminal operates in either the amplify-and-forward (AF) or decode-and-forward (DF) modes. For each protocol, we study the ergodic and outage capacity behavior (assuming Gaussian code books) under the AF and DF modes of relaying. We analyze the spatial diversity performance of the various protocols and find that full spatial diversity (second-order in this case) is achieved by certain protocols provided that appropriate power control is employed. Our analysis unifies previous results reported in the literature and establishes the superiority (both from a capacity, as well as a diversity point-of-view) of a new protocol proposed in this paper. The second part of the paper is devoted to (distributed) space-time code design for fading relay channels operating in the AF mode. We show that the corresponding code design criteria consist of the traditional rank and determinant criteria for the case of colocated antennas, as well as appropriate power control rules. Consequently space-time codes designed for the case of colocated multiantenna channels can be used to realize cooperative diversity provided that appropriate power control is employed.

High data rate wireless communications, nearing 1 Gb/s transmission rates, is of interest in emerging wireless local area networks and home audio/visual networks. Designing very high speed wireless links that offer good quality-of-service and range capability in non-line-of-sight (NLOS) environments constitutes a significant research and engineering challenge. Ignoring fading in NLOS environments, we can, in principle, meet the 1 Gb/s data rate requirement with a single-transmit single-receive antenna wireless system if the product of bandwidth (measured in hertz) and spectral efficiency (measured in bits per second per hertz) is equal to 10/sup 9/. A variety of cost, technology and regulatory constraints make such a brute force solution unattractive, if not impossible. The use of multiple antennas at transmitter and receiver, popularly known as multiple-input multiple-output (MIMO) wireless, is an emerging cost-effective technology that offers substantial leverages in making 1 Gb/s wireless links a reality. The paper provides an overview of MIMO wireless technology covering channel models, performance limits, coding, and transceiver design.

Cooperative diversity is a transmission technique where multiple users pool their resources to form a virtual antenna array that realizes spatial diversity gain in a distributed fashion. We examine space-time signal design for a simple amplify-and-forward relay channel. We show that the code design criteria for the relay case consist of the traditional rank and determinant criteria as well as appropriate power control rules. While proper signal design and power control can indeed achieve full spatial diversity gain, the potential benefit of relay-assisted communication over direct communication depends strongly on the channel conditions. In particular, we present a switching criterion based on which the source terminal may opt to forego relay-assisted communication and communicate with the destination terminal directly. The criterion is based on the cut-off rate of the effective channel - the physical channel in conjunction with finite constellation and maximum-likelihood (ML) decoding.

The use of spatial multiplexing (SM) in multiple-input multiple-output (MIMO) wireless systems promises a linear (in the minimum of the number of transmit and receive antennas) increase in data rate. In practice, the performance of SM depends critically on a variety of channel conditions, including antenna height and spacing, polarization of antennas, and richness of scattering. Transmit correlation has been shown to be detrimental to the performance of SM, since it leads to the existence of preferred spatial directions. In addition, the presence of an ill-conditioned fixed (possibly line-of-sight) component in the channel can severely degrade performance. We present a simple transmit optimization strategy to mitigate partially the impact of unfavorable channel statistics on the performance of SM. The proposed strategy takes the scalar symbol constellation and the channel statistics into account and relies on simple phase-shifting of the multiplexed symbol streams at the transmitter. The phase shifts are chosen such that the cut-off rate of the effective channel (physical channel in combination with finite constellation and ML decoding) is maximized. We find SNR gains of up to 4 dB over the case when no transmit optimization is employed.

We consider Gaussian multiple-input multiple-output (MIMO) fading channels assuming that the channel is unknown at the transmitter and perfectly known at the receiver. Using results from multivariate statistics, we derive a tight closed-form lower-bound for the ergodic capacity of such channels at any signal-to-noise ratio (SNR). Moreover, we provide an accurate closed-form analytical approximation of ergodic capacity in the high SNR regime. Our analysis incorporates the frequency-selective Rayleigh fading case and/or spatial fading correlation, and allows Important Insights Into optimal (ergodic capacity maximizing) MIMO configurations. Finally, we verify our analytical expressions through comparison with numerical results.

The performance of space-time block codes is well understood from an average (over the random channel) error point of view. However, inherent to the idea of diversity gain is the issue of reliability, which is better captured through an outage analysis indicating the quality of performance guaranteed with a certain level of reliability. In this paper, we study the outage performance of a simple space-time block code, the Alamouti scheme, in the presence of correlated Rayleigh or Ricean fading. We derive expressions for the cumulative distribution function of the uncoded symbol error rate and verify the accuracy of our analytical expressions through comparison with numerical results. In addition, we introduce a quantitative measure to compare the diversity gain offered by two channels at a given outage rate.

Multiple-input multiple-output (MIMO) wireless systems employ spatial multiplexing to increase data rate. The performance of spatial multiplexing is highly dependent on channel statistics which in turn depend on antenna spacing and richness of scattering. It has been shown Bolcskei and Paulraj, (see Asilomar Conf. on Signals, Systems, and Computers, Pacific Grove, CA, Oct./Nov. 2000) that the presence of transmit correlation can have a detrimental effect on the performance of multi-antenna signaling techniques. We present a novel scheme to (partly) mitigate the performance loss of spatial multiplexing in the presence of highly correlated fading at the transmitter. The adaption to be performed at the transmitter is a form of power allocation and/or relative phase adjustment between the different symbol streams to be multiplexed. We consider the cases of dual-polarized as well as uni-polarized antennas and derive estimates of the uncoded average symbol error rate as a function of channel statistics, power allocation and phase adjustment. We then optimize power allocation and phase adjustment and demonstrate that this form of preprocessing can yield SNR gains of up to 4 dB over the case where no precoding is employed.

In practice large antenna spacings are needed to achieve high capacity gains in multiple-input multiple-output (MIMO) wireless systems. The use of dual-polarized antennas is a promising cost effective alternative where two spatially separated antennas can be replaced by a single antenna element employing orthogonal polarizations. This paper investigates the performance of spatial multiplexing in MIMO wireless systems with dual-polarized antennas. We compute estimates of the symbol error rate as a function of cross-polarization discrimination (XPD) and spatial fading correlations. Using these estimates, we show that dual-polarized antennas can significantly improve the performance of spatial multiplexing systems. It is demonstrated that improvements in terms of symbol error rate of up to an order of magnitude are possible. We furthermore find that in general for a given SNR there is an optimum XPD for which the symbol error rate is minimum. Finally, we present simulation results and we show that our estimates closely match the numerical results.

Previous work has shown that the use of multiple antennas in a fading environment results in a linear increase in capacity. This paper examines the capacity of a multiple antenna element array (MEA) in a quasi-static flat fading environment with a rank deficient channel. We assume that the channel is known at the receiver and the existence of a feedback path to the transmitter. For a particular channel realization, we show that the judicious use of fewer transmit antennas when the channel matrix is ill-conditioned can increase system capacity. We develop a criterion for selecting an optimum set of transmit antennas. This selection is optimal in the sense that the capacity of the resulting MEA system is greater than that for any other configuration with the same number of transmit antennas chosen from the original set. The resulting channel is full rank.