Thursday, July 12, 2007

[WiMAX] Fundamentals of WiMAX - (5)

In this post, let's take a quick look of the standardizations of WiMAX.

IEEE 802.16 Group was formed in 1998, its inital focus is to develop an air-interface standard for a LOS-based point to mulitpoint wireless broadband system working in 10G-66GHz band. In 2001, the original 802.16 standard was completed. After that, the task group produced 802.16a, which is an amendment to 802.16. This 802.16a is for NLOS applications in the 2G-11GHz band. Further revision is IEEE 802.16-2004 (fixed WiMAX), which replaced all previous specifications and formed the first version of WiMAX standard. It targeted fixed applications. In order to support mobile services, in 2005 a new amendament to IEEE 802.16-2004 called IEEE 802.16e-2005 (Mobile WiMAX) was approved by IEEE.

In short, WiMAX uses OFDM in the physical layer. The difference of the two specifications is: IEEE 802.16-2004 uses fixed-FFT-size OFDM, but IEEE 802.16e-2005 uses scalable OFDMA. Therefore, for fixed WiMAX, because FFT size is fixed, the subcarrier spacing will increase with larger bandwidth, and then the symbol time will decrease. It implies that a larger portion of OFDM subcarriers is needed as guard time to overcome delay spread of the channel, which means spectral efficiency is lowered. In the contrast, for mobile WiMAX, the subcarrier spacing is fixed but the FFT size is adjustable. This scheme can be used to balance to requirements of delay spread and Doppler spread of the channel for different operating environments.

In addition to OFDM, in physical layer WiMAX also supports some optional advanced techniques for increasing the link performance and reliability. These techniques include powerful FEC - Forward Error Correction coding (such as Turbo coding and LDPC - Low Density Parity Check code, which is very close to the Shannon channel capacity), channel interleaving, multiple antenna techniques - MIMO (please refer to Fundamentals of WiMAX - (3) ) and adaptive modulation. From here you can see that WiMAX almost integreted the most popular and advanced techniques of digital communications together. This is an advantage of creating a new system, because you do not have too much back-compatibility issues to take into account.

Another big portion of WiMAX's standard is specifying the MAC layer functions between the lower physical layer and the upper network and transport layers. The MAC layer should be able to accommodate different traffic types, support QoS (Quality of Service), provide security and mobility and so on. Those are just common functions and are considered in almost every wireless system. The distinct part of WiMAX's MAC layer from other similar systems is on the scheduling and channel-access schemes. In downlink, the Base Station allocates dedicated or shared resources periodically to each subscriber station. Then each MS will use this allocated rresource to request bandwidth. This procedure is called Polling. Polling can be done with unicast or multicast. In uplink, WiMAX specifies a contention access and resolution scheme for multiple MSs accessing the shared resource. If one MS already has an allocation for traffic sending, it will not be polled. But it can be allowed to request more bandwidth.


david santos said...

Thanks for your work and have a good week

Jinghao Xu said...

I'm really glad to see you are interested in my blog. Thank you, David!