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# Cellular Wireless Networks - PowerPoint PPT Presentation

Cellular Wireless Networks. Chapter 10. Cellular Network Organization. Use multiple low-power transmitters (100 W or less) Areas divided into cells Each served by its own antenna Each served by a base station consisting of transmitter, receiver, and control unit

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## PowerPoint Slideshow about ' Cellular Wireless Networks' - isanne

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### Cellular Wireless Networks

Chapter 10

• Use multiple low-power transmitters (100 W or less)

• Areas divided into cells

• Each served by its own antenna

• Each served by a base station consisting of transmitter, receiver, and control unit

• Each with band of frequencies

allocated

• Cells are set up in a way such

that antennas of all neighbors

are equidistant (hexagonal

pattern)

• Adjacent cells assigned different frequencies to avoid interference or crosstalk

• Objective is to reuse frequency in nearby cells

• 10 to 50 frequencies assigned to each cell

• Transmission power controlled to limit power at that frequency escaping to adjacent cells

• The issue is to determine how many cells must intervene between two cells using the same frequency

D/R=sqrt(3N)

• D: 使用相同频率的蜂窝区的最小中心距离

• R: 一个蜂窝区的半径

• N: 一个重用模型（在模型中每个蜂窝使用唯一的频率）的蜂窝区个数，称重用因子。

N的值可以是1, 3, 4, 7, 9, 12, 13, 16, 19, 21….

• Frequency borrowing – frequencies are taken from adjacent cells by congested cells

• Cell splitting – cells in areas of high usage can be split into smaller cells

• Cell sectoring – cells are divided

into a number of wedge-shaped

(楔形) sectors, each with their

own set of channels

• 减少同道干扰和邻道干扰的影响

• Microcells （微蜂窝区） –

antennas move to buildings,

hills, and lamp posts

• Base Station (BS) – includes an antenna, a controller, and a number of receivers

• Mobile telecommunications switching office (MTSO) – 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

• Mobile unit initialization:信道扫描, 注册等

• Mobile-originated call

• Paging: MTSO试图寻找被叫

• Call accepted

• Ongoing call

• Handoff

• Call blocking

• Call termination

• Call drop

• Calls to/from fixed and remote mobile subscriber (呼叫固定电话用户, 或远端移动用户)

• Signal strength

• 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 with channels in another cell using the same frequency band

• Signal propagation effects may disrupt the signal and cause errors

• 设计蜂窝系统布局

• Hata考虑不同环境和条件的的路径损耗模型

• 大城市、中小城市、郊区、开放区域的经验计算公式

Handoff is used in North America while Handover is used in ITU documents.

• Cell blocking probability – probability of a new call being blocked (due to heavy load)

• Call dropping probability – probability that a call is terminated due to a handoff

• Call completion probability – probability that an admitted call is not dropped before it terminates

• Probability of unsuccessful handoff – probability that a handoff is executed while the reception conditions are inadequate

• Handoff blocking probability – probability that a handoff cannot be successfully completed

• Handoff probability – probability that a handoff occurs before call termination

• Rate of handoff – number of handoffs per unit time

• Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station

• Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur

• Relative signal strength

• 可能带来乒乓效应

• Relative signal strength with threshold

• handoff必要条件：

• 只有当前基站信号足够弱时（低于预定阈值），并且

• 另一个（小区的）信号是两者中较强的信号

• Relative signal strength with hysteresis (滞后）

• 只有新基站比当前基站的信号强且差值>H时才发生越区切换

• Relative signal strength with hysteresis and threshold

• Prediction techniques

• 越区切换取决于所收到信号强度的预期值

• 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 the transmitted signal from the mobile

• Reduce cochannel interference, alleviate health concerns, save battery power

• In SS systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS

• Open-loop power control

• Depends solely on mobile unit

• No feedback from BS

• Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength

• 应用于某些扩频系统中，BS（基站）发送一个未调制 的导频信号，CDMA接收机根据收到的导频信号强度调节自身发送功率

• 假定：正向和反向信道相关性强

• 反应速度快

• 如当一个终端从一个建筑物后面移出时

• 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 channel

• 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 – nonblocking system

• L > N – blocking system

• Probability that a call request is blocked? Or, alternatively, what capacity is needed to achieve a certain upper bound on probability of blocking?

• What is the average delay? Alternatively, what capacity is needed to achieve a certain average delay?

