1、1,Wireless Communication and Network,Chapter 4 Mobile Cellular Systems,2,蜂窝式网络,在数据通信和电信方面的所有重大进步中，或许最为革命性的进步要数蜂窝式网络的发展。蜂窝技术是移动无线通信的基础，它能为有线网络所难以服务的用户提供支持。蜂窝技术是移动电话、个人通信系统、无线因特网、无线Web应用以及其他更多技术的基础技术。 本章介绍所有蜂窝网络所需用到的基本原理，考察已经被分为三代的蜂窝技术和标准： 第一代是基于模拟的，尽管仍在广泛使用，却已在逐步淘汰中； 今天的主流技术是数字式的第二代系统； 第三代高速数字式系统也已开始
2、出现。,3,Cellular Concept Cellular Architecture Frequency Reuse Multiple Access Methods FDMA, TDMA, and CDMA In particular, we focus on CDMA.,Outline,4,Cellular Systems Terms,Base Station (BS) includes an antenna, a controller, and a number of receivers Mobile telecommunications switching office (MTSO)
3、 connects calls between mobile units Two types of channels available between mobile unit and BS Control channels used to exchange information having to do with setting up and maintaining calls Traffic channels carry voice or data connection between users,5,Steps in an MTSO Controlled Call between Mo
4、bile Users,Mobile unit initialization Mobile-originated call Paging Call accepted Ongoing call Handoff,6,移动设备初始化 移动设备发起的呼叫 寻呼 接收呼叫 接续呼叫 越区切换,7,Additional Functions in an MTSO Controlled Call,Call blocking(呼叫阻塞 ) Call termination(结束呼叫 ) Call drop(掉话 ) Calls to/from fixed and remote mobile subscriber(
5、拨往/来自固定用户和远程移动用户的呼叫 ),8,Mobile Radio Propagation Effects,Signal strength(信号强度 ) Must be strong enough between base station and mobile unit to maintain signal quality at the receiver Must not be so strong as to create too much cochannel interference(同信道干扰 ) with channels in another cell using the sam
6、e frequency band Fading(衰落 ) Signal propagation effects may disrupt the signal and cause errors,9,路径损耗的Hata模型举例：,设fc = 900MHz，ht = 40m，hr = 5m，且d=10km。试为一个中等规模的城市估计一下路径损失。A(hr) = (1.1lg9000.7)5 (1.56lg 9000.8) dB= 12.753.8 = 8.95 dB (移动天线高度的校正因子 )Ldb=69.55+26.16lg90013.82lg40-8.95+(44.96.55lg40) lg1
7、0= 69.55 + 77.28 22.14 8.95 + 34.4= 150.14 dB,10,Handoff Performance Metrics,Cell blocking probability(蜂窝阻塞概率 ) probability of a new call being blocked Call dropping probability(呼叫中断概率 ) probability that a call is terminated due to a handoff Call completion probability(呼叫完成概率 ) probability that an a
8、dmitted call is not dropped before it terminates Probability of unsuccessful handoff(不成功越区转换的概率 ) probability that a handoff is executed while the reception conditions are inadequate,11,Handoff Performance Metrics,Handoff blocking probability(越区切换阻塞概率 ) probability that a handoff cannot be successfu
9、lly completed Handoff probability(越区切换概率 ) probability that a handoff occurs before call termination Rate of handoff(越区概率 ) number of handoffs per unit time Interruption duration(中断历时 ) duration of time during a handoff in which a mobile is not connected to either base station Handoff delay(越区切换时延 )
10、 distance the mobile moves from the point at which the handoff should occur to the point at which it does occur,12,Handoff Strategies Used to Determine Instant of Handoff,Relative signal strength Relative signal strength with threshold Relative signal strength with hysteresis(滞后) Relative signal str
11、ength with hysteresis and threshold Prediction techniques,13,两蜂窝区之间的越区切换,14,Power Control,Design issues making it desirable to include dynamic power control in a cellular system Received power must be sufficiently above the background noise for effective communication Desirable to minimize power in
12、the transmitted signal from the mobile Reduce cochannel interference, alleviate(减轻) health concerns, save battery power In SS systems using CDMA, its desirable to equalize the received power level from all mobile units at the BS,15,Types of Power Control,Open-loop power control Depends solely on mob
