Sriram Mudulodu

A transmit diversity scheme for frequency selective fading channels

We propose a transmit diversity scheme for frequency selective fading channels using orthogonal frequency division multiplexing (OFDM). The transmit diversity scheme proposed by Alamouti (see IEEE JSAC, vol.16, no.8, p.1451-58, 1998) for flat fading channels is extended to the case when the channel has a delay spread. There is no loss in receive SNR due to the delay spread in the channel. A diversity order of 2N can be achieved by using two transmit and N receive antennas. We discuss several interesting aspects of the approach and compare it with other extensions of the Alamouti scheme for delay spread channels.

A simple multiplexing scheme for MIMO systems using multiple spreading codes

There has been a growing interest in the use of multiple transmit and receive antennas for wireless communications, due to the enormous increase in data rate that they promise over single antenna systems. Spatial multiplexing is a scheme that aims to achieve such high data rates by transmitting an independent substream of data from each transmit antenna. However when the receive spatial signatures are poor (i.e., when the channel is not favorable), one or more transmit antennas can not be used to transmit an independent stream. In fixed modulation systems this results in some loss in data rate. We consider such fixed modulation systems and propose a scheme that uses multiple spreading codes as in CDMA MIMO (multiple input multiple output) systems to smartly combine code multiplexing with spatial multiplexing in order to mitigate this loss in data rate. We also show that, in the presence of a low bandwidth feedback path, the proposed scheme, unlike the spatial-only multiplexing scheme, allows the transmitter to adapt the data, rate smoothly based on the receive spatial signatures.

An interference-suppressing RAKE receiver for the CDMA downlink

In this letter, we propose an interference-suppressing RAKE receiver for the code division multiple-access (CDMA) downlink. In the downlink, the received signal has a special structure that makes it possible for a RAKE receiver (which is a simple low-complexity linear receiver) with appropriately chosen weights to suppress interference efficiently. While there have been a few other interference-suppressing RAKE receivers proposed recently, our design is based on a different motivation, and we show that our approach significantly outperforms them especially when the number of active users in the cell is not large.

Cutoff rate analysis of MIMO wireless systems

We analyze the cutoff rates of MIMO wireless systems where the transmitter does not have channel knowledge. We also show how such analysis can be applied to practical systems.

CDMA multiuser detection based on state-space estimation techniques

We propose a new space-time multiuser receiver for CDMA wireless systems. The receiver is based on the fixed-lag smoothed estimator which occurs in the well-researched area of state-space estimation. The receiver can work with long codes as opposed to most other multiuser detectors that require cyclic spreading codes. It is also naturally suited for adaptation and can be used with time-varying (Doppler spread) multipath channels. All the desired users are decoded simultaneously with little decoding delay. The approach assumes that a reliable channel estimate is available which in practice can be obtained from training symbols. We also briefly study the computational complexity of the receiver.