Martin V. Clark

Adaptive frequency-domain equalization and diversity combining for broadband wireless communications

We introduce a new kind of adaptive equalizer that operates in the spatial-frequency domain and uses either least mean square (LMS) or recursive least squares (RLS) adaptive processing. We simulate the equalizers performance in an 8-Mb/s quaternary phase-shift keying (QPSK) link over a frequency-selective Rayleigh fading multipath channel with /spl sim/3 /spl mu/s RMS delay spread, corresponding to 60 symbols of dispersion. With the RLS algorithm and two diversity branches, our results show rapid convergence and channel tracking for a range of mobile speeds (up to /spl sim/100 mi/h). With a mobile speed of 40 mi/h, for example, the equalizer achieves an average bit error rate (BER) of 10/sup -4/ at a signal-to-noise ratio (SNR) of 15 dB, falling short of optimum linear receiver performance by about 4 dB. Moreover, it requires only /spl sim/50 complex operations per detected bit, i.e., /spl sim/400 M operations per second, which is close to achievable with state-of-the-art digital signal processing technology. An equivalent time-domain equalizer, if it converged at all, would require orders-of-magnitude more processing.

Theoretical reliability of MMSE linear diversity combining in Rayleigh-fading additive interference channels

We derive an exact closed-form solution for the reliability of an ideal M-branch MMSE (minimum mean-squared error) diversity combiner operating in a Rayleigh-fading channel with N interferers, each having some specified average power. The reliability is defined as the probability, taken over fading of the desired and interfering signals, that the combiners output signal-to-interference ratio (SINR) is greater than some specified threshold. This kind of metric is important in evaluating the potential capacity improvements of using diversity combining and adaptive array processing in interference-limited wireless systems. Our result is remarkably simple, fast, straightforward to compute, and numerically stable. We show a set of special cases, which relate to standard results and reveal valuable insights into how this type of array processing operates in interference-limited environments. We also present a set of numerical examples, which show that our calculated reliabilities agree with estimates from Monte Carlo simulation.

Distributed versus centralized antenna arrays in broadband wireless networks

This paper studies the potential benefits of antenna array processing-used in conjunction with adaptive data-rate control-in broadband wireless networks. We focus on distributed antenna arrays, i.e., combining signals from a group of microcells, rather than the more conventional centralized (macrocellular) antenna, array processing. We show that distributed arrays promise significant power and capacity gains over centralized arrays. Moreover, we show that even selection combining (though less effective than coherent combining) can be very successful in this architecture, offering a promising tradeoff between performance and complexity.

Outdoor IEEE 802.11 cellular networks: radio link performance

We explore the feasibility of designing an outdoor cellular network based on the IEEE 802.11 standard, which was developed originally for wireless local-area networks. For channels typical in cellular networks, we study the radio link power budget and, via simulation, the bit-error performance of three kinds of receiver: (1) the constrained RAKE, which is limited to a 1 /spl mu/s multipath span; (2) the full RAKE, which uses the full multipath channel information; (3) the ideal equalizer, the performance of which is represented by the matched filter bound. Our link budget reveals that the maximum cell radius in an outdoor 802.11 network ranges from 0.7 to 3 km, about half that supported by WCDMA and EDGE networks. For an RMS delay spread of 1 /spl mu/s, typical for urban-area cells of this size, our simulation results show that the conventional constrained RAKE receiver may yield a satisfactory performance. The improved receivers, however, yield 1-5 dB gain over the constrained implementation. Combining these results with those in a companion paper on the MAC protocol (see Leung, K.K. et al., ibid., p.595-599), we conclude that an 802.11 based cellular network with a cell radius of a few km is feasible.

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.

Design of a wireless backhaul network for microcells

The use of microcells is an attractive option for providing high data rate access for fixed and low mobility users. We propose and analyze a wireless backhaul network for connecting microcells to the wireline network. In the proposed network, several microcellular radio ports are connected to a so called hub through radio links. These hubs are then connected to the wireline network using cable or fiber. For ease of deployment, we propose that omnidirectional antennas be used at the radio ports and sectored antennas be used at the hubs. Our results show that for 95% of the links, a link reliability of 99% may be achieved with an effective frequency reuse of 3 by using site-selection, flexible channel assignment, power control and two-branch minimum mean square error combining. For the remaining 5% of the more vulnerable links, a direct wireline connection may be provided. We also find that power control plays a critical role in reducing the variation of the link performance over time. In addition, the best performance is achieved by jointly optimizing the receiver, i.e., the diversity combining weights, and the transmit power settings. We show that this joint optimization may be implemented in a distributed manner, making it attractive for use in real systems.

Performance bounds for MMSE linear macrodiversity combining in Rayleigh fading, additive interference channels

The theoretical performance of MMSE linear microdi-versity combining in Rayleigh fading, additive interference channels has already been derived exactly in the literature. In the macrodiversity case the fundamental difference is that any given source may well have different average received powers at the different antennas. This makes an exact analysis more difficult and hence for the macrodiversity case we derive a bound on the mean BER and a semi-analytic upper bound on outage probabilities. Hence we provide bounds on the performance of MMSE linear mi-crodiversity combining in Rayleigh fading with additive noise and any number of interferers with arbitrary powers.

The throughput of adaptive spread spectrum communication over multipath dispersive channels

We use analysis and simulation to investigate the downlink data rate of a wideband CDMA system, wherein the rate is adaptively varied based on the condition of the multipath fading channel. We use a previously published set of formulas for estimating output signal-to-distortion ratio which, in turn, permits us to compute the instantaneous allowed data rate per user for any fading channel response for three adaptive receivers. In addition, we use a previously published statistical model for path gain and delay spread to simulate a random population of downlink user terminals, including an ensemble of instantaneous channel responses for each. The result for each kind of receiver is a probability distribution, taken over the environment, of the multipath-averaged downlink data rate. We call the average of each such distribution the cell-averaged data rate (CADR), which we quantify for a variety of conditions (channel parameters, receiver type, cellular architecture). Our results demonstrate the superiority of the adaptive minimum mean square error (MMSE) receiver over the matched filter (MF) receiver, in terms of both data rate and robustness to the multipath parameters (e.g., median delay spread, delay profile shape). They also demonstrate the benefits against multipath fading of cell site diversity and the reductions in throughput due to multicell interference.

The relationship between fading and bandwidth for multipath channels

A signal transmitted over a multipath channel experiences fading, the variability of which is a function of: 1) signal bandwidth; and 2) power delay profiles of the channels specular and diffuse components. We analyze this general relationship and, for several important classes of multipath channel, we derive simple, closed-form approximations for what we call the stability bandwidth, W/sub 0/. For signal bandwidths greater than W/sub 0/, the local area variation in the received signal power is acceptably small, e.g., less than 1 dB standard deviation of decibel power. We demonstrate the accuracy of our W/sub 0/ approximations for some representative cases. Moreover, we show that the root mean square delay spread-a statistic commonly used to characterize multipath channels-has limited utility in estimating W/sub 0/.