Abstract
This paper investigates the capacity limit of an uplink WCDMA system considering a continuous-time waveform signal. Various realistic assumptions are incorporated into the problem, which make the study valuable for performance assessment of real cellular networks to identify potentials for performance improvements in practical receiver designs. An equivalent discrete-time channel model is derived based on sufficient statistics for optimal decoding of the transmitted messages. The capacity regions are then characterized using the equivalent channel considering both finite constellation and Gaussian distributed input signals. The capacity with sampling at the receiver is also provided to exemplify the performance loss due to a typical post-processing at the receiver. Moreover, we analyze the asymptotic capacity when the signal-to-noise ratio goes to infinity. The conditions to simultaneously achieve the individual capacities are derived, which reveal the impacts of signature waveform space, channel frequency selectivity and signal constellation on the system performance.
Highlights
Code division multiple access (CDMA) has become standard in several wireless communication systems from IS-95, UMTS wideband CDMA (WCDMA) to HSPA, and so on [1,2,3]
1.1 Contributions of this work Motivated by the fact that most existing research on multiuser CDMA capacity have focused on theoretical analysis with simplified system assumptions, in this work, we present a framework for capacity analysis of a WCDMA system with more realistic assumptions, which make the framework and results more valuable for the performance assessment of real cellular networks
2 Signal model Since the physical layer defines the fundamental capacity limit of the uplink WCDMA channel [2], we focus on a signal model reflecting the operations of the uplink
Summary
Code division multiple access (CDMA) has become standard in several wireless communication systems from IS-95, UMTS wideband CDMA (WCDMA) to HSPA, and so on [1,2,3]. The capacity analysis for a real cellular network with continuous-time waveform, timevariant-spreading, asynchronous CDMA is difficult due to the following reasons. − We derive sufficient statistics for decoding the transmitted symbols based on the continuous-time-received signal, which provides us an equivalent discrete-time signal model. − We analyze the asymptotic sum-capacity when the signal-to-noise ratio (SNR) goes to infinity, for which we derive the conditions on the signature waveform space so that on every link to the base station, the individual capacities are achieved simultaneously. A necessary condition to simultaneously achieve the individual capacities with a finite constellation input signal, which takes the signal constellation structure into account, is derived Those results are useful for spreading sequence design in a real WCDMA cellular network.
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