Zoran Kostic

Virtual path analysis of selective RAKE receiver in dense multipath channels

We develop an analytical framework to quantify the effects of spreading bandwidth on spread-spectrum systems operating in dense multipath environments. Closed-form expressions for the mean and variance of the total RAKE receiver output signal-to-noise ratio are derived. The proposed problem is made analytically tractable by transforming the physical RAKE paths into the virtual path domain.

Fundamentals of dynamic frequency hopping in cellular systems

We examine techniques for increasing spectral efficiency of cellular systems by using slow frequency hopping (FH) with dynamic frequency-hop (DFH) pattern adaptation. We first present analytical results illustrating the improvements in frequency outage probabilities obtained by DFH in comparison with random frequency hopping (RFH). Next, we show simulation results comparing the performance of various DFH and RFH techniques. System performance is expressed by cumulative distribution functions of codeword error rates. Systems that we study incorporate channel coding, interleaving, antenna diversity, and power control. Analysis and simulations consider the effects of path loss, shadowing, Rayleigh fading, cochannel interference, coherence bandwidth, voice activity, and occupancy. The results indicate that systems using DFH can support substantially more users than systems using RFH.

Impact of spreading bandwidth on RAKE reception in dense multipath channels

Spread spectrum (SS) multiple access techniques have been proposed for third generation broadband wireless access. We develop an analytical framework to quantify the effects of spreading bandwidth on SS systems operating in dense multipath environments in terms of the receiver performance, receiver complexity, and multipath channel parameters. In particular, we consider wide-sense stationary uncorrelated scattering (WSSUS) Gaussian channels with frequency-selective fading. The focus of the paper is to characterize the combined signal of the RAKE receiver fingers tracking the strongest multipath components. Closed form expressions for the mean and the variance of the total RAKE receiver output signal-to-noise ratio (SNR) are derived in terms of the number of RAKE fingers, spreading bandwidth, and multipath spread of the channel. The proposed problem is made analytically tractable by transforming the physical RAKE paths into the virtual path domain. A representative result indicates that for SS systems with 5 MHz signal bandwidth operating in a channel with constant power delay profile having 5 /spl mu/s spread, the average SNR gain from increasing the number of RAKE fingers from one to three is 3.8 dB and from three to five is 1.5 dB. Furthermore, the reduction in the variation of SNR is 1.1 dB and 0.4 dB for the same increments in the number of fingers.

Outdoor IEEE 802.11 Cellular Networks: Radio and MAC Design and Their Performance

This paper explores the feasibility of designing an outdoor cellular network based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, which was originally developed for wireless local area networks. Specifically, the performance of the 802.11 radio and medium access control (MAC) protocol in outdoor environments is investigated. For channels typical in cellular networks, we study the radio link power budget and the bit error performance of three kinds of receivers. We also propose a new timing structure for the MAC protocol to handle increased signal propagation delay and analyze its throughput performance in the outdoor network. Our analysis shows that the MAC protocol can handle a cell radius of 6 km without violating the 802.11 standard. However, the link budget reveals that the maximum cell radius in an outdoor 802.11 network ranges from 0.4 to 1.2 km, which is about one third that supported by wideband code-division multiple access and enhanced data rates for global system for mobile communications evolution networks. For a root-mean-square delay spread of 1 mus, which is typical for urban-area cells of this size, our simulation results show that the conventional urban-area cells RAKE receiver can yield a satisfactory performance. Combining these results, we conclude that using ordinary equipment, an 802.11-based cellular network with a cell radius up to 1.2 km is feasible. It is possible to further extend the service range by advanced techniques such as smart antennas.

Some performance results for the downlink shared channel in WCDMA

We study the data performance of WCDMA systems using the downlink shared channel (DSCH) by investigating the impact of loading, rate adaptation and power control for the typical urban (TU) channel. We conclude that, using DSCH, high speed data transmissions can be achieved if the offered traffic load is well controlled. We discuss a heuristic rate adaptation algorithm that adapts the transmission rate based upon the perceived user performance in previous frames. We show that the performance of the rate adaptation is significantly better than that for the fixed-rate allocation policy.

Interaction between fast scheduling diversity and RAKE receivers

Recent results indicate that multiuser diversity can provide significant improvements in total system capacity in fading multiuser channels. This paper investigates the simultaneous presence of link diversity and multiuser diversity and analyzes their mutual interaction. The results show that two types of diversity affect each other considerably. In particular, the use of link diversity reduces the capacity gains possible from multiuser diversity. We illustrate this by using selective RAKE receivers (with limited number of fingers) as an example of link diversity. We then present the performance trade-offs by showing the statistics of achieved SNR at the receiver as a function of the number of fingers, number of users, and number of available multipaths in the channel.