WiMAX and Its Applications （1）

Li Danpu,Hao Jianjun,Yue Guangxin

(Beijing University of Posts and Telecommunications, Beijing 100876, China)

1 WiMAX and IEEE 802.16 Standards

W ireless communications have been developing fastest among various communication ways.It aims at broadband,all-IP,multi-service and ubiquitous communications.Besides the cellular network technology being widely applied in live networks,broadband radio access technologies also include Wireless Local Area Network(WLAN),Wireless Metropolitan Area Network(WMAN),and emerging Wireless Wide Area Network(WWAN),among which WLANwas proposed earliest.Based on IEEE 802.11 standards,the WLANtechnology is getting mature and is being used commercially.In order to improve coverage,transmission rate and QoS,the IEEE 802.16 working group has specified a series of standards for WMAN since 1999.Learning from WiFi's great promotion of IEEE 802.11-based WLAN,several companies,including Intel,Fujitsu and Nokia,established an unprofitable industrial alliance—the Worldwide Interoperability for Microwave Access(WiMAX)Forum in 2001,with a goalof promoting and ensuring the compatibility and interoperability of IEEE 802.16 based broadband wireless devices from different vendors.

Thereafter,WiMAXbecame another name of the IEEE 802.16 standard family.

The IEEE 802.16(hereinafter 802.16)standards for the air interface include 802.16,802.16a,802.16d and 802.16e.802.16d and 802.16e are presently the leading standards,specified for fixed and mobile communications respectively.802.16d integrates and amends 802.16 and 802.16a.It defines the Media Access Control(MAC)layer and corresponding physicallayers in the fixed broadband radio access system that supports diversified service types.Based on 802.16d,802.16e have several new functions to support users'mobility.

In general,the 802.16 standard family is a wireless solution to“the-last-mile”broadband access;its specifications support low-latency applications such as voice and video,and help offer high-speed data,voice and video services to users with fixed,mobile and portable terminals.

Compared to other wireless communication technologies,WiMAXhas the following strengths:

(1)High-speed Transmission Rate and Large Coverage

The WiMAXsystem obviously improves frequency re-use by Orthogonal Frequency Division Multiplexing(OFDM),multi-antenna technology and flexible coding and modulation used on the physical layer.It can also support Non-Line of Sight(NLOS)and seamless coverage.When the bandwidth is 20 MHz,the WiMAX system has the highest transmission rate of 75 Mb/s,which triples the rate supported by a 3G system.Moreover,macro cells are used in the WiMAX system,and each Base Station(BS),with a typical coverage radius of 6 km-10 km,can have the largest coverage radius of 50 km for Line-of-Sight(LOS)communications.Therefore,not many BSs are needed for the system to fulfill full coverage of any city.

(2)Perfect QoSMechanism

WiMAXis connection-oriented.Its MAClayer defines a complete QoS mechanism and four service types.WiMAXcan offer video and voice services with either high or ordinary QoS,and services without QoSsuch as the Internet services.In addition,it has great flexibility to dynamically allocate bandwidth according to the actual requirement of a service.

(3)Good Data Security

Security plays an important role in wireless networks.802.16 defines an encryption sub-layer on the MAC layer to provide authorization,encryption and confidentiality for information transmission.

▲Figure 1. IEEE 802.16 protocol architecture model.

(4)All-IPArchitecture 802.16 equipment can access live IP networks as a router.It can not only interoperate with live networks,but also seamlessly converge with next-generation networks.

(5)Low Costs and Small Investment Risks

The standardized air interface and technical specifications make it possible for devices from different vendors to interoperate with each other.Therefore,operators need not use an equipment vendor only,which reduce their investment costs and risks,in return.

(6)Flexible Deployment,Scalable Configuration and Smooth Upgrade

Generally speaking,WiMAXsupports packetized data,broadband access and mobile terminal;it also provides better coverage,transmission rate and QoS than other wireless systems.Therefore,it has gotten wide attention and become a hot R&Dtopic in the broadband radio access field since its inception.This article will discuss the characteristics and key technologies for WiMAXphysical and MAC layers based on 802.16d/e,analyze the WiMAXnetwork architecture and its applications,and finally introduce the development tendency of WiMAX.

2 Reference Model of 802.16 Protocol Stack

The 802.16 defined air interface consists of the physical and MAC layers,as shown in Figure 1.The MAC layer is independent of the physical layer,and can support different physical-layer specifications for various application scenarios.

The physicallayer is composed of Transmission Convergence Sublayer(TCS)and Physical Medium Dependent(PMD)Sublayer,but usually,the physical layer only refers to PMD.TCSsegments the received MAC data,and then encapsulates them into TCSProtocol Data Unit(PDU).PMD executes a series of handling processes such as channel encoding,modulation and demodulation.

