Tutorial on WiMAX architecture
WiMAX stands for Worldwide Interoperability for Microwave Access. WiMAX is used for providing broadband internet using wireless medium mainly at 2.5GHz,3.5GHz and 5.8GHz radio frequencies. It is also known as 4G technology. It delivers about 4 times fast internet compare to its 3G counterpart. OFDM is behind this increase in speed as it carry multiple carriers, each carrying more than one data bits based on modulation techniques(QPSK,16QAM). This carriers are concisely packed together to save bandwidth.Intel is behind the developement and proliferation of wimax throughout the world.
WiMAX is referred as one of the metropolitan area network. It replaces DSL technologies for providing wireless internet at about 70Mbps upto the distance of 35 Km. There are two main versions of wimax viz. fixed wimax and mobile wimax. In Fixed wimax terminal or subscriber is fixed to a location and in mobile wimax they can move at various speeds. Fixed wimax is referred as 802.16d or 16d and mobile wimax is referred as 802.16e or 16e.
There are many physical layers as per IEEE 802.16 standard. The mainly used among them are Single carrier, OFDM and OFDMA. Single carrier is used for 10-66 GHz of frequency range while OFDM/OFDMA is used for 2-11 GHz frequency band. OFDM is employed in fixed wimax and OFDMA is employed in mobile wimax.
WiMAX network Architecture
The wimax architecture consists of various network elements that work together to deliver wireless connectivity. As shown in the block diagram, it houses SS (Subscriber Station) or CPE, BS (Base Station) or AP, Access Service Network (ASN), ASN Gateway, Connectivity Service Network (CSN), AAA server, Home Agent (HA), Foreign Agent (FA), Mobile IP Home Agent (MIHA), Mobile IP Foreign Agent (MIFA) and backhaul network.
Subscriber station (SS) : It is end user device that communicates with WiMax Base station. SS can be fixed or mobile device. It ia also known as CPE (Customer Premises Equipment).
Base Station (BS) : It is central component of wimax which serves multiple subscriber stations and connects with core network (CN).
It communicates with SSs using air interface.
ASN : It is responsible to manage radio resources and to provide connectivity with SSs.
It includes BS(s) and ASN-GW.
ASN-GW : It serves as aggregation point for multiple BSs and provides connectivity between ASN and CN. It handles
functions such as IP address assignment and security.
CSN : It is responsible for high level functions such as authentication, authorization, accounting (AAA) and IP connectivity with external networks.
AAA server : It is responsible to verify subscriber identities, authorize their access with network and to track their usage for
billing and accounting purpose.
Home Agent : It is used in mobile wimax. It is responsible for managing the mobility of subscriber stations
when they move between different BSs. It tracks location of mobile subscribers and help in maintaining
their connections.
Foreign Agent : Like home agent, it assists with mobility of subscribers in different network.
It communicates with home agent to manage seamless handoffs when subscribers move between different service areas.
MIHA : It is used in mobile wimax networks in which it is responsible to manage mobility of SSs that use mobile IP for
seamless handoffs.
MIFA : Like mobile IP Home Agent, it assists with mobility management for SSs moving between different networks.
Backhaul network : It helps to connect BSs to the CN (Core Network). It carries data traffic between the WiMAX access network and rest of the internet.
Fixed WiMAX System Features
Following are the features of Fixed WiMAX System.
| Feature | Fixed wimax support |
|---|---|
| IEEE Standard | 802.16-2004 OFDM PHY/MAC, 16d |
| Symbol Parameters | Cyclic Prefix (1/4,1/8,1/16,1/32),FFT size(256), Channel Bandwidth(1.75/2.5/3/3.5/5/5.5/7/10MHz) Long Preamble for Downlink sub frame and Short Preamble for Uplink Bursts |
| Frame duration | 2.5, 4, 5,8, 10, 12.5, 20 ms |
| Modulation | DL BPSK,DL QPSK, DL 16 QAM, DL 64 QAM,UL BPSK, UL QPSK,UL 16 QAM, UL 64 QAM |
| Channel Models | SUI channel models |
| Channel coding | Randomization, Concatenated RS-CC, Interleaving |
Mobile WiMAX System Specifications
| Feature | Mobile wimax support |
|---|---|
| IEEE Standard | 802.16-2005 OFDMA PHY/MAC,16e |
| FFT Size | 2048/1024/512/128 |
| Sampling Factor | If channel bandwidth is a multiple of 1.75MHz then n=8/7 else if channel bandwidth is a multiple of any of 1.25, 1.5,2 or 2.75 MHz then n=28/25 else if not otherwise specified then n=8/7. |
| Cyclic Prefix | 1/8 |
| Frame Size | 5 ms |
| DL Subcarrier allocation | PUSC,FUSC,AMC2x3 |
| UL Subcarrier allocation | PUSC,AMC2x3 |
| DL Modulation | QPSK,16QAM,64QAM |
| UL Modulation | QPSK,16QAM |
| Convolutional Coding | Tail biting/CTC |
| MAP Type | Normal/Compressed |
| DL MIMO | 2-antenna, matrix A, 2-antenna, matrix B vertical encoding |
| UL MIMO | Collaborative SM for two MS with single transmit antenna |
WiMAX OFDM PHY and MAC layer
Refer following links which describes WiMAX Physical layer transmitter step by step.
WiMAX OFDM PHY layer protocol➤
Fixed wimax MAC layer protocol➤
Conclusion : Overall, WiMAX architecture is designed to provide wireless broadband connectivity over large coverage area. It relies on combination of network elements to deliver seamless communication, mobility management and data transmission/reception for both fixed and mobile wimax deployments.