Cordless Systems and Wireless Local Loop

Chapter 11

张宝贤：《无线网络技术》

Cordless System Operating Environments Residential – a single base station can

provide in-house voice and data support Office

A single base station can support a small office Multiple base stations in a cellular

configuration can support a larger office Telepoint – a base station set up in a public

place, such as an airport

Design Considerations for Cordless Standards Modest range of handset from base station,

so low-power designs are used Inexpensive handset and base station,

dictating simple technical approaches Frequency flexibility is limited, so the

system needs to be able to seek a low-interference channel whenever used

Time Division Duplex (TDD) TDD also known as time-compression

multiplexing (TCM) Data transmitted in one direction at a time,

with transmission between the two directions Simple TDD TDMA TDD

Simple TDD Bit stream is divided into equal segments,

compressed in time to a higher transmission rate, and transmitted in bursts

Effective bits transmitted per second:

R = B/2(Tp+Tb+Tg)

R = effective data rate B = size of block in bits Tp = propagation delay Tb = burst transmission time Tg = guard time

每传输一个 burst 占用信道时间 :Tp+Tb+Tg

Simple TDD Actual data rate, A:

A = B /Tb

Combined with previous equation:

The actual data rate is more than double the effective data rate seen by the two sides

b

gp

T

TTRA 12

TDMA TDD Wireless TDD typically used with TDMA

A number of users receive forward channel signals in turn and then transmit reverse channel signals in turn, all on same carrier frequency

基本上消除了 Simple TDD 方案中 , 每次 burst 传输所占用的传播时延 Tp.

# 1 # 2 # 3 # 1 # 2 # 3用户端

基站

frame

……

UL DLTDMA TDD简化示意图

TDMA TDD Advantages of TDMA/TDD:

Improved ability to cope with fast fading 通过接收器配备两个间隔一个波长的天线——这

样做的好处在于 : 当其中一根天线遇到明显的快衰落 (fast fading) 时，另一根很可能不会。

虽然移动终端不太可能象基站那样安装两个天线，但基站可以选择信号强的那个天线进行信号发送——从而大大降低移动终端可能遭遇的衰落——因为前向和后向使用的频率相同

大大提高了带宽利用率（与 simple TDD 相比） 通过动态时隙分配——时隙变长、动态预约、帧

长可变 Tightly packed transmissions in each directions

DECT Protocol Architecture

DECT Protocol Architecture Physical layer – data transmitted in TDMA-TDD

frames over one of 10 RF carriers Medium access control (MAC) layer – selects/

establishes/releases connections on physical channels; supports three services: Broadcast Connection oriented Connectionless

Data link control layer – provides for the reliable transmission of messages using traditional data link control procedures

Differential Quantization Speech signals tend not to change much between

two samples Transmitted PCM values contain considerable

redundancy Transmit difference value between adjacent samples

rather than actual value If difference value between two samples exceeds

transmitted bits, receiver output will drift from the true value Encoder could locally replicate receiver output and

additionally transmit that difference

Differential PCM (DPCM) Since voice signals change relatively slowly, value

of kth sample can be estimated by preceding samples

Transmit difference between sample and estimated sample Difference value should be less than difference between

successive samples At the receiver, incoming difference value is

added to the estimate of the current sample Same estimation function is used

Adaptive Differential PCM (ADPCM) Improve DPCM performance using

adaptive prediction and quantization Predictor and difference quantizer adapt to the

changing characteristics of the speech Modules

Adaptive quantizer Inverse （反向） adaptive quantizer Adaptive predictor

ADPCM Encoder

ADPCM Decoder

Subject Measurement of Coder Performance（编码器性能的主观测量）

Subjective measurements of quality are more relevant than objective measures

Mean opinion score (MOS) – group of subjects listen to a sample of coded speech; classify output on a 5-point scale

MOS scale is used in a number of specifications as a standard for quality

Wireless Local Loop Wired technologies responding to need for

reliable, high-speed access by residential, business, and government subscribers ISDN, xDSL, cable modems…last mile techniques

Increasing interest shown in competing wireless technologies for subscriber access

Wireless local loop (WLL), also called fixed wireless access ， Wireless MAN

WLL 与移动电话服务的几个区别

通常要求高速数据通信支持能力 可以采用定向天线 用户是不移动的（如房子），所以没有了

handover 的问题 WLL ：源于 1969 年，但到 1998 年开

始运作 (serious operation)

