During recent years, the increasing demands for high-speed Internet access and multimedia services in the residential and small office sectors have expected the development of last-mile access technologies, especially through wireless.
In the mid-1990’s, various groups began to promote “last-mile” fixed wireless access solutions, they tried to achieve some goals. First, to provide the capacity and reliability of wireline but with the flexibility and ease of deployment of wireless. Second, to provide a hierarchical system for corporate or institutional backhaul / distribution networks. Third, to break the monopolies of incumbent carriers.
In 2001, the IEEE 802.16 standard for BWA (Broadband Wireless Access) systems which operate in the 10-66 GHz range was released. Although the single carrier modulation during 10-66 GHz features 30 miles transmission radius and 70Mbps transmission rate, but the Line-of-Sight (LOS) limitation is still a problem. Until January 2003, 802.16a was published. It does not only include new PHY layer specification and enhanced MAC functionality, but also add 2-11 GHz in spectrum. So the signal can be used in Non-Line-of-Sight (NLOS) environment and the single antenna can also serve multiple stations. Several years later, IEEE 802.16a / b / c and various updates were incorporated into IEEE 802.16-2004 [1].
The Worldwide Interoperability for Microwave Access (WiMAX) technology based on the IEEE 802.16-2004 Air Interface Standard is rapidly proving itself as a technology that will play a key role in fixed broadband wireless metropolitan area networks. The first certification lab, established at Cetecom Labs in Malaga, Spain is fully operational and lots of trials are underway in Europe, Asia, Africa and North and South America [21]. Unquestionably, Fixed WiMAX, based on the IEEE 802.16-2004 Air Interface Standard, has proven to be a
cost-effective fixed wireless alternative to cable and DSL services. In December, 2005 the IEEE ratified the 802.16e amendment to the 802.16 standard [2]. This amendment adds the features and attributes to the standard that is necessary to support mobility. The WiMAX Forum is now defining system performance and certification profiles based on the IEEE 802.16e Mobile Amendment. Beyond the air interface, the WiMAX Forum is defining the network architecture necessary for implementing an end-to-end Mobile WiMAX network. [3]
A network that utilizes a shared medium shall provide an efficient sharing mechanism.
The WiMAX architecture depends on point to multipoint (PMP) and Mesh networks to achieve medium sharing. The medium means the space which can propagate the radio waves.
The PMP mode is illustrated in Figure 1.1.
Figure 1.1 IEEE 802.16 PMP mode [22].
WiMAX provides fixed, nomadic, portable, and mobile wireless broadband connectivity without the need for line-of-sight with a base station. In a typical cell radius deployment of three to ten kilometers, WiMAX Forum Certified systems can be expected to deliver capacity of up to 40 Mbps per channel, for fixed and portable access applications. This bandwidth is sufficient to simultaneously support hundreds of T-1 speed connectivity and
thousands of residences with DSL speed connectivity. Mobile network deployments are expected to provide up to 15 Mbps of capacity within a typical cell radius deployment of up to three kilometers. It is expected that WiMAX technology will be incorporated in notebook computers and PDAs by 2007, allowing for urban areas and cities to connect with each other for portable outdoor broadband wireless access.
We know that the WiMAX has been analyzed in lots of countries and organizations, such as Nokia and Intel will cooperate to develop the WiMAX technologies. South Korea’s electronics and telecommunication industry spearheaded by Samsung Electronics and ETRI has developed its own standard, WiBro. In late 2004, Intel and LG Electronics have agreed on interoperability between WiBro and WiMAX. The telephone and telegraph office in Taiwan also work hard for opening the spectrum up.
We may ask whether one size of WiMAX transmission scope really fit all conditions.
Different applications have different requirements and constraints for spectrum and performance. So to allow options within a consistent framework is a suitable idea, because factories can choose serviceable standard profiles from limited set of standard profiles.
IEEE 802.16 seeks to provide the required features to serve users. One main element of WiMAX technology is the interoperability of WiMAX equipment that can integrate much wireless stations or products and let them work together. A common platform could achieve lower cost. Fixed wireless equipments will be able to use the same modem chipset used in personal computers (PCs) and PDAs. For short distance, the equipments will be similar to a cable or DSL and the base stations will be able to use the same chipsets developed for low-cost WiMAX access points. So they also provide lower cost equipments which support WiMAX and original functionality.
The technology behind WiMAX has been optimized to provide Non-Line-of-Sight (NLOS) coverage. Non-Line-Of-Sight advantages are coverage of wider area and better predictability of coverage. A key advantage of WiMAX is to use OFDM over single carrier
modulation schemes with the ability to deliver higher bandwidth efficiency and therefore higher data throughput, with more than 1 Mbps downstream and even much higher data rates, even in NLOS with multipath conditions. Adaptive Modulation also increases link reliability for carrier-class operation and the possibility to keep 64 QAM modulation at wider distance extend full capacity over longer distances. Optimized handover also ensures the time being less than 50ms.
QoS (Quality-of-Service) is an important issue in wireless network especially for those service flows belonging to voice and video. IEEE 802.16 has already specified complete construction, including how they achieve QoS requirement. In 802.16, there are five types of scheduling services specified for different traffic models, i.e. Unsolicited Grant Service (UGS), real-time Polling Services (rtPS), non-real-time Polling Service (nrtPS), Best Effort (BE), and Extended real-time Polling Service (ertPS) which was published in IEEE 802.16e.
Among the five service flows, the QoS support in UGS, rtPS, and ertPS is necessary. Here we briefly introduce what problem may occur in 802.16 and how can we solve it.
Before the uplink data transmission in IEEE 802.16 PMP mode, the subscriber station must send their bandwidth request first to base station and let it know how much bandwidth is required. When the whole bandwidth isn’t enough for all stations to send individual bandwidth request, the service flows belonging to ertPS, nrtPS, and BE have to count on contention to send their requests. There is certain collision probability during contention period and that would cause the request delay for the ertPS flows. When the request was delayed, the station couldn’t get proper bandwidth and as a request its QoS will degrade. So how to solve the collision problem will be discussed in this thesis.
The rest of this thesis is organized as follows. Chapter 2 provides an overview of the 802.16 system. Chapter 3 describes the detailed problem in 802.16 and our proposed scheme in detail. The numerical analysis and simulation evaluation are presented in Chapter 4.
Conclusions are stated in Chapter 5.