With a layered structure,the MAC layer has the service-specific Convergence Sublayer(CS),MAC Common Part Sublayer(CPS)and Security Sublayer.

(1)The service-specific CS converges various services from the upper layers.It converts and maps external network data received through Service Access Points(SAP)into MAC service data units,and then transfers them to the SAPon the MAC layer.The protocol provides multiple CS specifications to implement interfaces with various external protocols.It supports transparent transmission of such protocol data as Asynchronous Transfer Mode(ATM)and IP.

(2)The MAC CPSimplements major MAC functions,including network access,bandwidth allocation,connection establishment and connection maintenance.It receives data from various CSs through the SAPs on the MAClayer,and then classifies and distributes the data to specific MAC connections.Moreover,it controls QoSof data transported and scheduled by the physical layer.

(3)The security sublayer is mainly responsible for authentication,key exchange,encryption and decryption.It supports 128-byte,192-byte and 256-byte encryption system,and uses digital certificates for authentication.By these ways,the security sublayer guarantees the secure transmission of information.

3 Basic Features of 802.16 Physical Layer

802.16 defines five physical-layer technologies.They are Single-Carrier(SC)modulation,Single-Carrier access(SCa),Orthogonal Frequency Division Multiplexing(OFDM),Orthogonal Frequency Division Multiple Access(OFDMA)and High-speed Unlicensed Metropolitan Area Network(HUMAN).The SC technology is used for 10-66 GHz,SCa,OFDM and OFDMA for licensed bands below 11 GHz,while HUMAN for unlicensed bands below 11 GHz.OFDMand OFDMAare two leading technologies for licensed frequency bands below 11 GHz with high frequency efficiency and advantages in mitigation of the multipath effect,frequency selective fading and narrowband interference.

802.16 supports a system flexibly operating in bandwidths from 1.25 MHz to 20 MHz,which allows the system to balance the bandwidth and the number of users accessible.Since different countries have assigned different channel bandwidths for fixed radio access systems,802.16 specifies several bandwidth series,which are multiples of 1.25 MHz(1.25/2.5/5/10/20 MHz etc)or those of 1.75 MHz(1.75/3.5/7/14 MHz etc).The fixed radio access systems operating in 10-66 GHz can also use the channel bandwidth of 25 MHz and 28 MHz to provide a higher access rate.

With regard to modulation,SC supports Quadrature Phase Shift Keying(QPSK)and16-Quadrature Amplitude Modulation(16-QAM),and optionally 64-QAM.Among the five physical-layer technologies,SCa uses the most modulation methods,including Binary Phase Shift Keying(BPSK),QPSK,16-QAM,64-QAM and optionally 256-QAM.Every subcarrier on an OFD-based physicallayer can support BPSK,QPSK,16-QAM and 64-QAM,out of which 64-QAM is optional for unlicensed frequency bands.OFDMA supports QPSK,16-QAM and optionally 64-QAM.HUMAN can use either SCa,or OFDM,or OFDMA.Compared to 3GPP High Speed Downlink Packet Access(HSDPA)that only supports 16-QAM,WiMAXuses the modulation methods with higher orders to achieve higher transmission rates.Therefore,WiMAX aims to fulfilla very high peak rate when the channel condition is good.This is different from 3G systems that focus on full converage and high-speed mobility.

◀Figure 2.Frequency spectrum structure of an OFDM signal.

An 802.16 based system is enforced to support Reed-Solomon(RS)convolutional and block codes.The optionalchannelencoding methods include block and convolutional Turbo codes with strong error correction capability but long decoding delay,and Low Density Parity Check(LDPC)codes with low complexity and short delay.

The physicallayer defines both Time Division Duplex(TDD)and Frequency Division Duplex(FDD)to meet requirements of different national(or regional)telecommunications systems.FDD requires paired frequencies,but TDD does not.Moreover,TDDcan adaptively adjust uplink and downlink transmission time to achieve the goal of bandwidth adjustment.Both TDDand FDD use the transmission format of burst data to support adaptive burst service data,and with the assistance of the MAC layer,they can dynamically adjust transmission parameters(such as the modulation scheme,encoding scheme and transmitting power).In addition,FDD is also specified to support half duplex terminals in 802.16e,which has a lower requirement on terminals.

802.16d/e adopts Time Division Multiple Access(TDMA)in uplink channels and Time Division Multiplexing(TDM)in downlink channels.The uplink channels are divided into multiple time slots.The applications in the uplink channels like initialization,competition,maintenance and service transmission are fulfilled by using a certain number of time slots.The number of time slots an uplink application uses is controlled by the MAC layer of a base station,and is changed dynamically according to the requirements of system performance optimization.In the downlink channels,the messages a base station sends to different subscribers are multiplexed into a single data flow,and then broadcasted to allthe terminals in the cell.