WLL Configuration

Advantages of WLL over Wired Approach Cost – wireless systems are less expensive due to

cost of cable installation that’s avoided Installation time – WLL systems can be installed

in a small fraction of the time required for a new wired system

Selective installation – radio units installed for subscribers who want service at a given time With a wired system, cable is laid out in anticipation of

serving every subscriber in a given area 当然， WLL 也有劣势，如安装之后受环境因素影

响大，如楼群建设而导致的障碍物（未完待续）

Propagation Considerations for WLL Most high-speed WLL schemes use millimeter

wave frequencies (10 GHz to about 300 GHz) There are wide unused frequency bands available above

25 GHz At these high frequencies, wide channel bandwidths

can be used, providing high data rates Small size transceivers and adaptive antenna arrays can

be used

Propagation Considerations for WLL Millimeter wave systems have some undesirable

propagation characteristics Free space loss increases with the square of the

frequency; losses are much higher in millimeter wave range

Above 10 GHz, attenuation effects due to rainfall and atmospheric or gaseous absorption are large

Multipath losses can be quite high 受上述因素限制，通常， WLL 的服务半径在几公

里范围

Fresnel Zone菲捏耳区 How much space around direct path between transmitter and

receiver should be clear of obstacles? （发射器和接收器间的直线上多大的空间内的障碍物应该被清除？） Objects within a series of concentric circles around the line of sight

between transceivers have constructive/destructive effects on communication

For point along the direct path, radius of first Fresnel zone:

S = distance from transmitter D = distance from receiver

DS

SDR

Fresnel Zone菲捏耳区

DS

SDR

发现：若两收发器间任意点的第一菲捏耳区半径的 0.6 倍无障碍的话，由障碍而产生的衰减可以忽略

如：两个收发器相距 10km ，载频 2.4GHz, 则两收发器中间 点的第一菲捏耳区半径为 17.7 米，如果信号频率为 28GHz, 则同一点上 R=5.17 米

沿着或接近 LOS 直线的地方必须避免障碍物，包括树叶

Atmospheric Absorption Radio waves at frequencies above 10 GHz

are subject to molecular absorption （分子吸收） Peak of water vapor absorption at 22 GHz Peak of oxygen absorption near 60 GHz

Favorable windows for communication: From 28 GHz to 42 GHz From 75 GHz to 95 GHz

Effect of Rain Attenuation due to rain

Presence of raindrops can severely degrade the reliability and performance of communication links

The effect of rain depends on drop shape, drop size, rain rate, and frequency

Estimated attenuation due to rain:

A = attenuation (dB/km) R = rain rate (mm/hr) a and b depend on drop sizes and frequency

baRA

Effects of Vegetation植物效应 Trees near subscriber sites can lead to multipath

fading Multipath effects from the tree canopy are

diffraction and scattering （衍射和散射） Measurements in orchards ( 果园 ) found

considerable attenuation values when the foliage ( 树 ) is within 60% of the first Fresnel zone 但树的出现并没有阻止通信，但需要有充分的应对措施，如前向纠错。

Multipath effects highly variable due to wind

OFDM (正交频分复用 )也称多路载波调制 OFDM 在不同频率上使用多个载波信号，在每个信道

上发送一些位，这与 FDM类似。然而，在 OFDM 中，所有子信道为一个信息源所使用。

OFDM 中所有副载波都是某一基频的整数倍，即： fb, 2fb, 3fb,…

OFDM (正交频分复用 )

图中可以看出某个副载波的功率谱密度的峰值发生在其它副载波为 0 的点上——因而这些副载波具有最小的相互干扰。因此称这些副载波是正交的。

OFDM (正交频分复用 )

优点： 频率选择性衰落 (frequency-dependent fading)只

影响一些子信道而不是整个信号。 恰当的 FEC 可以很有效地保护这样的信号

在多径环境下，可以消除码间干扰 (intersymbol interference, ISI). 随着比特（或信号）之间距离变小， ISI对更高速率的数据流有更大的影响。

OFDM系统中，每个子信道上数据速率变为原始数据率的1/N, 则每个信号 (symbol) 占的时间变为原来的 N倍，这样可以大大降低 ISI 效应。

Multipoint Distribution Service (MDS) Multichannel multipoint distribution service

(MMDS) Also referred to as wireless cable Used mainly by residential subscribers and small

businesses About 50 km range

Local multipoint distribution service (LMDS) Appeals to larger companies with greater bandwidth

demands 2-5 km

Advantages of MMDS MMDS signals have larger wavelengths and

can travel farther without losing significant power

Equipment at lower frequencies is less expensive

MMDS signals don't get blocked as easily by objects and are less susceptible to rain absorption