13、ile unit(只依赖于移动单元) No feedback from BS Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength Closed-loop power control Adjusts signal strength in reverse channel based on metric of performance BS makes power adjustment decision and communicates to mobile on control
14、 channel,16,反向链路的功率控制,17,Traffic Engineering,Ideally, available channels would equal number of subscribers active at one time In practice, not feasible to have capacity handle all possible load For N simultaneous user capacity and L subscribers L N blocking system,18,Blocking System Performance Ques
15、tions,Probability that call request is blocked? What capacity is needed to achieve a certain upper bound on probability of blocking? What is the average delay? What capacity is needed to achieve a certain average delay?,19,Different Generations,1G analog 2G digital 3G Develop a single global standar
16、d for high speed data and high quality voice services. 4G Wireless broadband infrastructure to support multimedia information society,20,1G Cellular Systems,Many Different Standards: AMPS (US) NMT (Northern Europe) TACS (Europe) NTT (Japan) many others. Spectrum around 800 and 900 MHz Advanced Mobil
17、e Phone Service (AMPS) In North America, two 25-MHz bands allocated to AMPS One for transmission from base to mobile unit One for transmission from mobile unit to base Each band split in two to encourage competition Frequency reuse exploited,21,Frequency Division Duplex (FDD),Forward Link,Reverse Li
18、nk,Two separate frequency bands are used for forward and reverse links. Typically, 25 MHz in each direction. AMPS: 824-849 MHz (forward) 869-894 MHz (reverse),mobile,base station,22,Frequency Division Multiple Access (FDMA),The spectrum of each link (forward or reverse) is further divided into frequ
19、ency bands Each station assigned fixed frequency band,frequency bands,idle,idle,idle,23,AMPS Operation,Subscriber initiates call by keying in phone number and presses send key MTSO verifies number and authorizes user MTSO issues message to users cell phone indicating send and receive traffic channel
20、s MTSO sends ringing signal to called party Party answers; MTSO establishes circuit and initiates billing information Either party hangs up; MTSO releases circuit, frees channels, completes billing,图10.11 AMPS控制信道的帧格式,25,Number of User Channels in AMPS,Bandwidth allocated to each user in each link (
21、forward or reverse) is 30 KHz.No. of user channels = Total bandwidth / user bandwidth= 25 MHz / 30 kHz = 833 Is it enough?,26,Frequency Reuse,f,f,The same frequency can be reused in different cells, if they are far away from each other,Radio coverage, called a cell.,27,Cellular Architecture,MS Mobil
22、e Station BSC Base Station Controller MSC Mobile Switching Center PSTN Public Switched Telephone Network,MSC (MTSO),PSTN,BSC,segmentation of the area into cells,MS,28,Geometric Representation,Cells are commonly represented by hexagons.Why hexagon? How about circle? How about square, or triangle?,29,
23、Hexagonal Cells,30,Channel Reuse,The total number of channels are divided into K groups. K is called reuse factor or cluster size. Each cell is assigned one of the groups. The same group can be reused by two different cells provided that they are sufficiently far apart.,31,Example: K = 7,32,33,Reuse
24、 Distance,How far apart can two users share the same channel? It depends on whether signal quality is acceptable or not. The larger the distance between the two users, the better the signal quality.How to measure signal quality?,34,Review: Nyquist Bandwidth,For binary signals (two voltage levels) C