OFDMA is another scheme to multiple access,which will be discussed in detail in the following parts.

4 Key Technologies for 802.16 Physical Layer

4.1 OFDM and OFDMA

The basic idea of the OFDM technology is to divide the usable bandwidth of a channel into several orthogonal subcarriers,and data are transported on each subcarrier independently.In this way,OFDM can fulfilllow-speed parallel transmission of high-speed serial data flows.OFDM is evolved from legacy Frequency Division Multiplexing(FDM),but it implements the division of subcarriers by the Discrete Fourier Transform(DFT)and Inverse Discrete Fourier Transform(IDFT),rather than by the traditionalband-pass filter.The subcarriers,as shown in Figure 2,may partly overlap,but still keep their orthogonality.This greatly improves the frequency efficiency of the OFDM system.Moreover,the low-speed paralleltransmission of data enhances OFDM's capability of mitigating the multipath effect and frequency selective fading.Therefore,OFDM is adopted as a basic physical-layer technology by a large number of new generation radio communications systems,such as WLAN,Digital Audio Broadcasting(DAB),Digital Video Broadcasting(DVB)and B3G systems.

OFDMAintegrates Frequency Division Multiple Access(FDMA)into the OFDM technology.It allocates usable subcarrier resources in a channel to different users.According to specific methods of subcarrier allocation,OFDMA can be divided into subchannel OFDMA and hop frequency OFDMA.

The subchannel OFDMAdivides the bandwidth of an OFDM system into multiple subchannels;each subchannel has several subcarriers.Users are allocated with frequency resources in the unit of the subchannel.As shown in Figure 3,the subcarriers allocated to a subchannel can be either consecutive(i.e.,centralized)or alternate with the subcarriers allocated to other subchannels(i.e.,distributed).The feature of the subchannel OFDMA is that the subcarrier allocation is fixed.Therefore,the inter-cell coordination is complicated to avoid the interference caused by cells'resource scheduling in a multi-cell environment.

The hop frequency OFDMAquickly changes the allocation of subcarriers to each user by the hop frequency,as shown in Figure 4.It randomly distributes subcarriers,but the subcarrier sets allocated to different users are not permitted to overlap in the same time slot.Since the hop frequency OFDMA system has a short period for subcarrier source scheduling,the possible inter-cell interference is dispersed in both the time and frequency domains.

Therefore,in networks with light traffic load,the hop frequency OFDMA will effectively hold the interference down even if there is no inter-cell coordination.

▲Figure 3. Subcarrier allocation in subchannel OFDMA.

▲Figure 4. Subcarrier allocation in hop frequency OFDMA.

The OFDM physical layer of a WiMAX system has 256 subcarriers.But the OFDMA can fulfill flexible change between 128,512,1,024 and 2,048 subcarriers,and corresponding channel bandwidth can be adjusted from 1.75 MHz to 20 MHz.Therefore,the OFDMA system has more powerful capabilities of channel equalization and anti-rapid-fading to guarantee that WiMAXterminals in mobile environments are working.OFDMAimplements multiple user access by the hop frequency.For example,an OFDMA system with 2,048 subcarriers divides all the usable subcarriers into 32 subchannels;the subcarrier allocation can be either centralized or distributed;the frequency domain unit of the hop frequency is a subchannel,and the time domain unit is two,three or six OFDM symbol periods.

4.2 Multi-antenna

Multi-antenna technology can manifoldly improve the channel capacity without adding system bandwidth.It can accordingly fulfill a higher data rate and larger coverage,or improve the transmission quality of signals.Therefore,all kinds of new radio communication systems favor the multi-antenna technology in recent years,and WiMAX system is no exception.

The 802.16 standards support two types of multi-antenna technology:Multiple-Input Multiple-Output(MIMO)and Adaptive Antenna System(AAS).

(1)MIMO

The MIMO system sends/receives signals by using multiple antennas at both the base station and terminal in order to restrain channel fading and improve system performance.As an optional technology in the 802.16 protocols,MIMO has three application models:spatial diversity,spatial multiplexing and the combination of spatial diversity and spatial multiplexing.

The spatial diversity includes transmit diversity and receive diversity.It transmits signals with the same message through different antennas or combines the received signals from different antennas to obtain diversity gain for mitigating channel fading,improving communications quality and expanding system coverage.The transmit diversity is mainly implemented by space-time coding,and 802.16 adopts the classical Alamoutispace-time code for two transmit antennas to achieve the maximum diversity gain.

The spatial multiplexing model transmits signals separately through different antennas while it decodes signals at the receiving end by interference suppression.In this way,it obtains spatial multiplexing gain for improving the data rate and system capacity.This model is mainly implemented by various layered space-time codes,such as the Bell-labs Layered Space-Time(BLAST)algorithm.