Advantages of LMDS Relatively high data rates Capable of providing video, telephony, and

data Relatively low cost in comparison with

cable alternatives 但受 LOS 和雨的影响

802.16 Standards Development（ LMDS 的一个标准） Use wireless links with microwave or millimeter

wave radios Use licensed spectrum Are metropolitan in scale Provide public network service to fee-paying

customers Use point-to-multipoint architecture with

stationary rooftop or tower-mounted antennas Now starts to support mobile users and related

standardization is still in progress

802.16 Standards Development Provide efficient transport of heterogeneous traffic

supporting quality of service (QoS) Use wireless links with microwave or millimeter

wave radios Are capable of broadband transmissions (>2

Mbps)

802.16 Standard History 20062006

802.16a Fixed 802.16a Fixed Broadband Broadband

Wireless Standard Wireless Standard for 2-11 GHz Non-for 2-11 GHz Non-

LOS SystemsLOS Systems((Inactive)

65 members65 members19991999

2002200220032003

20042004

20052005

802.16c System Profiles 802.16c System Profiles for 10-66 GHz LOS for 10-66 GHz LOS

SystemsSystems(Inactive)(Inactive)

343 members343 members

Mem

bers

hip

Time

IEEE 802.16 IEEE 802.16 Working Working

Group StartedGroup Started

55 members55 members

802.16e 802.16e Combined Fixed Combined Fixed

and Mobile and Mobile Amendment Amendment

for <11 GHz for <11 GHz Licensed Systems Licensed Systems

(Formally approved (Formally approved in December 2005)in December 2005)

802.16 Fixed 802.16 Fixed Broadband Wireless Broadband Wireless Standard for 10-66 Standard for 10-66 GHz LOS SystemsGHz LOS Systems

((Inactive)

WiMAX Forum now certifying IEEE 802.16-2004 fixed products!WiMAX Forum now certifying IEEE 802.16-2004 fixed products!IEEE 802.16e mobile products will follow!IEEE 802.16e mobile products will follow!

802.16f Fixed MIBs802.16f Fixed MIBs

802.16-2004/Cor1 802.16-2004/Cor1 CorrigendumCorrigendum802.16-2004 Fixed 802.16-2004 Fixed

Broadband Wireless Broadband Wireless StandardStandard

(Revised : Covers <11 GHz NLOS (Revised : Covers <11 GHz NLOS & 10-66 GHz LOS Systems)& 10-66 GHz LOS Systems)

Standard Evolution

802.16 standard evolution based on latest technologies802.16 standard evolution based on latest technologies

802.16-2004802.16-2004Fixed ApplicationsFixed Applications

(licensed/unlicensed)(licensed/unlicensed)

Cor1 Cor1 CorrectionsCorrections

802.16e 802.16e Combined Fixed and Mobile ApplicationsCombined Fixed and Mobile Applications

(licensed)(licensed)

802.16f 802.16f Fixed MIBsFixed MIBs

802.16g 802.16g Fixed and Mobile ManagementFixed and Mobile Management(expected to complete in March 2007)(expected to complete in March 2007)

Planned 802.16j Planned 802.16j Multi-hop/Relay ProjectMulti-hop/Relay Project

(expected in 2008)(expected in 2008)

Planned 802.16i Planned 802.16i Mobile MIBs ProjectMobile MIBs Project(expected in March 2007)(expected in March 2007)

802.16h License Exempt 802.16h License Exempt Co-existence ProceduresCo-existence Procedures

(expected in March 2007)(expected in March 2007)

License-exempt TV bands, License-exempt TV bands, Non-exclusive licensed bandsNon-exclusive licensed bands

(US: FCC 3650-3700 MHz)(US: FCC 3650-3700 MHz)

StandardStandardUnder developmentUnder developmentFuture projectFuture project

Salient Features

ScalabilityScalabilityScalable PHY for flexible channel bandwidths (1.25-20 MHz)Scalable PHY for flexible channel bandwidths (1.25-20 MHz)as global RF band allocations vary (e.g.: 2.3, 2.5, 3.5 GHz)as global RF band allocations vary (e.g.: 2.3, 2.5, 3.5 GHz)Flexible frequency re-use schemes for network planningFlexible frequency re-use schemes for network planning

QoSQoS QoS with Service Flows, Advanced SchedulingQoS with Service Flows, Advanced Scheduling Framework, Adaptive Modulation & Coding, ARQ, H-ARQ Framework, Adaptive Modulation & Coding, ARQ, H-ARQ

