Tai-Ann Chen

On the design of MIMO block-fading channels with feedback-link capacity constraint

In this paper, we propose a combined adaptive power control and beamforming framework for optimizing multiple-input/multiple-output (MIMO) link capacity in the presence of feedback-link capacity constraint. The feedback channel is used to carry channel state information only. It is assumed to be noiseless and causal with a feedback capacity constraint in terms of maximum number of feedback bits per fading block. We show that the hybrid design could achieve the optimal MIMO link capacity, and we derive a computationally efficient algorithm to search for the optimal design under a specific average power constraint. Finally, we shall illustrate that a minimum mean-square error spatial processor with a successive interference canceller at the receiver could be used to realize the optimal capacity. We found that feedback effectively enhances the forward channel capacity for all signal-to-noise ratio (SNR) values when the number of transmit antennas (n/sub T/) is larger than the number of receive antennas (n/sub R/). The SNR gain with feedback is contributed by focusing transmission power on active eigenchannel and temporal power waterfilling . The former factor contributed, at most, 10log/sub 10/(n/sub T//n/sub R/) dB SNR gain when n/sub T/>n/sub R/, while the latter factors SNR gain is significant only for low SNR values.

A space-time model for frequency nonselective Rayleigh fading channels with applications to space-time modems

This paper extends the traditional Clarke/Jakes (1968, 1974) model for a frequency flat fading process in a land mobile radio system to facilitate the examination of coherent space-time demodulation systems. The work develops a space-time correlation function using a ring of scatterers model around the mobile unit. The resulting correlation function permits the investigation of a variety of issues concerning base station configurations in space-time systems. The interrelationship of the fading process between the space and the time domain is explored. A detailed example regarding the effects of antenna separation in a receiver diversity system is considered. A set of design rules for interleaving depth and antenna separation in a space-time modem is presented and quantified.

Capacity of memoryless channels and block-fading channels with designable cardinality-constrained channel state feedback

A coding theorem is proved for memoryless channels when the channel state feedback of finite cardinality can be designed. Channel state information is estimated at the receiver and a function of the estimated channel state is causally fed back to the transmitter. The feedback link is assumed to be noiseless with a finite feedback alphabet, or equivalently, finite feedback rate. It is shown that the capacity can be achieved with a memoryless deterministic feedback and with a memoryless device which select transmitted symbols from a codeword of expanded alphabet according to current feedback. To characterize the capacity, we investigate the optimization of transmission and channel state feedback strategies. The optimization is performed for both channel capacity and error exponents. We show that the design of the optimal feedback scheme is identical to the design of scalar quantizer with modified distortion measures. We illustrate the optimization using Rayleigh block-fading channels. It is shown that the optimal transmission strategy has a general form of temporal water-filling in important cases. Furthermore, while feedback enhances the forward channel capacity more effectively in low-signal-to noise ratio (SNR) region compared with that of high-SNR region, the enhancement in error exponent is significant in both high- and low-SNR regions. This indicates that significant gain due to finite-rate channel state feedback is expected in practical systems in both SNR regions.

The role of transmit diversity on wireless communications-reverse link analysis with partial feedback

Transmit diversity through multiple transmit antennas is generally regarded as beneficial to the link-level performance. In this paper, we shall investigate the role of transmit diversity with respect to the link level and the system-level performance. We focus on the reverse link analysis, where mobile users are assumed to have independent fading, and they are equipped with multiple transmit antennas (n/sub T/). Each mobile station is assumed to have an average power constraint. The base station is assumed to have n/sub R/ receive antennas. We consider two levels of partial feedback, namely, scalar feedback and per-antenna vector feedback. In both cases, the transmitter does not have full knowledge of the channel matrix. Based on the information theoretical analytical model, it is shown that transmit diversity could enhance link-level performance, but is harmful to the multiuser system performance when there is insufficient feedback information.

Optimal multi-user space time scheduling for wireless communications

The multi-user MIMO space time scheduling problem is the prime focus of the paper. Optimizing the link level performance of the MIMO system does not always imply achieving scheduling level optimization. Therefore,design optimization across the link layer and the scheduling layer is very important to exploit fully the temporal and spatial dimensions of the communication channel. We address the design of the optimal space time scheduler for a multi-user MIMO system based on an information theoretical approach. We assume a partial feedback channel, namely a scaler rate-feedback channel, is available to the transmitter, but the full channel matrix is unknown to the transmitter. With the partial feedback, we find that the optimal resource allocation strategy is water-filling in both the temporal domain and spatial domain. The optimal scheduler should allocate all the power to at most n/sub R/ users at any particular instant. The optimal scheduler performance depends on the total number of users (K), the number of receive antennas at the base station (n/sub R/), and the number of transmit antennas at the mobile (n/sub T/). The scheduling gain increases with n/sub R/ due to the distributed MIMO configuration formed between mobile users and base station. With a single power feedback channel, the scheduling gain reduces as n/sub T/ increases, illustrating that transmit diversity hurts the scheduling performance in the case of scalar feedback. Finally, scheduling performance improves as K increases due to the multi-user selection diversity.

