WiMAX
Background
WiMAX stands for Worldwide Interoperability for Microwave Access and is a wireless broadband communication system defined in the IEEE 802.16 industry standards. The
WiMAX Forum is an organization formed to promote conformance and interoperability of the IEEE 802.16 standards.
The IEEE 802.16-2004 (802.16.d) standard, also known as the fixed WiMAX standard, was approved by IEEE in June 2004. It provides fixed, point-to-multi point broadband wireless access service and is primarily used as a wireless metropolitan area network
(WMAN).
In 2005 IEEE approved the mobile WiMAX amendment, IEEE 802.16e-2005 (802.16.e).
In the 802.16e-2005 amendment the most important addition to the previous standards is mobility with specifications of mobility management, extensible authentication protocol
(EAP) and handover.
Mobile WiMAX is based upon Orthogonal Frequency Division Multiple Access
(OFDMA).
Some of the features of Mobile WiMAX are:
• High Data Rates: Can theoretically support peak download data rates up to 63
Mb/s per sector and peak upload data rates up to 28 Mb/s per sector in a 10 MHz channel, when MIMO 2*2 is used and DL:UL ratio is 1:0 and 0:1 respectively.
•
Quality of Service (QoS): QoS measures for WiMAX include service availability, data throughput, delay, jitter, and error rate.
•
Scalability: Mobile WiMAX has been designed to be able to work in different channel bandwidths from 1.25 to 20 MHz.
•
Mobility: Optimized handover schemes with latencies less than 50 milliseconds and flexible key management schemes are supported. Mobile WiMAX has been tested for speeds up to 120 km/h.
[5]
These features make the technology suitable for a range of applications. Among the applications that have been suggested for mobile WiMAX is to provide high speed data services and other Internet services like VoIP for fixed, nomadic and mobile terminals, to provide a wireless alternative to cable or DSL for last mile broadband access and to connect hotspots with each other and the rest of the Internet. The only application we will consider is the high speed Internet service to fixed, nomadic and mobile users.

2.1.2 Technology
The IEEE 802 standards are a family of standards dealing with local area networks
(LANs) and metropolitan area networks (MANs). The standards mainly focus on the lowest 2 layers of the Reference Model for Open Systems Interconnection (OSI)[6], the
Data Link layer and the Physical (PHY) layer. Furthermore, IEEE 802 splits the Data
Link Layer into two sub-layers, the Logical Link Control (LLC) sub layer and the Media
Access Control (MAC) sub layer. In broadband wireless networks, most of the things happen in the MAC layer and the PHY (Air Interface) layer.
Figure 1. The layers of the OSI model, and the Data Link sublayers, LLC and
MAC
.
The Physical layer
At the first level of the seven level OSI model there is the Physical layer, the most basic network layer. Its main functions are providing an interface to physical transmission media, modulation, coding, bit synchronization, flow control and circuit mode multiplexing. [7]
Mobile WiMAX (802.16e-2005) uses a physical layer mode called scalable Orthogonal
Frequency Division Multiple Access (sOFDMA PHY) mode.
OFDM
Orthogonal Frequency Division Multiplexing is a multiplexing technique that divides the input data stream into a large number of parallel substreams of reduced data rate and each substream is modulated and transmitted on a separate orthogonal subcarrier. Each subcarrier is modulated with a conventional modulation scheme. The reduced data rate causes the symbol duration to increase and hence improves the robustness of OFDM to delay spread. Because of the subcarriers orthogonality there will be no cross talk between them, even though their spectra is very close or overlapping. By allowing the subcarriers to overlap, the spectrum required is reduced and the bandwidth efficiency is increased.
OFDM signals are generated with efficient Inverse Fast Fourier Transform (IFFT), which enables a large number of subcarriers (up to 2048) with low complexity. In the time