The combination model has the strengths of both the spatial diversity and multiplexing.It makes a compromise between the spatialdiversity and spatial multiplexing.Therefore,it can not only improve transmission quality,but also expand system capacity.However,its processing is more complicated than the other two models.

In order to further improve system performance,802.16e also supports the closed-loop MIMO technology,with which the transmit end can acquire all feedback information about channel environments.Then,the system in operation can make conversions among different MIMO models according to the application environments and channel conditions.

(2)AAS

AASis a mobile communications technology based on the adaptive antenna concept.With Space Division Multiple Access(SDMA)technique,it generates a directional beam in space by digital signal processing.The main lobe of the antenna is placed toward the Direction of Arrival(DOA)of the Signalof Interest(SOI),and side lobes or nulls toward the DOA of the Signal not of Interest(SNOI).In this way,it restrains co-channel interference,and fulfills the optimal signal receiving.

AASis optionalin the WiMAXsystem,available for both the uplink and downlink.It is used to mitigate the deep fading caused by Obstructed-Line-Of-Sight(OLOS)and Non-Line-Of-Sight(NLOS)propogation environments.The design and application of AASare based on TDD,because the frequency resources are shared by the uplink and downlink in the TDD system,which facilitates both the base station and terminal to compute beam-forming weights.In the FDD system,however,the uplink and downlink use different channels,so beam-forming weights can only be acquired through feedback,which will increase the system overhead.

4.3 Adaptive Modulation and Coding(AMC)

The time-varying and fading features of a wireless channel results in a random channelcapacity which changes with time.Therefore,to change the transmit rate accordingly is necessary for the full use of the channelcapacity,that is to say,the modulation and coding mode should be adaptive.The AMC technology dynamically adjusts the modulation and coding mode according to channel conditions,so as to improve data rate or system throughput.When channel conditions are good,it uses high-order modulation and high coding rate(such as 64-QAM and the coding rate of 5/6)to achieve a high peak rate;when channel conditions are bad,it uses low-order modulation and low coding rate(such as QPSK and the coding rate of 1/2)to guarantee the transmission performance.

The performance improvement by adjusting the modulation and coding mode,rather than by changing the transmit power,may,to a great extent,avoid extra interference caused by the increase of the transmit power.

The physical layer of the 802.16 system provides channel quality feedback mechanism.The base station and terminal can measure the Received Signal Strength Indication(RSSI)and Carrier-to-Interference-and-Noise Ratio(CINR)in uplink and downlink channels if necessary,and then report the measurements through the feedback channel.RSSIand CINRare main parameters to measure channel conditions.It is notable that channel changes should not be very quick,otherwise the adjustment of the measurements will fail to follow the channelchanges.Therefore,the AMC technology is only applicable to environments with small Doppler spread,especially the indoor environment,where AMC can be used frame by frame.

4.4 Hybrid-Automatic Repeat Request(H-ARQ)

H-ARQ is a physical-layer technology that integrates Automatic Repeat Request(ARQ)and Forward Error Correction(FEC)codes.It is used to mitigate the negative impact on data transmission caused by channel and interference jitter.

Basically,H-ARQ works as follows:

(1)One or more data units waiting to be sent to the MAC layer are serially encoded into four H-ARQ sub-packets.

(2)The base station just sends an H-ARQ sub-packet one time.Since the four sub-packets are closely related,the receiving end can correctly decode with no need to get all the sub-packets.Therefore,once the receiving end receives the first sub-packet,it will start to decode it.If the decoding is successful,the receiving end will immediately send an ACK message back to the base station;if the decoding fails,the receiving end will send back an NACKmessage to ask the base station to send the next sub-packet.

(3)The terminal will decode all the sub-packets it has received to increase the success probability of decoding.

The H-ARQ working process shows that it uses the simplest stop-and-wait ARQ to reduce both the control overhead and transmit/receive cache.However,the stop-and-wait protocols have a drawback of low channel efficiency.If an OFDMAphysicallayer is used together with H-ARQ,this drawback will be perfectly improved.Therefore,the 802.16 protocols only specify the OFDMA physical layer to support H-ARQ.

4.5 Power Control

802.16e specifies that power control is necessary for both the uplink and downlink,in order to improve system performance.The overall transmit power consists of fixed and dynamic parts.

On the uplink,open-loop power control is used at the moment a terminal is accessing to a WiMAXnetwork;the downlink loss is used to estimate the uplink loss,and then the transmit power is adjusted to achieve expected Signal-to-Noise Ratio(SNR).

After a terminal accesses the WiMAX network,the default power controlmode becomes closed loop.The base station measures the SNRin the uplink channel and compares it with the target SNR.

Then,it sends a command to the terminal to adjust its transmit power on a real-time basis.

On the downlink,the power control is closed loop,and combined with AMC to perform. (to be continued)