Quality of service (QoS) provisioning is an important issue in the deployment of broadband wireless access networks e.g. WiMAX (IEEE Std 802.16-2004, 2004) networks with real-time and non-real-time traffic integrated. To design a QoS support framework tailored for WiMAX networks is more challenge as wireless channel has unique characteristics such as time-varying channel and limited channel capacity. This chapter presents various QoS support mechanisms in WiMAX networks. Existing proposals with the state-of-the-art technology have been classified into three main categories: QoS support architecture, bandwidth management mechanism, and packet scheduling schemes. Representative schemes from each of the categories have been evaluated with respect to major distinguishing characteristics of the WiMAX MAC layer and PHY layer as specified in the IEEE 802.16d standard. Suggestions and research trends on QoS support in WiMAX networks are highlighted.
TopIntroduction
The Worldwide Interoperability for Microwave Access (WiMAX) system has been specified by the IEEE 802.16d and its amendment IEEE 802.16e standard. The IEEE 802.16d and IEEE 802.16e standard defines the physical (PHY) and medium access control (MAC) layer of the fixed and mobile broadband wireless access systems, respectively.
Broadband wireless access (BWA) systems like WiMAX networks have been deployed not only to be complement and extension of existing last mile wired networks such as cable modem and xDSL but also to be competitor to wired broadband access networks. Due to the upcoming air interface technologies, which promise to deliver high transmission data rates, BWA systems become an attractive alternative.
IEEE 802.16d supports both frequency division duplex (FDD) and time division duplex (TDD) PHYs. IEEE 802.16d specifies 4 different PHY specifications, namely, WirelessMAN-SC PHY specification, WirelessMAN-SCa PHY specification, WirelessMAN-OFDM PHY specification, WirelessMAN-OFDMA PHY specification. At the PHY layer, a WiMAX system can take an adaptive modulation policy selecting 1 from 3 different modulation schemes. On the uplink (UL), QPSK is mandatory, while 16-QAM and 64-QAM are optional. The downlink (DL) can support QPSK and 16-QAM, while 64-QAM is optional. To fully utilize the flexible and robust PHY layer, a WiMAX system equips a flexible radio link control (RLC) scheme, which is responsible for transition from one PHY scheme to another. The system also uses the receiver sensitivity (RS) as a parameter together with the signal to interfernece and noise ratio (SINR) thresholds of receivers used by an adaptive modulation and coding (AMC) scheme to select the different burst profiles in order to maximize the network throughput and maintain the bit error rate (BER) under a preset level e.g. BER<=10-5.
WiMAX transceivers support different transmission modes with different modulation and coding schemes corresponding to different data transmission rates. For each modulation scheme, there is one relationship between the theoretical bit error rate and the ratio of energy per bit (Eb) to the spectral noise density (No) (Eb/No). Eb/No can be expressed in terms of SINR as:
(1) where
W is the channel bandwidth,
Rb is the transmission bit rate of a transmission mode corresponding to a modulation and coding scheme. Different modulation schemes have different curves of theoretical bit error rates versus E
b/N
0.
The range of the received SINR values will be classified into seven non-overlapping scales corresponding to an adaptive modulation and coding index AMC(q) where q=1, 2,…,7 on a target prescribed BER e.g. BER<=10-5 in a WiMAX PMP network. Further compiling with the RS requirement specified by IEEE 802.16d standard, a lookup table shows the relationship among AMC, SINR and RS requirement with its corresponding data rate as shown in Table 1. It is obvious that the data transmission rate can be dynamically changed from 20 Mbps to 180 Mbps if the channel capacity of WiMAX network is set to 40Mbps.
Table 1.
AMC vs. receiver SINR min sensitivity requirement
AMC Index
AMC(q) | Receiver SINR(dB) γ(q) | Receiver
Min Sensitivity RSS(q) (dBm) | Modulation/
Coding Scheme | E(q)
(bit/symbol) | Bit Rate
R(q) (Mbps) |
| AMC(1) | 8.3-11.6 | - 80 | BPSK1/2 | 0.5 | 20 |
| AMC(2) | 11.7-13.2 | - 80 | QPSK1/2 | 1 | 40 |
| AMC(3) | 13.3-18.9 | - 78 | QPSK3/4 | 1.5 | 60 |
| AMC(4) | 19.0-21.9 | - 73 | 16-QAM1/2 | 2 | 80 |
| AMC(5) | 21.0-28.0 | -71 | 16-QAM3/4 | 3 | 120 |
| AMC(6) | 28.1-29.1 | - 66 | 64-QAM2/3 | 4 | 160 |
| AMC(7) | >=29.2 | -65 | 64-QAM3/4 | 4.5 | 180 |