Umer Salim

How many users should inform the BS about their channel information

Simultaneous transmission of multiple data streams from a multiple antenna base station (BS) to multiple single antenna users gives significant gain in spectral efficiency as compared to when a single such user is being served. This simultaneous transmission to multiple users is realizable only if BS knows the forward channels linking its transmitting antennas to these users which requires channel feedback from these users. This feedback overhead could be prohibitively large especially in large user systems, limiting the multi-user transmission gains. Exploiting the channel reciprocity in a time-division duplexed (TDD) broadcast channel, we give a simple transmission strategy, where users feedback independent of their channel realizations. We analyze the sum rate of this multi-user system when the channel acquisition load is completely accounted for. We derive a novel lower bound of the sum rate which allows us to optimize over how many users should inform the BS about their channel information, solving the intriguing trade-off of multiuser diversity, interference cancellation and feedback overhead.

Hybrid Pilot/Quantization Based Feedback in Multi-Antenna TDD Systems

The communication between a multiple-antenna transmitter and multiple receivers (users) with either a single or multiple-antenna each can be significantly enhanced by providing the channel state information at the transmitter (CSIT) of the users, as this allows for scheduling, beamforming and multiuser multiplexing gains. The traditional view on how to enable CSIT has been as follows so far: In time-division duplexed (TDD) systems, uplink (UL) and downlink (DL) channel reciprocity allows for the use of a training sequence in any given uplink slot, which is exploited to obtain an uplink channel estimate. This estimate is in turn recycled in the next downlink slot. In frequency-division duplexed (FDD) systems, which lack the UL and DL reciprocity, the CSIT is provided via the use of a dedicated feedback link of limited capacity between the receivers and the transmitter. In this paper, we focus on TDD systems and show that the traditional TDD CSIT acquisition fails to fully exploit the channel reciprocity in its true sense. In fact, we show that the system can benefit from a combined CSIT acquisition strategy mixing the use of limited feedback and that of a training sequence. We demonstrate the potential of our approach in terms of improved CSIT quality under a global training and feedback resource constraint.

Asymptotic capacity of underspread and overspread stationary time- and frequency-selective channels

In this paper, we consider stationary time- and frequency-selective channels. No channel knowledge neither at the transmitter nor at the receiver is assumed to be available. We investigate the capacity behavior of these doubly selective channels as a function of the channel parameters delay spread, Doppler bandwidth and channel spread factor (the product of the delay spread and the Doppler bandwidth). We shed light on different capacity regimes at high values of signal to noise ratio (SNR) in which the dominant capacity term is either of order log(SNR) or log(log(SNR)), depending on the channel conditions (delay spread, Doppler Bandwidth and channel spread factor). For critically spread channels (channel spread factor of 1), it is widely believed that the dominant term of the high-SNR expansion of the capacity is of order log (log(SNR)) or in other words, that the pre-log (the coefficient of log(SNR)) is zero. We provide a very simple scheme that shows that even for critically spread channels a non-zero pre-log might exist under certain conditions. We also specify these conditions in terms of Doppler bandwidth and delay spread. We also show that a nonzero pre-log might exist even for over-spread channels (channel spread factor greater than 1). We specify the channel conditions which govern the range of existence of the log(SNR) regime. At higher channel spread factor, the log(SNR) term vanishes and a log(log(SNR)) term becomes the dominant capacity term. We specify the range of this log(log(SNR)) regime and also provide bounds for the coefficient of this log(log(SNR)) term (the pre-loglog).

MIMO capacity pre-log for flat fading stationary channels with no CSIR

The asymptotic capacity for the non-coherent MIMO stationary channels having n_t transmit and n_r receive antennas with flat fading is the focus of this paper. Fading processes of concern are bandlimited. These non-coherent MIMO channels were studied by Etkin and Tse and lower bound of capacity was shown to grow with min(n_t, n_r)[1 - min(n_t, n_r)mu] log(SNR) where mu is the normalized Doppler bandwidth. The contribution of this paper is to specify the pre-log for MIMO channels and this is done by giving matching upper and lower bounds of the pre-log. Moreover min(n_t, n_r) factor in the pre-log term bears a small modification. The actual pre-log is min(n_t, n_r, 1/2mu)[1 - min(n_t, n_r, 1/2mu)mu] and takes into account the optimal number of streams that should be activated as a function of the Doppler bandwidth.

Asymptotic capacity of underspread and overspread doubly selective MIMO channels

In this paper, we consider stationary time- and frequency-selective MIMO channels. No channel knowledge neither at the transmitter nor at the receiver is assumed to be available. We investigate the capacity behavior of these doubly selective channels as a function of one of the system parameters, the number of transmit antennas and channel parameters as delay spread, Doppler bandwidth and channel spread factor (the product of the previous two parameters). For critically spread channels (channel spread factor of 1), it is widely believed that the dominant term of high-SNR expansion of the capacity is log(log(SNR)) or in other words, the pre-log (the coefficient of log(SNR)) is zero. We provide a very simple scheme showing that for critically spread and mildly overspread channels a non-zero pre-log exists under certain conditions. We specify these conditions in terms of the Doppler bandwidth and the delay spread. We reason that for nearly critically spread channels, MIMO systems exhibit same degrees of freedom as that of a SISO system. At higher channel spread factor (overspread case), the log(SNR) term vanishes and log(log(SNR)) term becomes the dominant capacity term. We specify the range of existence for log(SNR) regime.