25、= 2B With multilevel signaling C = 2B log2 M M = number of discrete signal or voltage levels,35,Signal Quality,The signal quality depends on the ratio between signal power and interference (noise) power.This is called signal-to- noise (interference) ratio (SNR or SIR).,Interference from the i-th int
26、erfering BS.,36,Signal-to-Noise Ratio,Ratio of the power in a signal to the power contained in the noise thats present at a particular point in the transmission Typically measured at a receiver Signal-to-noise ratio (SNR, or S/N)A high SNR means a high-quality signal, low number of required intermed
27、iate repeaters SNR sets upper bound on achievable data rate,37,Shannon Capacity Formula,Equation:Represents theoretical maximum that can be achieved In practice, only much lower rates achieved Formula assumes white noise (thermal noise) Impulse noise is not accounted for Attenuation distortion or de
28、lay distortion not accounted for,38,Example of Nyquist and Shannon Formulations,Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dBUsing Shannons formula,39,Example of Nyquist and Shannon Formulations,How many signaling levels are required?,40,Propagation Model,The received signal power de
29、pends on the distance between the transmitter and the receiver.P0 is the power received at a reference distance d0. is called the path loss exponent. Typically, 3 5.,41,Worst-case Analysis,Assumption: The user is located at the corner of a cell.,Di is the distance between the center of the reference
30、 cell and the i-th interfering cell.,42,Signal-to-Noise (Interference) Ratio (SNR),Consider only the 1st tier of interfering cells,R,D,43,Example,Suppose the SNR is required to be greater than 18 dB. What is the minimum reuse factor? (assume = 4)Before answering this question, we need some results o
31、btained by geometry,44,Coordinate System,Use (i,j) to denote a particular cell.Example: Cell A is represented by (2,1).,A,45,R,D,Distance Formula,Note: i and j are integers,where,Reuse factor,46,Solution,Choose i = 2, j = 1 or i = 1, j = 2.,Ans: K = 7,47,2G Cellular Systems,Four Major Standards: GSM
32、 (European) IS-54 (later becomes IS-136, US) JDC (Japanese Digital Cellular) IS-95 (CDMA, US),48,Differences Between First and Second Generation Systems,Digital traffic channels first-generation systems are almost purely analog; second-generation systems are digital Encryption all second generation
33、systems provide encryption to prevent eavesdropping Error detection and correction second-generation digital traffic allows for detection and correction, giving clear voice reception Channel access second-generation systems allow channels to be dynamically shared by a number of users,49,GSM-全球移动通信系统
34、,在全球移动通信系统开发出来之前，欧洲各国使用大量不兼容的第一代蜂窝电话技术。 GSM被开发出来后，为欧洲提供了一个普遍的第二代技术，于是同一个用户设备在整个大陆都可使用。 该技术特别成功，且很可能成为新应用中最通用的全球标准。 GSM最初在1990年出现于欧洲。类似系统已在北美、亚洲、北非、中东和澳大利亚得到应用。 GSM协会声称至2000年底全球有将近7.5亿用户，其中主要在欧洲和亚太，但北美有超过800万。,50,Example: GSM,Frequency Band 935-960, 890-915 MHz Two pieces of 25 MHz band (same as AMPS
35、) AMPS has 833 user channels How about GSM?,51,Time Division Multiple Access (TDMA),The mobile users access the channel in round-robin fashion. Each station gets one slot in each round.,Slots 2, 5 and 6 are idle,52,FDMA/TDMA, example GSM,1,2,3,7,8,f,t,124,1,124,1,20 MHz,200 kHz,890.2 MHz,935.2 MHz,9
36、15 MHz,960 MHz,Each freq. carrier is divided into 8 time slots.,53,Number of channels in GSM,Freq. Carrier: 200 kHz TDMA: 8 time slots per freq carrierNo. of carriers = 25 MHz / 200 kHz= 125 No. of user channels = 125 * 8 = 1000,54,Capacity Comparison,Reuse factor 7 for AMPS 3 for GSM (why smaller r
