Wimax Technology

WiMAX

Naveen M B, Nidhish N, Prasanna M, Varun V

INDEX

* Introduction

* Why WiMAX

* Applications

* WiMAX Spectrum Allocation and Standardization

* PHY Layer Details

* Randomizer

* Forward Error Correction

* Interleaver

* Constellation Mapping

* Frequency Processing

* Time Processing

* Pulse Shaping Filter

* Diversity in WiMAX

* Challenges facing WiMAX

Introduction

WiMAX is defined as Worldwide Interoperability for Microwave Access by the WiMAX Forum. The WiMAX Forum has more than 522 members comprising the major communication players, component and equipment companies in the communications field including Accenture, Agilent, Acer, Intel, Fujitsu, Motorola, Samsung, Sprint and AT&T. The WiMAX forum was formed in April 2001 to promote conformance and interoperability of the IEEE 802.16 standard, officially known as Wireless MAN. The Forum describes WiMAX as "a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL".

It should be noted that, WiMAX is not a technology that is used but is more specifically a Certification or an Approval stamp given to devices that are in conformity with certain standards of the IEEE 802.16 family of standards. WiMAX supports data rate of up to 75 Mbps and provides a range of about 50 kms for LOS propagation.

Why WiMAX?

Wi-Fi devices operate in the unlicensed ISM band (Industrial Scientific and Medical Band) centered at 2.4GHz. This frequency band is also used by other devices. Hence the Wi-Fi devices are allocated a maximum power limit by FCC which limits their range or coverage. WiMAX was designed keeping in mind a WMAN. It is allocated a licensed frequency band and hence interference from other devices is relatively less. Correspondingly the range for WiMAX is more.

Further, in Wi-Fi, the media access controller ("MAC") uses CSMACA (Carrier Sense Multiple Access Collision Avoidance) scheme. In this media access scheme, a station first senses the medium, if it finds the medium busy; it backs off, waits for a random period of time and tries accessing the medium again. A packet is allowed up to maximum of about 10 retransmission attempts before it is dropped. Thus all subscriber stations that wish to pass data through a wireless "access point" (AP) are competing for the AP's attention on a random interrupt basis. This can cause distant nodes from the AP to be repeatedly interrupted by closer nodes, greatly reducing their throughput. This makes delay sensitive services such as Voice over IP (VoIP) or IPTV, which depend on a predetermined type of "quality of service" (QoS), difficult to maintain for large numbers of users. In contrast, the 802.16 MAC uses a scheduling algorithm, where the subscriber station only has to compete once (for initial entry into the network). After that it is allocated a time slot by the base station. The time slot can enlarge and contract, but it remains assigned to the subscriber station, meaning that other subscribers cannot use it. This scheduling algorithm is stable under overload and over-subscription (unlike 802.11). It can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control Quality of Service parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

Another limitation of Wi-Fi is that the devices need to be in Line Of Sight of each other. However WiMAX provides connectivity between network endpoints without the need for direct line of sight in favorable circumstances. It relies upon clever use of multi-path signals using Cyclic Prefix.

Applications

The most basic use of WiMAX is to provide Wireless broadband services at all times and at all places. It provides fixed and wireless connectivity to all devices in a cell range of 3 to 10 kilometers.

During natural disasters like tsunami in Aceh, Indonesia (Dec 2004) and Katrina in US, all communication infrastructure was destroyed making the survivors unable to communicate with people outside the disaster area and vice versa. WiMAX provided broadband access that helped regenerate communication to and from the affected area.

Figure 1: Typical Network using WiMAX [1]

The above figure shows typical applications supported by WiMAX. The products incorporated in the WiMAX family range from Mobile phones, Laptops, PDAs and many more. Some of the products having WiMAX technology built in within them are as shown below.

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(a) (b)

Figure 2: Typical products that use WiMAX technology [2]

(a) PDA and (b) Mobile WiMAX Measurement.

WiMAX Spectrum Allocation and standardization

Unlike Wi-Fi which is operated in an