domain the resources of an OFDM system are available by means of OFDM symbols and in the frequency domain by means of subcarriers. For allocation to individual users the time and frequency resources can be organized into subchannels. [5]
OFDMA
Orthogonal Frequency Division Multiple Access is a multiple-access/multiplexing scheme that divides the channel in such a way that many users can share it. An individual subcarrier or a group of subcarriers are assigned to different users and thus the users can all use the same channel simultaneously.[8]
In other words OFDMA is a multi-user version of OFDM where subsets of subcarriers can be assigned to different users. Different number of subcarriers can be assigned to different users according to channel conditions and the type of service the user is requesting. This is comparable to CDMA and how spreading codes can be used to assign different users with different data rates.
Scalable OFDMA (S-OFDMA) is the modulation/multiple-access method used in
Mobile WiMAX to address the problem of different channel sizes in different countries.
With the scalability IEEE 802.16-2005 can support channel sizes ranging from 1.25 MHz to 20 MHz. This is done by adjusting the FFT size based on channel size or bandwidth while keeping the subcarrier frequency spacing fixed at 10.94 kHz.
Since the resource unit subcarrier bandwidth and symbol duration is fixed, the impact to higher layers is minimal when scaling the bandwidth. [5]
Time Division Duplex (TDD) frame structure
The 802.16-2005 PHY does support TDD and Full and Half-Duplex FDD operation but
TDD is the preferred duplexing mode and FDD will only be considered to address specific market opportunities.
When using TDD only one channel is required for both downlink and uplink as opposed to the two separate frequency channels of FDD. The advantages of TDD are that it enables adjustment of the downlink/uplink ratio to efficiently support asymmetric downlink/uplink traffic instead of fixed and equal DL and UL bandwidths. The ratio between the uplink and downlink determines how time is split between them, for example

the ratio 2:1 means that the UL gets two times more time for transmission than the uplink. TDD also supports channel reciprocity, which is important when using MIMO and other closed loop advanced antenna technologies. Transceiver designs are also simpler for TDD implementations than FDD. On the other hand the discontinuous transmission and reception of TDD reduces the average power of the system. [5], [9]
Modulation and Coding
In Mobile WiMAX support for QPSK, 16 QAM and 64 QAM is mandatory in the DL but in the UL 64 QAM is optional. Both Convolutional Code and Convolutional Turbo
Code are supported.
The coding and modulation schemes supported in Mobile WiMAX can be seen in table 1.
To show how it is possible to vary the data rates by choosing different MCS levels, the different PHY DL data rates for different MCS levels are also shown in Table 1 (for
PUSC mode and 10 MHz channel).
Modulation Code rate DL PHY data rate (10 MHz)
QPSK 1/2 (CTC) 6,34 Mb/s
QPSK 3/4 (CTC) 9,50 Mb/s
16 QAM 1/2 (CTC) 12,67 Mb/s
16 QAM 3/4 (CTC) 19,01 Mb/s
64 QAM 1/2 (CTC) 19,01 Mb/s
64 QAM 2/3 (CTC) 25,34 Mb/s
64 QAM 3/4 (CTC) 28, 51 Mb/s
64 QAM 5/6 (CTC) 31,68 Mb/s
Table 1. MCS levels and DL PHY data rates in Mobile WiMAX [5]
Other features of the PHY layer
The term Adaptive Modulation and Coding (AMC) is used to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link.
Hybrid Auto Repeat Request (HARQ) is a feature that enables fast response to packet errors and asynchronous operation, with a variable delay between retransmissions.
Fast Channel Feedback (CQICH): A Channel Quality Indicator (CQI) channel is used to provide channel-state information from the user terminals to the base station scheduler.
.

.
The MAC (Media Access Control) Layer:
At the second level of the OSI model there is the Data Link layer that can be divided into two sub-layers, MAC and LCC. The MAC sub-layer serves as an interface between the physical layer and the LCC sub-layer. It provides channel access control mechanisms and addressing mechanisms (MAC address) that make communication between terminals and/or networks possible. Also it provides the protocol and control mechanisms that are required for several stations to share the same physical medium.
The MAC layer of the IEEE 802.16-2005 is quite complex and advanced. The main features are the following:
Quality of Service (QoS):
Is a term that refers to control mechanisms that can provide different priority to different users or data flows, or guarantee a certain level of performance in accordance with requests from the application program.
For certain types of network traffic a defined QoS is required. For example:
•
Voice over IP (VOIP): Requires maximum latency tolerance, jitter tolerance and maximum sustained rate.
• Streaming multimedia: Minimum reserved rate, maximum sustained rate, latency tolerance and traffic priority.
• File Transfer Protocol (FTP): Maximum sustained rate, minimum reserved rate and traffic priority.
•
Data transfer, web browsing: Maximum sustained rate, traffic priority.
In the Mobile WiMAX MAC layer, QoS is provided via service flows. First the base station (BS) and the user-terminal establish a unidirectional logical link between the peer
MACs. The QoS parameters associated with the specific kind of data to be transmitted define the transmission ordering and scheduling on the air interface. For whatever service is in use the parameters can be dynamically managed through Media Access
Control (MAC) messages. This QoS mechanism applies to both DL and UL for a better
QoS in both directions. [5]

