WiMAX is designed with QoS in mind and it has some underlying technological strengths that help it offer improved QoS. Some of these strengths are outlined in this section.
1 WIMAX PHY LAYER
In WiMAX, the upstream PHY layer consists of time division multiple access (TDMA) and demand assigned multiple access (DAMA). For TDMA, the channel for upstream communication is divided into multiple time slots and the access of time slots for various clients is governed by the MAC layer at the receiver end. The time slots allocated for various clients can be varied depending on demands. The downstream traffic can be continuous time division multiplexing (TDM) or burst mode transfer. In continuous TDM, data for various clients is multiplexed onto the same stream and is received by all clients at the same coverage sector. For bursty data, bursts are sent to the receiver in a similar fashion to the TDMA upstream burst. With time slots-based communication, overheads due to contentions and collisions can be reduced significantly, which can improve the QoS.
The modulation used in WiMAX is the orthogonal frequency division multiplexing (OFDM). WiMAX OFDM features multiple subcarriers ranging from a minimum of 256 up to 2048, each modulated with either BPSK, QPSK, 16 QAM, or 64 QAM modulation. The advantage of orthogonality is that it minimizes self-interference, a major source of error in received signals in wireless communications. WiMAX supports different signal bandwidths ranging from 1.25 to 20 MHz to facilitate transmission over longer ranges in different multipath environments. Multipath signals, another limiting factor for higher sustained throughput in wireless communications, specially when the terminal nodes have the mobility, are caused by reflections between a transmitter and receiver whereby the reflections arrive at the receiver at different times.
Interference caused by multipath tends to be highly problematic when the delay spread, the time span separating the reflection, is on the order of the transmitted symbol time. For WiMAX, due to its OFDMA, symbol times tend to be in the order of 100 µs, which makes multipath less of a problem. Moreover, in WiMAX, a guardband of about 10 µs, called the cyclic prefix, is inserted after each symbol to mitigate the effect of multipath. Another feature of WiMAX PHY is the use of advanced multiantenna signal processing techniques, mainly in the form of multiple input multiple output (MIMO) processing and beamforming. For MIMO, the received signal from one transmitting antenna can be quite different to the received signal from a second antenna, a common scenario in indoor or dense metropolitan areas where there are many reflections and multipaths between the transmitter and the receiver. In such cases, a different signal can be transmitted from each antenna at the same frequency and still be recovered at the receiver by signal processing.
Beamforming, on the other hand, attempts to form a coherent construction of the multiple transmitters at the receiver, which can ultimately offer a higher SNR at the receiver resulting in higher bandwidth or longer range communication. In WiMAX, it is also possible to combine both MIMO and beamforming in cases like 4-antenna systems.
All these features in the WiMAX PHY layer contribute to higher throughput and stability at the receiver end, which makes WiMAX an excellent platform to deliver a predefined level of QoS. With improved throughput and stability, management of QoS is considerably easier in WiMAX compared to other similar wireless standards. Increased throughput, however, does not ensure guaranteed QoS, and bandwidth management is another crucial part that plays a big role for maintaining QoS. This is where WiMAX MAC comes into action.
2 WIMAX MAC
WiMAX MAC is designed for the point-to-multipoint wireless communication with the capability to support higher-layer protocols including ATM, IP, and other future protocols. One of the design considerations of WiMAX MAC is to accommodate very high bit rates of the broadband PHY layer, while delivering ATM-compatible QoS at the same time. A connection oriented MAC architecture in WiMAX provides a platform for strong QoS control. MAC uses a scheduling algorithm that enables the subscriber station (SS) to only compete once for initial entry into the network and upon successful entry, the SS is allocated a time slot by the BS. The time slot can increase or decrease according to the needs and it remains assigned to the SS for the whole communication period. The time slot assigned to an SS cannot be used by other subscribers, which makes WiMAX MAC increasingly stable under overload and over-subscriptions. It also works as a key tool for the BS to control QoS by adjusting the time-slot assignments according to the applications’ needs of the SSs.
