Kamyar Rohani

A comparison of base station transmit diversity methods for third generation cellular standards

Code division multiple access (CDMA) is a common access scheme among all major proposals for third generation cellular standards. The objective of this paper is to show the performance benefits of open loop (which do not adapt transmission based on the channel to the mobile) and closed loop (which do adapt based on the channel to the mobile) transmit diversity methods for CDMA. We examine orthogonal transmit diversity (OTD) (an open loop method), and transmit adaptive arrays (TXAA) (a closed loop method).

Performance analysis of CDMA transmit diversity methods

The link performance of transmit diversity (TD) schemes for third generation CDMA systems is analyzed. Orthogonal transmit diversity (OTD) and space-time transmit diversity (STTD) are compared. It is shown that the link performances of OTD and STTD are nearly the same. However, STTD requires additional receiver processing, and STTD is not directly extendable to more than 2 antennas, because rate one space-time block codes do not exist for greater than 2 antennas. Also, an interleaving approach is shown that works for various TD schemes as well as non-TD.

Analysis of CDMA range extension due to soft handoff

We provide an analysis of CDMA range extension due to soft handoff. A simulation of GSM hard handoff and CDMA soft handoff is presented. In these simulations, a collection of 90 second calls are tracked through the system and the margin required to meet a constant call reliability is determined. A comparison of these margins for both GSM and CDMA indicate that, under a variety of conditions, the shadow fade margin required by the GSM system is about 2.6-3.6 dB higher than that needed by the CDMA soft handoff.

On application of multiple-input multiple-output antennas to CDMA cellular systems

Our focus in this paper is on applying multiple-input multiple-output (MIMO) antenna systems to the time division multiplexed (TDM) shared channel on the WCDMA forward link. We describe cellular system-level simulations with and without MIMO. These simulations include multipath channel models and they account for other-cell interference. We compute the SINR at the output of a linear MMSE receiver and map these values to throughput using simulated turbo code results. Simulation results are presented with various numbers of antennas at the transmitter and receiver. We show that with two transmit and two receive antennas, system capacity can be doubled relative to one transmit and one receive antenna. Two transmit and four receive antennas can provide three times the capacity of a one transmit and one receive antenna system. Also, results are presented that compare three sector systems to six sector systems.

Neural subnet design by direct polynomial mapping

In a recent paper by M. Chen and M. Maury (1990), it was shown that multilayer perceptron neural networks can be used to form products of any number of inputs, thereby constructively proving universal approximation. This result is extended, and a method for the analysis and synthesis of single-input, single-output neural subnetworks is described. Given training samples of a function to be approximated, a feedforward neural network is designed which implements a polynomial approximation of the function with arbitrary accuracy. For comparison, example subnets are designed by classical backpropagation training and by mapping. The examples illustrate that the mapped subnets avoid local minima which backpropagation-trained subnets get trapped in and that the mapping approach is much faster.

Application of MIMO and proportional fair scheduling to CDMA downlink packet data channels

There are many capacity enhancement techniques in wireless networks. Multiple-input-multiple-output (MIMO) antenna systems, channel-state based packet data scheduling algorithms and adaptive modulation and coding are examples of such techniques. In this paper, we provide the simulation results for various transmission schemes (including MIMO) in conjunction with a channel-state-based scheduling algorithm and an adaptive modulation and coding scheme Consequently, we demonstrate the cellular system capacity gain resulting from interaction between these three techniques. Specifically, system-level simulations are presented for the wideband CDMA (WCDMA) downlink shared channel.

CDMA receiver performance for multiple-input multiple-output antenna systems

We consider the performance of a forward link (base-to-mobile) shared channel with and without a multiple-input multiple-output (MIMO) antenna system. A CDMA shared channel is analyzed and simulated for which users are served in a time division multiplexed (TDM) manner. The slot structure and chip rate are based on the wideband CDMA standard. Link-level simulations are presented to compare the performances of two different receivers: linear minimum mean-square-error (MMSE), and nonlinear (turbo/iterative) MMSE. In addition, MIMO performance is compared with and without antenna array weight feedback.

QoS and fairness for CDMA packet data

Motivated by higher bandwidths and the need to handle a range of packet data services, several enhancements over the IS 95A/B packet interface have been proposed for third generation (3G) CDMA systems: enhanced physical layer to provide data rates greater than 64 kbps, enhanced MAC layer to support concurrent operation of any combination of voice, circuit and packet data, each with varying QoS and enhanced signaling to enable efficient resource management. The objective of this paper is to introduce concepts from queueing theory that address radio resource management issues for supporting packet data services over 3G CDMA systems.

The design of multi-layer perceptrons using building blocks

A building-block approach for constructing large backpropagation (BP) neural networks is described. This results in considerably less training time than conventional BP, which starts from random initial weights. Unlike previous approaches, this approach involves the mapping of conventional algorithms onto neural network structures. This has several benefits. First, it produces alternative parallel structures for implementation of conventional signal processing algorithms. Second, it produces a good set of initial weights for BP training. An example is given in which a randomized initial weight network fails to learn, but the assembled network succeeds.<<ETX>>

Impact of imperfect estimators on W-CDMA receiver performance with MIMO antenna systems

Multiple-input multiple-output (MIMO) antenna systems have been proposed to enhance the capabilities of the high-speed downlink-shared channel (HS-DSCH) of WCDMA. To date, link simulations have been used to demonstrate the performance of this system. However, these studies have generally relied upon ideal solutions for determining the channel impulse response, minimum mean-squared-error (MMSE) filter coefficients, and SINR. In this paper, we present practical solutions for estimating the channel, MMSE filter coefficients, and SINR, and then compare the link level performance using ideal solutions to that with estimated solutions.