MAC Scheduling Service
Is a scheduling service that is designed to deliver broadband data services in an efficient way over time-varying broadband wireless channel. These data services include voice, data and video. To achieve this the scheduling service uses the following properties:
•
Fast Data Scheduler: To efficiently allocate available resources in response to bursty data traffic and time-varying channel conditions. Located at each BS for faster response. In order for the scheduler to correctly determine the packet transmission ordering, the data packets are associated to service flows with defined QoS parameters. Because of the CQICH channel feedback the scheduler can choose the best coding and modulation for each allocation.
•
Scheduling for both DL and UL
•
Dynamic resource allocation: Frequency-time resource allocation on a perframe basis is supported in both DL and UL. As the resource allocation is delivered in Media Access Protocol (MAP) messages in the beginning of each frame it can be changed on a frame-by-frame basis in response to traffic and channel conditions. The amount of resource can range from one slot to entire frame.
•
QoS Oriented: Data transport is handled on a connection-by-connection basis.
For each connection there is a single data service with a set of QoS parameters that define its behavior.
•
Frequency Selective Scheduling: Provides different services for different types of subchannels.
Power Management
To enable power-efficient MS operation Mobile WiMAX supports Sleep Mode and Idle
Mode. In Sleep Mode the MS conducts pre-negotiated periods of absence from the
Serving Base Station air interface to minimize power usage and usage of the Serving
Base Station air interface resources. In Idle Mode the MS is periodically available for
DL broadcast traffic messaging without registration at a specific BS.

Network Architecture
The end-to-end network architecture is not part of the IEEE 802.16e-2005 standard. As
IEEE usually only defines the PHY and MAC layers. However operators and vendors have formed working groups to define standard network reference models. In Figure 2 a basic view of the all-IP Mobile WiMAX network can be seen.
Figure 2. Network Architecture of Mobile WiMAX
The three basic domains are:
20

User Terminals : Mobile, portable and fixed devices with WiMAX support. This could be for example cellular phones, MIDs (Mobile Internet Devices), UMPCs (Ultra Mobile
PCs), laptops or Desktops.
Access Service Network (ASN) : Provides a way to connect user terminals using
OFDMA air interface to an IP backbone with session continuity (a session is not drop when user moves between wireless environment). An ASN consists of BSs and Access
Gateways. The set of network functions in ASN include:
Network discovery and selection of the preferred CSN/NSP (Network Service Provider)
Network entry with IEEE 802.16e-2005 based layer 2 connectivity and AAA proxy.
Relay function for IP Connectivity.
Radio Resource Management.
Multicast and Broadcast Control.
Foreign agent functionality for inter-ASN mobility
Paging and Location Management.
Data forwarding.
Service flow authorization.
Quality of Service.
Admission Control and Policing.
Connectivity Service Network (CSN) : is defined as a set of network functions that provide IP connectivity services to the mobile subscribers. A CSN consists of network elements such as routers, AAA(Authentication, Authorization, Accounting) proxy/servers, user databases and Interworking gateway devices. A CSN is deployed by a
NSP. AAA or Home Agent residing in CSN allocates the IP address. AAA also performs authentication, authorization, and accounting. Communication is through a RADIUS protocol. The policy server residing in the CSN is responsible to store the policy and QoS info of each subscriber, which is communicated to ASN during service flow creation.
CSN is also responsible to access other IP networks.
Other features of Mobile WiMAX
Smart Antenna Technologies
Using OFDMA makes it possible for Mobile WiMAX to use smart antenna operations in rather simple way as they can be performed on vector-flat subcarriers and no complex equalizer are required to compensate for frequency selective fading. The MIMO technologies can be divided into the following three categories:
• Spatial Multiplexing: To enhance the peak data transmission rate and overall throughput by using multiple streams with multiple antennas.
•
Space-Time Coding: A transmission diversity method for enhancing transmission robustness.
•
Beamforming: The received signal gain is improved and interference reduced with better coverage, higher capacity and reduced outage probability.

By using Adaptive MIMO Switching (AMS) between multiple MIMO modes, the MIMO system can use the transmission techniques one at a time or all together as is best at each time. In that way the spectral efficiency can be maximized with no reduction in coverage area.