Two-dimensional space-time pilot-symbol assisted demodulation for frequency-nonselective Rayleigh fading channels

A two-dimensional space-time pilot-symbol assisted demodulation system is proposed in this paper. Since complex scattering environments can produce nontrivial space-time correlation characteristics, this paper proposes coherent demodulation by estimating the fading channel distortion using pilot observations across both space and time. The optimum interpolation filter takes in multiple signals from receiving antennas and exploiting the space-time channel correlation to produce multiple channel-estimate outputs corresponding to every antenna. The optimal channel-estimation filter, its mean-square error performance, and the pairwise symbol-error probability are derived for realistic scattering models to characterize the system performance. A design criterion is proposed for the pilot-symbol spacing and the filter length. Since the filter is a function of the statistical description of the channel, we investigate if this statistical description and the resulting optimal filters can be estimated simply from the pilot observations.

Optimal space-time scheduling for block fading channels with partial power feedbacks

The multi-user multiple-input-multiple-output (MIMO) space-time scheduling problem is the prime focus of the paper. Using multiple transmit (nT) and receive antennas (nR), it is well known that the link-level throughput will be increased linearly with respect to min[nT, nR] without increasing the bandwidth and power budget. However, optimizing the link-level performance of the MIMO system does not always imply achieving scheduling-level optimization. Therefore, the design optimization across the link layer and the scheduling layer is very important to fully exploit the temporal and spatial dimensions of the communication channel. In this paper, we address the design of the optimal space-time scheduler for the multi-user MIMO system based on an information theoretical approach. Since full knowledge of channel matrix at the transmitter requires a feedback channel capacity not scalable with respect to nT (number of transmit antennas at mobiles) and nR (number of receive antennas at base station), we will assume two partial feedback models.

Two dimensional space-time pilot symbol assisted demodulation over frequency nonselective Rayleigh fading channels

Pilot symbol assisted modulation (PSAM) is known to provide good performance in estimating the multiplicative distortion of fading channels. The 2-D space-time PSAM is derived and analyzed in this paper by applying the multiple-antenna reception at the base station. The interrelationship between temporal and spatial correlations is shown to provide better channel information than using the temporal correlation solely. The average mean square error (MSE) per antenna and the binary phase shift keying (BPSK) bit error probability (BEP) are used to characterize the performance analytically and draw comparisons with the traditional 1-D PSAM. When 5 antennas are used, the 2-D space-time PSAM has better average performance over the l-D PSAM for about 4 dB of MSE or 1 dB of BPSK BEP.

Iterative detection for BLAST systems with an arbitrary number of receive antennas

Turbo-coded Vertical Bell Labs Layered Space-Time Architecture (V-BLAST) is a multiple-antenna system employing turbo codes and bit-interleaving to offer flexible rates and high-speed wireless data communications. The conventional detection techniques based on one-shot demodulation and decoding algorithms may suffer poor performance or simply fail when there are fewer receive antennas than transmit antennas or when the multiple-input-multiple-output (MIMO) channel exhibits strong spatial correlations. We propose employing an iterative demodulation and decoding algorithm that solves these problems and greatly extends the applicability of V-BLAST. Simulations demonstrate that iterative V-BLAST offers the best-known performance in a wide range of settings. Through complexity analysis, we show that (a) the incremental computing complexity is small, although additional memory is required, compared with the conventional one-shot maximum likelihood (ML) decoding method, and (b) an iterative algorithm employing generalized sphere decoding will be a low-complexity, memory-reduced, and high-performance solution. We also give extensions to the technique beyond simple V-BLAST. In addition, when feedback from the receiver to the transmitter is available, we illustrate how to adapt code rate, modulation, and the power distribution to take advantage of the channel knowledge. With a combination of the “best in class” receiver performance and the ability to decode V-BLAST transmissions with fewer receive antennas than transmit antennas, the techniques described here can be viewed as key “technology enablers” for BLAST/MIMO in next-generation systems.

Space-time characteristics of frequency-flat Rayleigh fading channels and applications on MIMO systems

We discuss the interrelationship between the spatial and the temporal correlation of the frequency-flat Rayleigh fading channels, and propose a joint design of the mobile station (MS) antenna separation (AS), base station (BS) antenna separation, and the channel interleaving depth (ID) for achieving the spatial, temporal, and space-time channel independence. The proposed design is based on a model generalized from the Jakes/Clarke (1968, 1974) ring-of-scatterer fading model. A space-time channel correlation function of two BS and two MS antennas is derived and examined to produce the AS and ID design rule. Analytical binary phase shift keying (BPSK) bit error probability with perfect channel state information is used to compare the performance.