Cordless Systems and Wireless Local Loop

1. 张宝贤：《无线网络技术》 Cordless Systems and Wireless Local Loop Chapter 11

2. 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

3. 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

4. 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

5. 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

6. 每传输一个burst占用信道时间:Tp+Tb+Tg

7. 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

8. frame 基站 …… 用户端 # 2 # 3 # 1 # 1 # 2 # 3 UL DL 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. TDMA TDD 简化示意图

9. 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

10. DECT Protocol Architecture

11. 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

12. 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

13. 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

14. 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

15. ADPCM Encoder

16. ADPCM Decoder

17. 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

18. 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

19. 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)

20. WLL Configuration

21. 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）

22. 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

23. 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

24. 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

25. Fresnel Zone菲捏耳区 沿着或接近LOS直线的地方必须避免障碍物，包括树叶 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

26. 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

28. 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

29. 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

30. 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,…

31. 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。

32. 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。

33. 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

34. 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

35. 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

36. 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

37. 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)

38. 802.16f Fixed MIBs 802.16-2004/Cor1 Corrigendum 343 members 802.16-2004 Fixed Broadband Wireless Standard (Revised : Covers <11 GHz NLOS & 10-66 GHz LOS Systems) Membership 2005 802.16e Combined Fixed and Mobile Amendment for <11 GHz Licensed Systems(Formally approved in December 2005) 802.16c System Profiles for 10-66 GHz LOS Systems(Inactive) 2004 55 members 65 members 802.16a Fixed Broadband Wireless Standard for 2-11 GHz Non-LOS Systems(Inactive) 802.16 Fixed Broadband Wireless Standard for 10-66 GHz LOS Systems(Inactive) IEEE 802.16 Working Group Started 2002 1999 2003 WiMAX Forum now certifying IEEE 802.16-2004 fixed products! IEEE 802.16e mobile products will follow! Time 2006 802.16 Standard History

39. License-exempt TV bands, Non-exclusive licensed bands (US: FCC 3650-3700 MHz) 802.16-2004 Fixed Applications (licensed/unlicensed) Cor1 Corrections 802.16h License Exempt Co-existence Procedures (expected in March 2007) 802.16e Combined Fixed and Mobile Applications (licensed) Planned 802.16j Multi-hop/Relay Project (expected in 2008) 802.16f Fixed MIBs Planned 802.16i Mobile MIBs Project (expected in March 2007) 802.16g Fixed and Mobile Management(expected to complete in March 2007) Standard Under development Future project Standard Evolution 802.16 standard evolution based on latest technologies

40. Scalability 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) Flexible frequency re-use schemes for network planning High Data Rates Larger MAC frames with low overhead, Adaptive Modulation, Advanced FEC, H-ARQ, Beamforming (AAS), Space-Time Transmit Diversity, MIMO QoS QoS with Service Flows, Advanced Scheduling Framework, Adaptive Modulation & Coding, ARQ, H-ARQ Mobility Secure Handover, Optimized Hard Handover, Multicast, Broadcast, Paging, Power Management with Sleep and Idle Modes EAP authentication, Encryption with AES-CCM, CMAC Authentication, X.509 Certificates, Key Binding, Mutual Authentication, Device and User Authentication Security Salient Features

41. 802.16传输环境

42. 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

43. WiMAX Forum Technical Liaisons • IETF • 16ng (i.e., IP over IEEE 802.16(e) NetworksWG) – 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

44. Industry Mobile WiMAX Solutions Roadmap 2006 2007 2008 2009 2007 2008 * Names and Brands may be claimed as property of others

45. Mobile WiMAX = Mobile Internet

46. 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

47. IEEE 802.16 Protocol Architecture -Tanenbaum, computer networks, 第四版

48. 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

49. 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

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