分布式天线系统中IEEE802.11n MIMO接入点性能实验研究

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"分布式天线系统中商用多输入多输出接入点的性能分析" 这篇研究论文探讨了在分布式天线系统(Distributed Antenna System, DAS)中，采用商业化的多输入多输出(Multiple-Input Multiple-Output, MIMO)技术的接入点(Access Point, AP)的性能表现。研究人员通过实验方法分析了IEEE 802.11n标准下的2x2 MIMO信号在基于光纤的DAS中的吞吐量，并考虑了不同光纤长度和功率不平衡的影响。 MIMO技术是现代无线通信系统中的关键组成部分，它通过使用多个天线来提高数据传输速率和系统可靠性。在分布式天线系统中，MIMO技术能够增强覆盖范围，特别是在室内环境如办公室等地方，能够有效地克服多径衰落和阴影效应。实验中，研究人员对比了两种类型的AP：支持MIMO的AP和采用空间分集(spatial diversity)的AP。他们分别在不同的无线用户位置进行吞吐量测量，以模拟实际应用环境。对于不同光纤长度的影响，研究结果显示，即使在光纤长度差异较大的情况下，MIMO信号仍然可以保持较高的吞吐量。这意味着MIMO技术具有一定的抗光纤延迟和失真能力。此外，论文还关注了功率不平衡问题，这是实际部署中常见的挑战。功率不平衡可能导致各个天线接收信号强度不一致，从而影响MIMO系统的性能。实验数据表明，尽管存在功率不平衡，MIMO系统的吞吐量仍能相对稳定，这进一步证明了MIMO技术的鲁棒性。该研究为理解并优化分布式天线系统中MIMO接入点的性能提供了有价值的数据和见解。其结果对无线网络设计者和运营商具有重要意义，有助于他们在设计和部署DAS时更好地利用MIMO技术，提升室内无线网络的服务质量和用户体验。同时，这项工作也为未来更深入的研究，如多用户MIMO (MU-MIMO) 和更高阶的MIMO配置在DAS中的应用奠定了基础。

propagation distance is much shorter and radio signals are presumed to experience better

channel quality since there is lower path loss, penetration loss and shadowing, which can

significantly improve the system capacity and coverage. Furthermore, upgrading and re-

configuration can be more easily performed in this centralized architecture. These features

make RoF-based DAS attractive in achieving a cost-effective, scalable and flexible network.

With the shared RoF-based DAS infrastructure, it is preferred that WLAN signals are

distributed in the same buildings and public venues as an important complement to telephony

signals [8]. IEEE 802.11 is a series of standards for implementing WLAN products [9]. In the

last five years, almost every newly-shipped WLAN product supports the IEEE 802.11n

standard. More recently, we have seen a dramatic increase in the shipment of WLAN products

compliant with IEEE 802.11ac. One distinct characteristic of IEEE 802.11n/ac standards is

the use of multiple-input-multiple-output (MIMO) technique. Moreover, the latest mobile

telephony standards also include the use of MIMO techniques. The integration of MIMO and

RoF in a DAS architecture where any antenna port in CU could connect to any RAU, can

improve the system capacity and extend the wireless coverage by increasing the received RF

power and the antenna separation [10–13]. Therefore, the spatial streams could be flexibly

allocated to any adjacent RAUs depending on the bandwidth demands, as seen in Fig. 1. One

major issue with DAS when distributing WLAN signals over a fiber system [14] is the effect

of different fiber lengths on the received OFDM signal. This was shown in a simulcast DAS,

when a single AP (BS) feeds multiple RAUs. A similar situation should occur in Distributed-

MIMO antenna systems (D-MIMO), where multiple transmitters and receivers are employed

in the system. The CU or the clients need to align the OFDM signals and combine the

different spatial streams before demodulation. For the design of a practical, re-configurable

in-building network, it would be necessary to ensure equivalent fiber lengths for every link,

which may prove difficult considering the reuse of the large installed base of fiber links.

Fig. 1. RoF-based Distributed Antenna System.

Some parameters are helpful in discussing the issue above. OFDM in both IEEE 802.11g

and 802.11n has a symbol duration of 4 µs. The cyclic prefix (CP) as the guard interval is 0.8

µs for 802.11g and 0.4 µs for 802.11n in default mode. The CP helps the receiver to recognize

and align the signals. In an ideal wireless environment, it theoretically allows a path

difference up to 400 feet (122m) for 802.11n [15], but one might expect the practical limit to

be less than this. In a mixed optical-wireless channel, as the speed of light in vacuum is higher

than in the fiber, the ideal maximum path-length difference will be less than 400 feet. We first

identified and preliminarily analyzed the fiber-length different effect in WLAN-over-fiber

DAS in [16]. In this paper, we also analyzed the received power imbalance problem to

reinforce the conclusion made easier, and correspond to the fiber-length results.

Another critical feature of MIMO systems is the spatial independence of the wireless

channels, which generally requires an antenna separation of at least ¼ the radio signal

wavelength [17]. With optical feeding of the antennas, significantly larger antenna spacing

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Received 11 Dec 2014; revised 23 Feb 2015; accepted 2 Mar 2015; published 13 Mar 2015

9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.007500 | OPTICS EXPRESS 7502

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