2.2. Fundamental Technical Specifications of WiMAX

Technical specifications of WiMAX will be introduced in this section. WiMAX is Worldwide Interoperability for Microwave Access which based on IEEE 802.16 standard. Firstly, I will go indepth to the fundamental technologies of WiMAX; secondly, frequency selection is interesting to be focused on; thirdly related techniques of required bandwith practically is going to be indicated; then, spectrum theme for WiMAX about increasing capacity and service demand will be shown and technical improvements or addings in the standards will be mentioned; finally, a typical WiMAX network configuration will be illustrated.

Fundamental Technologies

This part is specially for the fundamental technologies of WiMAX. As we know, 802.16 operates at up to 124Mbps in the 28MHz channel (in 10-66GHz), 802.16a at 70Mbps in lower frequency, 2-11GHz spectrum. It will specify two flavors of OFDM systems: one simply identified as OFDM, the other OFDMA.

OFDM
Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels.

OFDM has been recently recognized as an excellent method for high speed bi-directional wireless data communication. Its history dates back to the 1960s, but it has recently become popular because economical integrated circuits that can perform the high speed digital operations necessary have become available. OFDM effectively squeezes multiple modulated carriers tightly together, reducing the required bandwidth but keeping the modulated signals orthogonal so they do not interfere with each other. Today, the technology is used in such systems as asymmetric digital subscriber line (ADSL) as well as wireless systems such as IEEE 802.11a/g (64 subcarriers) and IEEE 802.16 (WiMAX). In OFDM we have 256 sub-carriers with 192 data sub-carriers, 8 pilot sub-carriers and 56 nulls. In its most basic form, each data sub-carrier could be on or off to indicate a one or zero bit of information. However, either phase shift keying (PSK) or quadrature amplitude modulation (QAM) is typically employed to increase the data throughput. So in this case, a data stream would be split into n (192) parallel data streams, each at 1/n (1/192) of the original rate. Each stream is then mapped to the individual data sub-carrier and modulated using either PSK or QAM. Pilot subcarriers provide a reference to minimize frequency and phase shifts during the transmission while null carriers allow for guard bands and the DC carrier (center frequency).All subcarriers are sent at the same time. Actually, 802.16a has three PHY options: an OFDM with 256 sub-carriers – the only option supported in Europe by the ETSI, whose rival HiperMAN standard is likely to be subsumed into WiMAX; OFDMA, with 2048 sub-carriers; and a single carrier option for vendors that think they can beat multipath problems in this mode. OFDM will almost certainly become dominant in all wireless technologies including cellular and its industry body, the OFDM Forum, is a founder member of WiMAX Forum.

OFDMA
Orthogonal frequency division multiple access (OFDMA) allows some sub-carriers to be assigned to different users. For example, sub-carriers 1, 3 and 7 can be assigned to user 1 and sub-carriers 2, 5 and 9 to user 2. These groups of sub-carriers are known as sub-channels. Scalable OFDMA allows smaller FFT sizes to improve performance (efficiency) for lower bandwidth channels. This applies to IEEE 802.16-2004 which can now reduce the FFT size from 4096 to 128 to handle channel bandwidths ranging from 1.25–20 MHz. This allows sub-carrier spacing to remain constant independent of bandwidth which reduces complexity while also allowing larger FFT for increased performance with wide channels.

A great advantage of OFDM and OFDMA modulation is tolerance to multipath propagation and selective fading. It can overcome its negative influence utilizing parallel, slower bandwidth nature. This has made it not only ideal for such new technologies like WiMAX, but also currently one of the prime technologies being considered for use in future fourth generation (4G) networks.

802.16-2004 was updated by 802.16e-2005 in 2005 and uses scalable orthogonal frequency-division multiple access (SOFDMA) as opposed to the OFDM version with 256 sub-carriers (of which 200 are used) in 802.16d

PHY Layer
Physical layer was defined for a wide range of frequency from 2 up to 66 GHz. In sub-range 10-66 GHz system there is an assumption of Line-Of-Sight propagation. In this scheme single carrier modulation was chosen, because of low complexity of system. Downlink channel is shared among users with TDM signals. Subscriber unit are being allocated individual time slots. Access in uplink is being realized with TDMA. Channel bandwidths are 20 or 25 MHz in USA and 28MHz (Europe). Duplex can be realized with either TDD or FDD scheme. In the 2-11 GHz bands communication can be achieved for licensed and non-licensed bands. The communication is also available in NLOS conditions. The 802.16a Draft3 air interface specification describes three formats: Single Carrier modulation (SC), OFDM with 256 point transform, and OFDMA with 2048 point transform. The Forward Error Correction (FEC) is used with Reed-Salomon Codes GF(256). It is also paried inner block convolutional code to robustly transmit critical data, like Frame Control or Initial Access.

