Chandrasekar Sankaran

Solving a class of optimum multiuser detection problems with polynomial complexity

We identify a class of optimum multiuser detection problems which can be solved with polynomial complexity in the number of users. The identification is based on transforming a quadratic 0-1 programming problem into an equivalent problem in graph theory. For a synchronous direct sequence code-division multiple access (CDMA) system, the result translates to designing a set of pseudorandom codes with the property that the cross correlation between every pair of codes in the set over one symbol period is nonpositive. We give two sets of codes with good correlation properties that fall within this class. Finally, we derive a bound on the cardinality of a signal set in an n-dimensional space, having the property that the cross correlation between every pair of signals in the set is nonpositive.

The use of multiuser detectors for multicasting in wireless ad hoc CDMA networks

In this paper, we address the issue of performance of linear multiuser detectors for a multicasting application in an ad hoc wireless network. Using a code-division multiple-access (CDMA) framework, we demonstrate how capacity results for multiuser detectors can be adapted to do session admission control for the multicasting problem. We then develop a multicast routing algorithm for ad hoc wireless networks. Using the session admission control mechanism and the multicast routing algorithm, we evaluate the performance of three different linear multiuser detectors for the multicasting application.

Multicasting with multiuser detection in ad-hoc wireless networks

In this paper, we develop several multicast routing algorithms for wireless networks and compare their performance. Using a CDMA framework, we demonstrate how capacity results for multiuser detectors can be used to develop link power metrics for the routing problem.

Performance of Frequency Selective Scheduling and Fractional Frequency Reuse Schemes for WIMAX

WiMaX is the commercial name for the 4G OFDM based wireless broadband data transmission standard 802.16, developed by IEEE ([1] and [2]). It is currently being deployed in several countries around the world. Sub-channels (which are the basic unit of resource allocation) are formed by grouping together sub-carriers in one of two formats - PUSC or AMC. In PUSC the set of sub-carriers in a sub-channel are distributed throughout the bandwidth while in AMC the sub-carriers in a subchannel are contiguous within a portion of the bandwidth. In this paper we study two candidates that offer the potential to improve the system performance of PUSC and identify scenarios when this improvement is possible: (i) compare the performance of PUSC with two flavors of AMC; and (ii) study how the PUSC performance can be improved by a Fractional Frequency Reuse (FFR) scheme that tries to perform interference mitigation/co-ordination and improve the throughput of users at the edge of the cell.

Performance of OFDM based broadcast schemes for EV-DO systems

In this paper salient details of the Enhanced Broadcast Multicast (EBM) scheme (1) and the Harmonized Broadcast Multicast (HBM) scheme (2) that have been proposed in the 3GPP2 standards for supporting broadcast and multicast services in EV-DO (HRPD-A) systems are presented. Link and system simulation results for the two schemes are presented based on the system performance evaluation methodologies proposed in the 3GPP2 standards (3), (4) and the 3GPP standards (5). It is shown that for a data rate of 1.8 Mbps, at 1% PER, cell coverage of ~ 90% can be obtained using the 360 tone/symbol broadcast configuration.

Wireless multicasting using CDMA and multiuser detection

We consider the problem of multicasting in a fixed wireless network, that uses CDMA for channel access and multiuser detection techniques for decoding the signals. In order to provide a better coupling between physical layer and network layer issues, we develop link metrics from capacity expressions for multiuser detectors. Using the signal to interference plus noise ratio (SINR) as a QoS parameter, we do admission control by developing lower and upper bounds for the power level at which a signal can be received at a node. In this ongoing research, we develop efficient multicast routing algorithms for wireless networks.