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Use MIMO and DAS to optimise mobile networks

Posted: 15 Nov 2012     Print Version  Bookmark and Share

Keywords:Multiple input/multiple output  Distributed Antenna System  4G 

With the expansion of wireless usage due to smart phones, tablet computers and a growing applications developer base, mobile operators are constantly searching for ways to deliver more capacity in their networks without using precious spectrum resources.

Multiple input/multiple output (MIMO) technology is now gaining substantial momentum in wide area mobile wireless network architectures with the launch of LTE services. MIMO is a key technology that substantially improves network capacity and data throughput in LTE networks while conserving spectrum resources.

MIMO stands for "multi-input, multi-output." It made its first broad commercial appearance in 802.11n systems. Input refers to the number of transmitter antennas and output refers to the number of receiver antennas (figure 1). SISO mode (single input, single output) is the classical mode of communication architecture, where there is one antenna transmitting and one antenna receiving.

Figure 1: SISO and MIMO modes.

MIMO uses multiple transmit antennas and multiple receive antennas. Multi-antenna configurations have been around for years, but with advances in signal processing and silicon, MIMO is now economically possible in many small form factor devices such as handsets and data cards. All practical LTE devices support MIMO, as required by the 3GPP standard. While initial LTE networks use downlink 2x2 MIMO (where there are two transmit antennas and two receive antennas), future LTE systems will use downlink 4x2 or even 4x4 MIMO and even higher dimensions of antenna configurations. LTE Advanced systems will have the nominal ability to use up to 8x8 MIMO downlink antenna configurations and up to 4x4 MIMO in the uplink, although actual device implementations supporting these modes may take some years to come to market.

For MIMO to work, a rich scattering environment (with many different paths between transmitter and receiver) as well as a high signal-to-noise ratio (SNR) are needed. Rather than being a detriment to network performance, a multi-path environment is actually exploited by MIMO processing to increase the capacity or the coverage of the network. The key is that each path must be independent and look different to the receiver. The differences in the multi-path are used to create orthogonal communication channels analogous to the orthogonal spreading codes in CDMA-based systems. In addition to being required for the higher orders of modulation, such as 16-QAM and 64-QAM, a high signal-to-noise ratio is also required to properly exploit the MIMO wireless channel, and to allow MIMO systems to algorithmically separate the multiple spatial transmission paths, which overlap one another in frequency and time.

An in-building Distributed Antenna System (DAS) is ideal for MIMO because it provides very good SNR, and in-building environments provide a rich scattering environment.

MIMO types
There are two major categories of MIMO – spatial diversity, in which the same data is transmitted over each of the multiple paths, and spatial multiplexing, in which each of the paths carries different data. In 2x2 MIMO with spatial diversity, for example, each of the two antennas is essentially transmitting and receiving the same data although the data is coded differently and each channel is separable. This mode is primarily used to improve signal quality or to increase the coverage area. In 2x2 MIMO with spatial multiplexing, a different stream of data is transmitted over each of the two sub-channels, and this can increase throughput by a factor of up to 2x over the single sub-channel throughput as shown in figure 2, depending on the SNR of each sub-stream, and the base station rate adaptation procedure.

Figure 2: Spatial diversity versus spatial multiplexing.

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