MIMO
More advanced versions, including 802.16e, also bring Multiple Antenna Support through Multiple-input multiple-output communications (MIMO), referring to WiMAX MIMO. This brings potential benefits in terms of coverage, self installation, power consumption, frequency re-use and bandwidth efficiency. 802.16e also adds a capability for full mobility support. The WiMAX certification allows vendors with 802.16d products to sell their equipment as WiMAX certified, thus ensuring a level of interoperability with other certified products, as long as they fit the same profile.

Dynamic Frequency Selection in Unlicensed Spectrum

Mesh
Mesh Mode is an optional topology for subscriber-to-subscriber communication in non-line of sight 802.16a. It is included in the standard to allow overlapping, ad hoc networks in the unlicensed spectrum and extend the edges of the WMAN’s range at low cost. Mesh support has recently been extended into the licensed bands too. Although it has highly complex topology and messaging, mesh is a good alternative to the usual NLOS, as it scales well and addresses license exempt interference. It allows a community to be densely seeded with WiMAX connections at low cost, with robust communications as there are multiple paths for traffic to take

Spectral efficiency
Spectrum efficiency measures the maximum total amount of data that can be carriedby a cell per unit of time, normalized with the occupied system bandwidth. For anygiven traffic load per user, spectral efficiency can be used to determine the numberof users a cell can support. For example, 802.16-2004 (fixed) has a spectral efficiency of 3.7 (bit/s)/Hertz, and other 3.5–4G wireless systems offer spectral efficiencies that are similar to within a few tenths of a percent. The notable advantage of WiMAX comes from combining SOFDMA with smart antenna technologies. This multiplies the effective spectral efficiency through multiple reuse and smart network deployment topologies. The direct use of frequency domain organization simplifies designs using MIMO-AAS compared to CDMA/WCDMA methods, resulting in more-effective systems

Protocol independent core
WiMAX can transport IPv4, IPv6, Ethernet or ATM and others, supporting multiple services simultaneously and with quality of service.

Bandwidth on Demand

Bandwidth is significant aspect in WiMAX. Related consideration such as Quality of Service, adaptive modulation, and duplexing, etc. are need to be taken into account.

QoS
The ‘b’ extension to 802.16 is concerned with quality of service (QoS), which enables NLOS operation without severe distortion of the signal from buildings, weather and vehicles. It also supports intelligent prioritization of different forms of traffic according to its urgency. Mechanisms in the Wireless MAN MAC provide for differentiated QoS to support the different needs of different applications. For instance, voice and video require low latency but tolerate some error rate, while most data applications must be error-free, but can cope with latency. The standard accommodates these different transmissions by using appropriate features in the MAC layer, which is more efficient than doing so in layers of control overlaid on the MAC. Later amendments such as 802.16-2004, 802.16e are also based on this scheme.

Adaptive Modulation
Many systems in the past decade have involved fixed modulation, offering a trade-off between higher order modulation for high data rates, but requiring optimal links, or more robust lower orders that will only operate at low data rates. 802.16a supports adaptive modulation, balancing different data rates and link quality and adjusting the modulation method almost instantaneously for optimum data transfer and to make most efficient use of bandwidth.

FDD and TDD
The standard also supports both frequency and time division duplexing (FDD and TDD) to enable interoperability with cellular and other wireless systems. FDD, the legacy duplexing method, has been widely deployed in cellular telephony. It requires two channel pairs, one for transmission and one for reception, with some frequency separation between them to mitigate self-interference. In regulatory environments where structured channel pairs do not exist, TDD uses a single channel for both upstream and downstream transmissions, dynamically allocating bandwidth depending on traffic requirements.

Future Spectrum for WiMAX – More Room and Service Options

Additional bands are being considered today by different regions around the world for the deployment of WiMAX and other similar broadband wireless access services. In Japan the 5.47GHz – 5.725GHz band is being considered for future use. The North American market is indicating some interest in deploying WiMAX in the 4.9GHz broad-spectrum public safety band. There is even some interest in using lower frequency bands such as the licensed 800MHz and he unlicensed 915MHz ISM bands for WiMAX and similar types of services and deployments. The WiMAX standard is set to bring the long-awaited spectral efficiency and throughput to meet users’ needs for combined mobility, voice services and high data rates. It will enable access for more users due to its non-line-of-sight capability, lower deployment costs, wide range capability and penetration into the mass consumer market with lower CPE costs as a result of standardization and interoperability. Since October 2007, the Radiocommunication Sector of the International Telecommunication Union (ITU-R) has decided to include WiMAX technology in the IMT-2000 set of standards. This enables spectrum owners (specifically in the 2.5-2.69 GHz band at this stage) to use Mobile WiMAX equipment in any country that recognizes the IMT-2000.

