ADAPTIVE ANTENNA AND BEAMFORMING SYSTEMS IN WIMAX

1 Beamforming

Beamforming takes advantage of interference to change the directionality of an antenna array system. A beamformer controls the amplitude and phase of the signal at each transmitting antenna element, to create a pattern of constructive interference (beamspots) and destructive interference (null) in the wavefront. To create a beamspot, the beamformer uses an array of closely spaced antennas, often enclosed in a single enclosure as illustrated in Figure 1. λ/2 antenna spacing between the antenna elements is commonly used (where λ is the wavelength of the transmitted signals, given by λ = c/f; f is the frequency of the transmitted signals, c is the speed of light). By varying the amplitude and phase of each antenna element, the beamformer is able to focus electromagnetic energy (beam) in the desired directions. The beams are directed to intended users, while nulls are focused on other unintended users reducing interference to the unintended users while increasing received SNR for the intended user. This provides a stronger link to the intended user and improves reach and capacity.

 
Figure 1: Beamforming technique.

2 Adaptive Antenna System

AAS is one of the advanced antenna technologies specified in the WiMAX standard to improve performance and coverage. In the AAS system, the transmitter (base station, BS) adaptively tracks a mobile receiver as it moves around the coverage area of the transmitter (BS), and steers the focus of the beam (beam spot) on the receiver unit as it moves. The beam steering method can either be mechanical or electronic. Thus AAS creates narrow beams to communicate with desired user device, which helps to reduce interferences to unintended user devices and improves carrier-to-interference (C/I) and frequency reuse, giving rise to high spectral efficiency. Thus, through the use of adaptive processing (beam steering), AAS improves performance and coverage of the system significantly. In WiMAX networks, AAS will find wide applications both in the point-to-multipoint (PMP) as well as mesh network deployments. In the PMP mode, AAS operates in similar way as in the current 3G cellular system, and is used for enhancing coverage and performance. In the mesh mode, AAS is used to form physical or directed mesh links. Physical or directed mesh is a form of mesh where substantially directional antennas are used to create physical links between neighboring devices. Mesh nodes adaptively steer antennas towards other nodes in their neighborhood and direct the focus of the electromagnetic radiations accordingly, to create the physical link with the intended neighboring device. One of the main drawback of AAS however is the high complexity involved in designing antenna systems capable of adaptively switching (steering) antenna directionality toward users who may be highly mobile. The use of AAS technology in mobile network deployment is therefore very challenging from a complexity perspective. Another drawback of AAS technology is that in an urban environment, with rich scatterer, the beams get blurred at the receiver and are not focused as expected, due to the reflections of waves as it propagates from the transmitter to the receiver. This effect is known as angle spread and it impacts significantly the performance of AAS in urban areas with cluttered structures. The gains achieved using AAS in such places thus reduce considerably from the theoretical expectations. For example, an AAS system using an eight-column array would have an ideal gain of 6.9 dB but angle spread would reduce this to only 3.2 dB in an urban environment and 4.7 dB in a suburban environment. There are some techniques however to mitigate the effects of angle spread. Active research works are ongoing in this area

ANTENNA TECHNOLOGIES IN WIMAX

Advanced antenna technologies specified in the WiMAX system to mitigate the non-LOS propagation problems and ensure high quality signal receptions include Diversity and multiple-input multiple-output (MIMO) systems, adaptive antenna systems (AAS), as well as beamforming systems.

1 Diversity Systems

Diversity technique provides the receiver with multiple copies of the transmitted signal, each of them received over independently fading wireless channel. The notion of diversity relies on the fact that with M independently fading replicas of the transmitted signals available at the receiver, the probability of an error detection is improved to p^M, where p is the probability that each signal will fade below a usable level. The link error probability is therefore improved without increasing the transmitted power. Recently, the use of diversity technique at the transmitter side also gained wide attentions, and has resulted in the consideration of the more general case of multiple transmit–multiple receiving antennas or MIMO systems.

2 MIMO Systems

The two options for MIMO transmissions in the WiMAX standard are space-time codes and multiplexing. For space-time codes, both space-time trellis codes and Alamouti space-time block codes are specified. However, it is the Alamouti space-time block codes that has yet been implemented by vendors due to its reduced complexity (eventhough space-time trellis code has better link performance improvements). In the Alamouti scheme designed for two transmitting antennas, a pair of symbol is transmitted at a time instant, and a transformed version of the symbols are transmitted in the next time instant. At the receiver, the decoder detects the four symbols transmitted over two time slots and processes them to obtain 2-branch diversity gain. Thus the Alamouti scheme achieves full diversity, with a rate-1 code. For the multiplexing option, the multiple antennas are used for capacity increase. In this option, original high-rate stream is partitioned into N low-rate substreams and each substream is transmitted in parallel over the same channel, using different antennas. If there are enough scatterers between the transmitter and the receiver, adequate MIMO detection algorithms like zero-forcing, minimum mean-square error (MMSE), or vertical Bell labs Layered Architecture for space-time codes (V-BLAST), etc., can be designed to separate the substreams. Thus the link capacity (theoretic upper-bound on the throughput) is increased linearly with min(N,M), where N is the number of transmit and M is the number of receiving antennas.

3 MIMO Systems with Antenna Selection

For MIMO systems to be deployed on mobile WiMAX devices, the concept of antenna selection is very essential. Because RF chain dominates the link budget in wireless systems, mobile devices are unable to implement large numbers of RF chains to incorporate high order MIMO systems. For such systems therefore, a reduced numbers of RF chains are implemented and antennas with the best received energies are adaptively selected and switched on to the implemented RF chains for MIMO signal processings, as illustrated in Figure 1. The performance of such system has been studied quite elaborately in the literature, in comparison to the full complexity system that utilizes all available antennas. It was shown that the diversity gain performance is maintained in the reduced-complexity system despite the use of antenna selection, while the coding gain deteriorates proportional to the ratio of the selected antennas to the total available antennas.

 
Figure 1: MIMO subset antenna selection.

4 MIMO Technologies in IEEE 802.16m Standard

The IEEE 802.16 standards committee has recently initiated the process of extending the existing IEEE 802.16e standard (mobile WiMAX) for high capacity, high-QoS mobile application. The new standard was dubbed IEEE 802.16m at the IEEE January session in London, 2008. The working group tasked with the responsibility of producing the working documents for the new standard was named task group m (TGm). The group hopes to complete the specification for the new standard by the end of 2009. When completed, the standard will be backward compatible with IEEE 802.16e, and interoperable with 4G cellular standards supporting the IMT-advanced technologies. Although the details of the IEEE 802.16m standard is not available at the moment, the most important features being touted for the standard include

• Target downstream speed of 100 Mbps in highly mobile mode, and upto 1 Gbps in normadic mode (upstream rate are not yet known, but would be at least at par with 802.16e).
• Channel sizes upto 40 MHz (802.16e currently supports upto 20 MHz channel size).
• Use of TDD and FDD.
• Backward-compatibility with 802.16e.
• OFDMA radio (same as in 802.16e).
• Mandatory MIMO antenna technology of size 4 × 4 (four transmitting, and four receiving antennas).

In contrast to the IEEE 802.16e, which supports mandatory MIMO antenna technology of size 2 × 2 (two transmitting, and two receiving antennas), the use of mandatory higher-capacity MIMO technology in 802.16m will provide extra capacity to support the targeted high-speed in the downstream. Since downstream has been the bottleneck in wireless services, this improvement will provide significant boost in system capacity, to enable the system support wide range of multimedia services expected in 4-G compatible technologies.