SecuritySecurityEAP authentication, Encryption with AES-CCM, EAP authentication, Encryption with AES-CCM,

CMAC Authentication, X.509 Certificates, Key Binding,CMAC Authentication, X.509 Certificates, Key Binding,Mutual Authentication, Device and User AuthenticationMutual Authentication, Device and User Authentication

MobilityMobilitySecure Handover, Optimized Hard Handover, Secure Handover, Optimized Hard Handover,

Multicast, Broadcast, Paging, Multicast, Broadcast, Paging, Power Management with Sleep and Idle ModesPower Management with Sleep and Idle Modes

High DataHigh DataRatesRates

Larger MAC frames with low overhead, Larger MAC frames with low overhead, Adaptive Modulation, Advanced FEC, H-ARQ, Adaptive Modulation, Advanced FEC, H-ARQ,

Beamforming (AAS), Space-Time Transmit Diversity, MIMOBeamforming (AAS), Space-Time Transmit Diversity, MIMO

802.16 传输环境

WiMAX Forum

•The WiMAX Forum, founded in April 2001, brings together leaders in the communications and computing industries to drive a common platform for the global deployment of IP-based broadband wireless services.

•The WiMAX Forum certifies products for conformance and interoperability based upon the IEEE 802.16-2004 and ETSI Hyperman standard.

•www.wimaxforum.org

WiMAX: Worldwide interoperatability for Microwave Access

WiMAX Forum Technical Liaisons

• IETF– 16ng (i.e., IP over IEEE 802.16(e) Networks WG) – Will publish RFCs for IPv6

and Ethernet CS (Convergence Sublayer) operation based on NWG (Network Working Group) specification

– Other dependencies – AAA, RADEXT, MIPv4/v6– Evaluating NETLMM

• IEEE– Close coordination with 802.16 TGs

• 3GPP– Liaison with TSG-SA for mobile WiMAX Interworking

• 23.882, 23.203

• 3GPP2– Liaison proposal under discussion

Industry Mobile WiMAX Solutions Roadmap

20072007 2008200820062006 20072007 20082008 20092009

* Names and Brands may be claimed as property of others

Mobile WiMAX = Mobile Internet

Pre-commercial Services Overview• Media service

– T-DMB service– M-log service– VOD/MOD service– Entertainment service

• Communication service– PTA (Push-to-all) service– Multi-party video conferencing service– Community service

• Data service– Internet access service– Search service

• Specialized service– PIS (Personal Intelligent Service)– Mydesk service– @mint service

Web Storage

PC at home

PC at office

Manage my data/contents anywhere anytime

IEEE 802.16 Protocol Architecture

-Tanenbaum, computer networks, 第四版

Protocol Architecture Physical and transmission layer functions:

Encoding/decoding of signals Preamble generation/removal Bit transmission/reception

Medium access control layer functions: On transmission, assemble data into a frame with

address and error detection fields On reception, disassemble frame, and perform address

recognition and error detection Govern access to the wireless transmission medium

Protocol Architecture Convergence layer functions:

Encapsulate PDU framing of upper layers into native 802.16 MAC/PHY frames

Map upper layer’s addresses into 802.16 addresses

Translate upper layer QoS parameters into native 802.16 MAC format

Adapt time dependencies of upper layer traffic into equivalent MAC service

IEEE 802.16.1 Services Digital audio/video multicast Digital telephony ATM Internet protocol Bridged LAN Back-haul Frame relay

MAC Management Messages Uplink and downlink channel descriptor Uplink and downlink access definition Ranging request and response Registration request, response and acknowledge Privacy key management request and response Dynamic service addition request, response and

acknowledge

MAC Management Messages Dynamic service change request, response,

and acknowledge Dynamic service deletion request and

response Multicast polling assignment request and

response Downlink data grant type request ARQ acknowledgment

Physical Layer – Upstream Transmission Uses a DAMA-TDMA technique Error correction uses Reed-Solomon code Modulation scheme based on QPSK, QAM-

16, or QAM-64

Physical Layer – Downstream Transmission Continuous downstream mode

For continuous transmission stream (audio, video) Simple TDM scheme is used for channel access Duplexing technique is frequency division duplex (FDD)

Burst downstream mode Targets burst transmission stream (IP-based traffic) DAMA-TDMA scheme is used for channel access Duplexing techniques are FDD with adaptive modulation,

frequency shift division duplexing (FSDD), time division duplexing (TDD)