本文为新西兰坎特伯雷大学（作者：Wan Amirul Wan Mohd Mahyiddin）的博士论文，共147页。
由于大规模多输入多输出（MIMO）系统具有实现高频谱效率的潜力，近年来引起了人们的广泛兴趣。尽管具有很大的潜力，但也伴随着一些问题，如导频污染。当小区同时传输相同的导频序列而产生干扰时，就会发生导频污染。不同步的导频可以减少导频污染，但它产生的数据对导频是有干扰的。
本文研究了导频污染和其他干扰（即数据对导频干扰）对具有同步/非同步导频的有限大容量MIMO系统性能的影响。研究了两种非同步导频方案，第一种是基于现有的时移导频方案，其中导频与来自附近小区的下行数据重叠。第二种时移方法将导频与来自邻近小区的上行链路数据重叠。结果表明，在用户数量较少的情况下，第一种时间移位法具有最佳的和速率性能。但是，对于更多的用户，第二种时间移位方法提供了比其他方法更好的性能。
我们的研究还表明，在考虑阴影效应的情况下，时间同步导频不一定是最坏的和速率情况。由于移动用户设备（UE）和多径信道的多普勒效应，无线信道可以是时频变化的。这些变化可以通过使用选择性信道模型来模拟，模拟信道可以在相干数据块的时间域和频率域内发生变化。块衰落信道模型通过假设信道在一个相干块内保持特性不变，但在块之间独立变化来近似模拟。由于块衰落模型的简单性，在大规模MIMO研究中得到了广泛应用。
我们的研究比较了块衰落和时间选择性衰落信道模型对大规模MIMO系统的影响。为了实现这一点，我们推导了一种新的时间选择信道的和速率闭式表达。结果表明，这些模型在和速率性能上存在显著差异。除了多普勒效应的时间变化外，信道还可能由于多径信号传播的延迟而经历频率变化。时间和频率选择信道的组合可以描述为双选择信道。因此，时间选择信道的和速率表达式也可以扩展到双选择信道。我们研究了两种导频序列，即双选择信道中的等幅导频和零填充导频。结果表明，在天线数目和时频相关性较大的情况下，零填充导频比等幅导频具有更好的和速率性能。研究了两种不同类型的训练优化算法，即平均最优训练和自适应最优训练，两种方法显示出相似的和速率性能。此外，我们还研究了增加频率复用和导频复用因子的效果。尽管这些方法可以减少小区间干扰，但由于时频资源的利用效率低下，也会导致总的和速率降低。
Massive multiple-input multiple-output(MIMO) systems have been gaining interest recently due to their potential toachieve high spectral efficiency [1]. Despite their potential, they come withcertain issues such as pilot contamination. Pilot contamination occurs whencells simultaneously transmit the same pilot sequences, creating interference.Unsynchronizing the pilots can reduce pilot contamination, but it can producedata to pilot interference. This thesis investigates the impact of pilotcontamination and other interference, namely data to pilot interference, on theperformance of finite massive MIMO systems with synchronized and unsynchronizedpilots. Two unsynchronized pilot schemes are considered. The first is based onan existing time-shifted pilot scheme, where pilots overlap with downlink datafrom nearby cells. The second timeshifted method overlaps pilots with uplinkdata from nearby cells. Results show that if there are small numbers of users,the first time-shifted method provides the best sum rate performance. However,for higher numbers of users, the second time-shifted method provides betterperformance than the other methods. We also show that time-synchronized pilotsare not necessarily the worst case scenario in terms of sum rate performancewhen shadowing effects are considered. The wireless channel can be time andfrequency varying due to the Doppler effect from mobile user equipment (UE) anda multipath channel. These variations can be simulated by using a selectivechannel model, where the channel can vary within the coherence block in bothtime and frequency domains. The block fading channel model approximates thesevariations by assuming the channel stays constant within a coherence block, butchanges independently between blocks [2]. Due to its simplicity, the block fadingmodel is widely used in massive MIMO studies [3–8]. Our research compares theimpact of block fading and time-selective fading channel models in massive MIMOsystems. To achieve this, we derive a novel closed form sum rate expression fortime-selective channels. Results show that there are significant differences insum rate performance between these models. In addition to time variation fromDoppler effect, the channel can also experience frequency variation due todelay spread from multipath signal propagation. The combination of time andfrequency selective channels can be described as a doubly-selective channel.Hence, the sum rate expression for time-selective channels can also be extendedto doubly-selective channels. We investigate two types of pilot sequences,namely constant amplitude pilots and zero padded pilots in doubly-selectivechannels. Results show that a zero padded pilot has a better sum rateperformance than a constant amplitude pilot for a wide range of antenna numbersand time-frequency correlation values. Two different type of trainingoptimization, namely average optimum training and adaptive optimum training,are investigated. Both methods shows similar sum rate performance. In addition,we also study the effect of increasing frequency reuse and the pilot reusefactor. Even though these methods can reduce intercell interference, they alsoresult to lower sum rate due to inefficient use of time-frequency resources.
