Smart Antenna Technology for Mobile WiMAX

Smart antenna systems refer to a class of antenna technologies designed to improve the received signal strength in a wireless access network. The intent is to improve the carrier to-interference plus noise ratio (CINR). The use of ‘smart’ antenna technologies can both increase received signal strength and decrease interference levels to greatly enhance performance in a mobile communication network

Smart antenna technologies involve complex vector or matrix operations on signals due to the use of multiple antennas. OFDMA allows smart antenna operations to be performed on vector-flat sub-carriers. Mobile WiMAX supports a full range of smart antenna technologies to enhance system performance. The smart antenna technologies supported are:

• Space-Time Code (STC)
• Beam forming
• Spatial Multiplexing (SM)

1 Space-Time Coding

Space-time coding (STC) is a family of techniques for implementing transmission diversity. Mobile WiMAX uses transmit diversity in the downlink direction to provide spatial diversity that enhance the signal quality to a specific subscriber located anywhere within the range of the antenna beam. Although providing less signal gain than beam-forming, transmit diversity is more robust for mobile users since it does not require prior knowledge of the path characteristics of a subscriber’s particular frequency channel. One such STC technique, known as the Alamouti Code, was published in 1998 and has been incorporated in the WiMAX standard

2 Beam-Forming

The transmission of signals from several antennas at specific relative phases can be used to create a much narrower antenna beam giving rise to the name ‘beam-forming’. Beam-forming provides substantial improvement in the link budget in both the downlink and uplink directions by increasing the effective antenna gain in addition to reducing fade margin requirements due to interference. Beam-forming does require knowledge of a subscriber’s location making it more challenging to implement for subscribers moving at high speeds. According to cellular network statistics however, the majority of subscribers are either stationary or only moving at pedestrian speeds thus enabling beam-forming to provide significant benefits for most usage models.

3 Spatial Multiplexing (SM)

Spatial multiplexing [30, 31] is supported to take advantage of higher peak rates and increased throughput. With spatial multiplexing, multiple streams are transmitted over multiple antennas. If the receiver also has multiple antennas, it can separate the different streams to achieve higher throughput compared to single antenna systems. With 2x2 Multiple Input Multiple Output (MIMO), SM doubles the peak date rate by transmitting two data streams. In up-load direction, each user has only one transmit antenna, two users can transmit in the same slot as if two streams are spatially multiplexed from two antennas of the same user.