37、euse factor?)Whats the capacity of GSM relative to AMPS?A. one half of AMPS B. the sameC. 3 times larger D. 10 times larger,55,Answer,AMPS reuse factor = 7 no. of users / cell = 833 / 7 = 119 GSM reuse factor = 3 no. of users / cell = 1000 / 3 = 333 almost 3 times larger than AMPS!,56,Multiple Acces
38、s Methods,Three major types: Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Frequency hopping (FH-CDMA) Direct sequence (DS-CDMA),57,Frequency-Time Plane,Time,Frequency,Partition of signal space into time slots and frequency bands,
39、58,FDMA,Time,Frequency,Different users transmit at different frequency bands simultaneously.,59,TDMA,Time,Frequency,Different users transmit at different time slots. Each user occupy the whole freq. spectrum.,60,Frequency Hopping CDMA,Frequency,Time,At each successive time slot, the frequency band a
40、ssignments are reordered. Each user employs a code that dictates the frequency hopping pattern.,61,Synchronization,The previous figure implies that each signal synchronizes with each of the other signals. In practice, this is not the case. Frequency hops may collide, but it does not occur frequently
41、. How often collisions occur depends on the choice of codes.,62,Direct Sequence CDMA,Time,Frequency,All users occupy the whole bandwidth all the time. Signals of different users overlap with one other. How can it be done?,63,CDMA Encoding,Each user is assigned a unique signature sequence (or code),
42、denoted by (c1,c2,cM). Its component is called a chip.Each bit, di, is encoded by multiplying the bit by the signature sequence:Zi,m = di cm,64,Encoding Example,Data bit d1 = 1 Signature sequence (c1,c2,c8) = (+1,+1,+1,1,+1,1,1,1) Encoder Output (Z1,1,Z1,2,Z1,8) = (1,1,1,+1,1,+1,+1,+1),65,Bandwidth,
43、Note that the chip rate is much higher than the data rate. Consider our previous example. Suppose the original data signal occupies a bandwidth of W. What is the bandwidth of the encoded signal?,66,Spread Spectrum Technique,Time,Frequency,Time,Frequency,Encoding,The bandwidth expands by a factor of
44、M. M is called spreading factor or processing gain.,67,CDMA Decoding,Without interfering users, the receiver would receive the encoded bits, Zi,m , and recover the original data bit, di, by computing:,68,CDMA Decoding Example,(c1,c2,c8) = (+1,+1,+1,1,+1,1,1,1)(Z1,1,Z1,2,Z1,8) = (1,1,1,+1,1,+1,+1,+1)
45、(1,1,1,1,1,1,1,1)di = 1,multiply,add and divide by M,69,70,Multiuser Scenario,If there are N users, the signal at the receiver becomes:How can a CDMA receiver recover a users original data bit?,71,Multiplied by the signature sequence of user 1,2-user example,72,Signature Sequences,In order for the r
46、eceiver to be able to extract out a particular senders signal, the CDMA codes must be of low correlation. Correlation of two codes, (cj,1, cj,M) and (ck,1, ck,M) , are defined by inner product:,73,The Meaning of Correlation,What is correlation? It determines how much similarity one sequence has with
47、 another. It is defined with a range between 1 and 1.,Other values indicate a partial degree of correlation.,74,Generation of Signature Sequences,How to generate signature sequences of low correlation?There are two classes of signature sequences that are widely used in CDMA systems. Orthogonal Codes
48、 Pseudo Noise Sequences (PN Sequences),75,Orthogonal Codes,Two codes are said to be orthogonal if their correlation is zero. no interference between the two users. In our previous two-user example, the codes are orthogonal. How to generate orthogonal codes?,76,Walsh Codes,The most common orthogonal codes used in CDMA systems. A set of Walsh codes of length n is defined by the rows of an n n Hadamard matrix. Hadamard matrix can be constructed by an iterative method.,