Technical Improvement & Development of Standards

The current WiMAX incarnation, Mobile WiMAX, is based upon IEEE Std 802.16e-2005, approved in December 2005. It is a supplement to the IEEE Std 802.16-2004, and so the actual standard is 802.16-2004 as amended by 802.16e-2005 — the specifications need to be read together to understand them.

IEEE Std 802.16-2004 addresses only fixed systems. It replaced IEEE Standards 802.16-2001, 802.16c-2002, and 802.16a-2003.

IEEE 802.16e-2005 improves upon IEEE 802.16-2004 by [2]:

- Adding support for mobility (soft and hard handover between base stations). This is seen as one of the most important aspects of 802.16e-2005, and is the very basis of 'Mobile WiMAX'.

- Scaling of the Fast Fourier Transform (FFT) to the channel bandwidth in order to keep the carrier spacing constant across different channel bandwidths (typically 1.25 MHz, 5 MHz, 10 MHz or 20 MHz). Constant carrier spacing results in a higher spectrum efficiency in wide channels, and a cost reduction in narrow channels. Also known as Scalable OFDMA (SOFDMA). Other bands not multiples of 1.25 MHz are defined in the standard, but because the allowed FFT subcarrier numbers are only 128, 512, 1024 and 2048, other frequency bands will not have exactly the same carrier spacing, which might not be optimal for implementations.

- Improving NLOS coverage by utilizing advanced antenna diversity schemes, and hybrid-Automatic Retransmission Request (HARQ).

- Improving capacity and coverage by introducing Adaptive Antenna Systems (AAS) and Multiple Input Multiple Output (MIMO) technology

- Increasing system gain by use of denser sub-channelization, thereby improving indoor penetration

- Introducing high-performance coding techniques such as Turbo Coding and Low-Density Parity Check (LDPC), enhancing security and NLOS performance

- Introducing downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa

- Enhanced Fast Fourier Transform algorithm can tolerate larger delay spreads, increasing resistance to multipath interference

- Adding an extra QoS class (enhanced real-time Polling Service) more appropriate for VoIP applications.

802.16d vendors point out that fixed WiMAX offers the benefit of available commercial products and implementations optimized for fixed access. It is a popular standard among alternative service providers and operators in developing areas due to its low cost of deployment and advanced performance in a fixed environment. Fixed WiMAX is also seen as a potential standard for backhaul of wireless base stations such as cellular, Wi-Fi or even Mobile WiMAX. SOFDMA (used in 802.16e-2005) and OFDM256 (802.16d) are not compatible so most equipment will have to be replaced if an operator wants or needs to move to the later standard. However, some manufacturers are planning to provide a migration path for older equipment to SOFDMA compatibility which would ease the transition for those networks which have already made the OFDM256 investment. Intel provides a dual-mode 802.16-2004 802.16-2005 chipset for subscriber units. This affects a relatively small number users and operators.

Typical WiMAX Network Configuration



The figure (Source:[3]) above shows a typical WiMAX network congiguration solution. Where SS is subscriber station; CPE is Customer premises equipment such as router; MS and BS are Mobile station and base station respectively; ASN is access service network; CSN, obviously, is connectivity service network; AAA is authentication, authorization and accounting entity; HA is home agent; and DHCP is dynamic host control protocol.This typical structure is basd on Fujitsu WiMAX solution of Japan.


Overall, this section mainly focuses on the technology details of WiMAX. With the technologies improvements and development, the standards will also be amended based on the core techniques. In the coming section, I would like to show the relationship with other wireless technologies.

Reference:
[1] Carl Eklund, Roger B. Marks, Kenneth L. Stanwood and Stanley Wang, "IEEE Standard 802.16:A Technical Overview of theWirelessMAN™ Air Interface forBroadband Wireless Access", IEEE Communications Magazine, June 2002.
[2] http://en.wikipedia.org/wiki/WiMAX
[3] http://www.fujitsu.com/img/TELCOM/wireless/wimax/wimax_network.jpg