5.3.2 Multiple Antenna Systems
Multiple antenna systems have been extensively used due to the advantages of diversity, especially of the spatial kind. Currently, demand for high-data-rate wireless communications is still growing rapidly. In the past technologies with the greatest media hype promised more value to the end user, but so far success has not been attained. Limiting factors in wireless environments are the lack of wide network coverage, vehicular mobility, and inadequate spectral resources.
For many years now, multiple antenna techniques - especially multiple-input, multiple-output (MIMO) technology - have gained much research attention. MIMO is very promising for technologies such as 4G and 802.11n wireless LANs, and it will probably be deployed in environments where several users require high downlink data rates at single base stations. We provide an introduction to the topic by developing discussion from the traditional single-input singleoutput case to the MIMO case. In this section, we provide a brief summary of MIMO systems.
For wireless services demand will likely continue to grow due to multimedia services for mobile terminals such as live video streaming, video conferencing, mobile games, and mobile television. These services not only require high data rates from wireless networks but also support implementation of QoS schemes.
Development of higher and higher data rates has been gradual. The 2.5G technologies included high-speed circuit-switched data (HSCSD) and general packet radio service (GPRS) , of which the former is able to provide 57.6 Kbps in good radio conditions using four time slots, and the latter is, depending on the channel coding, capable of rates up to 114 Kbps. However, the GPRS has been offering peak data rates of only 53.6 Kbps (4 times 13.4 Kbps) in commercial networks, and overall performance is even lower due to packet retransmissions and connection establishment (i.e., reserving time slots from the network).
After introducing 2.5G technology and while standardization work for 3G systems such as the Universal Mobile Telecommunication System (UMTS) was going on, a bolt-on enhancement for GSM systems called enhanced data rates for GSM evolution (EDGE) was introduced by 3GPP—also responsible for GSM and GPRS. The primary target group for EDGE is the operators, who lost the competition of large frequency bands for 3Gtechnology. EDGE is able to provide 236.8 Kbps with four time slots in perfect radio conditions.
As deployment began, the 3G technologies became available in an increasing number of countries. For example, in Mexico the CDMA-2000 standard has been deployed providing services such as TV. During the auction of frequency bands in Europe, there was enormous media hype about the 3G/UMTS technology with promises of video conferencing and other applications. However, as the implementations lack QoS features and the pricing of data transfer is still high, the popularity of 3G/UMTS is growing at a much slower pace than expected, and there is still a great void to be filled with mobile services using the faster transfer rates of 3G (up to 384 Kbps in commercial WANs).
Additionally, maximum goodput is achievable only with contents, which require longer download times and somewhat perfect signaling conditions. To increase data rates even more, 3GPP has defined a technology called high-speed downlink packet access (HSDPA) in its UMTS/WCDMA standard. It promises data rates exceeding 10 Mbps .
With this short historical review, it is clear that there is still a great need for faster transmission speeds inWANs and WLANs. Despite huge effort, video conferencing and mobile TV still require faster Internet access for the end user. MIMO technology is a very promising answer to this demand. It is often suggested as the basis not only of future 4G technology in WANs, but also of the new WLAN technology with over 100-Mbps Internet access—referred to as the 802.11n standard.
The rest of this section discusses the wireless channel in general with the single-input single-output (SISO) case, and step by step we develop the discussion toward the MIMO case. The complete summary is in Perala and Vargas .