Srikrishna Bhashyam

Feedback gain in multiple antenna systems

Multiple antenna transmission and reception have been shown to significantly increase the achievable data rates of wireless systems. However, most of the existing analysis assumes perfect or no channel information at the receiver and transmitter. The performance gap between these extreme channel assumptions is large and most practical systems lie in between. Therefore, it is important to analyze multiple antenna systems in the presence of partial channel information. We upper bound the outage probability performance of multiple antenna systems with preamble-based channel estimation and quantized feedback. We design causal feedback and power control schemes to minimize this upper bound on outage probability. We consider the following practical issues in our analysis and design: (1) the channel information is imperfect both at the receiver and at the transmitter and (2) part of the total available resources for the system need to be used for estimation and feedback. Our results demonstrate that for block fading channels, sending a periodic preamble and causally receiving channel state information via a feedback channel can lead to substantial gains in the outage performance over any nonfeedback scheme. Most of the gains achieved by perfect feedback can be achieved by very few bits of feedback. Furthermore, it is demonstrated that these outage probability gains can be translated into improvements in frame error rate performance of systems using space-time codes. Thus, implementing a power control, even at the cost of reduced spectral resources for the forward channel is beneficial for block fading channels.

A sub-optimal joint subcarrier and power allocation algorithm for multiuser OFDM

This paper investigates subcarrier and power allocation in multiuser OFDM. The aim is to maximize the overall rate while achieving proportional fairness amongst users under a total power constraint. Achieving the optimal solution is computationally demanding thereby necessitating the use of sub-optimal techniques. Existing sub-optimal techniques either use fixed power allocation and perform only subcarrier allocation or handle subcarrier and power allocation separately. In this paper, we propose an algorithm that performs joint subcarrier and power allocation. Simulation results are shown to compare the performance of the proposed algorithm with that of existing algorithms.

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.

Joint Subcarrier and Power Allocation in Channel-Aware Queue-Aware Scheduling for Multiuser OFDM

In an orthogonal frequency division multiplexing (OFDM) downlink scenario, we propose joint subcarrier and power allocation for channel-aware queue-aware scheduling while allowing multiple users to share a single OFDM symbol. Our approach is to combine subcarrier and power allocation by optimizing a users power allocation immediately after the user has been allocated a subcarrier. Simulation results show that joint subcarrier and power allocation yields a significant performance improvement compared to other existing schemes which perform subcarrier allocation with a fixed (uniform) power allocation assumption. Joint subcarrier and power allocation is also extended to band-wise allocation of subcarriers in order to help reduce signaling overhead in time varying channels. We examine the trade-off between increasing the sub-band size and the corresponding degradation in system performance for different values of the channel multipath delay spread.

Time-selective signaling and reception for communication over multipath fading channels

The mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code-division multiple-access systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel, which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the intersymbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the intersymbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudorandom codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel, thereby facilitating the use of powerful existing coding techniques for Gaussian channels.

A subcarrier allocation algorithm for OFDMA using buffer and channel state information

We propose a new subcarrier allocation algo- rithm for Orthogonal Frequency Division Multiple Access (OFDMA) that gives fair allocation of capacity to multiple users with different channel and traffic characteristics. This is achieved by utilizing buffer state information and mea- sured traffic statistics in addition to channel state feedback. Multiuser diversity gains are achieved in the proposed al- gorithm by scheduling across both time (buffering) and fre- quency (subcarriers). Simulation results are shown to illus- trate (a) improved capacity allocation and throughput, and (b) larger admissible traffic when compared to the existing algorithms.

Using delayed feedback for antenna selection in MIMO systems

Antenna selection in multiple-input-multipleoutput (MIMO) systems preserves diversity gain while significantly reducing hardware complexity. However, imperfect channel state information (CSI) affects performance. In this paper, we first analyze the performance of a MIMO system employing antenna selection at the transmitter and maximal ratio combining (MRC) at the receiver in the presence of feedback delay and channel estimation errors. Then, we determine whether channel prediction can compensate for the effect of feedback delay. Outage probability is analyzed as a function of ¿, the correlation coefficient between the CSI used at the receiver for decoding (CSIR) and the CSI used at the transmitter for selection (CSIT). Analytical results show that the effect of feedback delay is more significant than the effect of estimation error. In order to overcome the effect of delay, ¿ should increase with SNR. For a given SNR, the length of the linear minimum mean square error (LMMSE) prediction filter required is calculated and shown to increase with SNR. Finally, we determine the asymptotic diversity order as a function of the feedback quality. Results show that if 1 - ¿ ¿ SNR-1, the diversity order with imperfect CSI is same as that with perfect CSI.

Real-time algorithms and architectures for multiuser channel estimation and detection in wireless base-station receivers

This paper presents algorithms and architecture designs that can meet real-time requirements of multiuser channel estimation and detection in future code-division multiple-access-based wireless base-station receivers. Sophisticated algorithms proposed to implement multiuser channel estimation and detection make their real-time implementation difficult on current digital signal processor-based receivers. A maximum-likelihood based multiuser channel estimation scheme requiring matrix inversions is redesigned from an implementation perspective for a reduced complexity, iterative scheme with a simple fixed-point very large scale integration (VLSI) architecture. A reduced-complexity, bit-streaming multiuser detection algorithm that avoids the need for multishot detection is also developed for a simple, pipelined VLSI architecture. Thus, we develop real-time solutions for multiuser channel estimation and detection for third-generation wireless systems by: (1) designing the algorithms from a fixed-point implementation perspective, without significant loss in error rate performance; (2) task partitioning; and (3) designing bit-streaming fixed-point VLSI architectures that explore pipelining, parallelism, and bit-level computations to achieve real-time with minimum area overhead.

Outage probability of multiple-input single-output (MISO) systems with delayed feedback

We investigate the effect of feedback delay on the outage probability of multiple-input single-output (MISO) fading channels. Channel state information at the transmitter (CSIT) is a delayed version of the channel state information available at the receiver (CSIR). We consider two cases of CSIR: (a) perfect CSIR and (b) CSI estimated at the receiver using training symbols. With perfect CSIR, under a short-term power constraint, we determine: (a) the outage probability for beamforming with imperfect CSIT (BF-IC) analytically, and (b) the optimal spatial power allocation (OSPA) scheme that minimizes outage numerically. Results show that, for delayed CSIT, BF-IC is close to optimal for low SNR and uniform spatial power allocation (USPA) is close to optimal at high SNR. Similarly, under a longterm power constraint, we show that BF-IC is better for low SNR and USPA is better at high SNR. With imperfect CSIR, we obtain an upper bound on the outage probability with USPA and BF-IC. Results show that the loss in performance due to imperfection in CSIR is not significant, if the training power is chosen appropriately.

Self-interference cancellation models for full-duplex wireless communications

In this paper, we study two models for self or loopback interference cancellation in full-duplex wireless communications. Both models are based on an underlying Z-channel with side information. We obtain achievable rate regions with suitable coding schemes under both models. Under model 1, where the self-interference channel gain is random, we employ training to estimate the unknown gain, and optimize the required training time. Under model 2, where the self-interference gain is exactly known, we show that the capacity of an ideal full-duplex node can be realized even when the side information is low rate and quantized. Our results show that loopback interference, rather than being treated as noise, can be effectively dealt with by suitable coding.