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首页> 外文期刊>Information Theory, IEEE Transactions on >Asynchronous CDMA Systems With Random Spreading—Part I: Fundamental Limits
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Asynchronous CDMA Systems With Random Spreading—Part I: Fundamental Limits

机译：具有随机扩频的异步CDMA系统-第一部分：基本限制

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摘要

Spectral efficiency for asynchronous code division multiple access (CDMA) with random spreading is calculated in the large system limit allowing for arbitrary chip waveforms and frequency-flat fading. Signal-to-interference and noise ratios (SINRs) for suboptimal receivers, such as the linear minimum mean square error (MMSE) detectors, are derived. The approach is general and optionally allows even for statistics obtained by undersampling the received signal. All performance measures are given as a function of the chip waveform and the delay distribution of the users in the large system limit. It turns out that synchronizing users on a chip level impairs performance for all chip waveforms with bandwidth greater than the Nyquist bandwidth, e.g., positive roll-off factors. For example, with the pulse shaping demanded in the UMTS standard, user synchronization reduces spectral efficiency up to 12% at 10 dB normalized signal-to-noise ratio. The benefits of asynchronism stem from the finding that the excess bandwidth of chip waveforms actually spans additional dimensions in signal space, if and only if the users are desynchronized at chip-level. The analysis of linear MMSE detectors shows that the limiting interference effects can be decoupled both in the user domain and in the frequency domain such that the concept of effective interference spectral density arises. This generalizes and refines Tse and Hanly's concept of effective interference. In Part II, the analysis is extended to any linear detector that admits a representation as multistage detector and guidelines for the design of low complexity multistage detectors with universal weights are provided.
机译：在较大的系统限制下计算出具有随机扩展的异步码分多址（CDMA）的频谱效率，从而允许任意码片波形和频率平坦衰落。得出了次优接收器（如线性最小均方误差（MMSE）检测器）的信号干扰和噪声比（SINR）。该方法是通用的，并且可选地甚至允许通过对接收信号进行欠采样获得的统计信息。给出的所有性能指标均取决于芯片波形以及在较大系统限制下用户的延迟分布。事实证明，在芯片级别上同步用户会损害带宽大于奈奎斯特带宽（例如正滚降因子）的所有芯片波形的性能。例如，通过UMTS标准中要求的脉冲整形，用户同步可在10 dB归一化信噪比下将频谱效率降低多达12％。异步的好处来自于这样的发现，即当且仅当用户在芯片级失同步时，芯片波形的额外带宽实际上才跨信号空间的其他维度。线性MMSE检测器的分析表明，可以在用户域和频域中将限制干扰效应解耦，从而产生了有效干扰频谱密度的概念。这概括和完善了谢和汉利的有效干预概念。在第二部分中，分析扩展到了任何可以表示为多级检测器的线性检测器，并提供了具有通用权重的低复杂度多级检测器设计指南。

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