• Load presented to a system: (到达流量）

•  = mean rate of calls attempted per unit time

• h = mean holding time per successful call

• A = average number of calls arriving during average holding period, for normalized …(单位Erlang)

• Manner in which blocked calls are handled

• Lost calls delayed (LCD) – blocked calls put in a queue awaiting a free channel

• Blocked calls rejected and dropped

• Lost calls cleared (LCC) – user waits before another attempt

• Lost calls held (LCH) – user repeatedly attempts calling

• Number of traffic sources

• Whether number of users is assumed to be finite or infinite

• 当潜在用户数量是小区容量的5-10倍时，假定无限信息源是合理的

• 无限信息源(即：无限潜在用户数）、呼损清除(Lost calls cleared)

A: 到达流量 (offered traffic)

N: 服务器数量

P: 阻塞率

C=A(1-P)

C: 系统承载的流量(carried traffic, or admitted traffic)

• 假定信道总数Stotal, N为重用因子

• S=total # of channels per cell =Stotal/N

(1) 降低N似乎可以使S增大，但同频干扰也增加了

(2) 按照每个小区的负载分配信道——难点在于无法有效地预测瞬时流量，即使可以统计过去的平均信息

(3)因此为每个小区分配相同的信道数是合理的

• 固定分配方案

• 动态分配方案

• 混合分配方案

——见《无线与移动系统导轮》

D. P. Agrawal, et al.

• 每个小区永久性地分配一组信道

• 信道借用方案——当一个小区信道不足时

• Borrow from the richest

• Borrowed Channel returned

• Borrow first available (channel)

• 相临小区搜索遵循一定的次序

• 复杂的借用方案

• 信道分为两类：永久性分配的和用于借用目的的

• 如何有效的分配这两类信道和负载、信道利用率、分配复杂性等都有关系

• 当一个呼叫请求(call request)到达时，系统按照某种准则从一个central pool里动态选择一个可用信道

• 选择第一个可用信道

• 选择最小化未来阻塞概率的可用信道

• 集中动态信道分配机制

• 信道分配是一个中心控制节点完成的

• 性能优化、但时延大，不现实

• 分布式态信道分配

• 每个小区保持周临范围内的小区信道使用情况；

• 在这种情况下，一个小区可以根据上述信息为一个new request选择一个可用信道

混合机制

• 固定+动态信道分配机制

• Advanced Mobile 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

• Assigned to two companies

• Frequency reuse exploited

• Subscriber initiates call by keying in phone number and presses send key

• MTSO verifies number and authorizes user

• MTSO issues message to user’s cell phone indicating send and receive traffic channels

• 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

• Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital

• Encryption – all second generation 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

• Number of logical channels (number of time slots in TDMA frame): 8

• Maximum cell radius (R): 35 km

• Frequency: region around 900 MHz

• Maximum vehicle speed (Vm):250 km/hr

• Maximum coding delay: approx. 20 ms

• Maximum delay spread (m): 10 s

• Bandwidth: Not to exceed 200 kHz (25 kHz per channel)

• Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit

• Mobile equipment (ME) – physical terminal, such as a telephone or PCS

• ME includes radio transceiver, digital signal processors and subscriber identity module (SIM)

• GSM subscriber units are generic until SIM is inserted

• SIMs roam, not necessarily the subscriber devices

• BSS consists of base station controller and one or more base transceiver stations (BTS)

• Each BTS defines a single cell

• BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging

• NS provides link between cellular network and public switched telecommunications networks

• Controls handoffs between cells in different BSSs

• Authenticates users and validates accounts

• Enables worldwide roaming of mobile users

• Central element of NS is the mobile switching center (MSC)

• Home location register (HLR) database – stores information about each subscriber that belongs to it

• Visitor location register (VLR) database – maintains information about subscribers currently physically in the region

• Authentication center database (AuC) – used for authentication activities, holds encryption keys

• Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station

• Trail bits – allow synchronization of transmissions from mobile units

• Encrypted bits – encrypted data

• Stealing bit - indicates whether block contains data or is "stolen"

• Training sequence – used to adapt parameters of receiver to the current path propagation characteristics

• Strongest signal selected in case of multipath propagation

• Guard bits – used to avoid overlapping with other bursts

• Protocols above the link layer of the GSM signaling protocol architecture provide specific functions:

• Mobility management

• Connection management

• Mobile application part (MAP)

• BTS management

• Frequency diversity – frequency-dependent transmission impairments (e.g., noise burst, selective fading) have less effect on signal

• Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation

• Delayed signal by more than one chip interval does not interference with the dominant signal as much as other multipath environments

• Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals

• TDMA/FDMA systems are with fixed max # of users

• Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized

• So, certain level of cross correlation can be observed

• Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away

• Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

• RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them

• This method achieves better performance than simply recovering dominant signal and treating remaining signals as noise

• Soft Handoff – mobile station temporarily connected to more than one base station simultaneously

• 当一个移动单元进入软切换状态时，它将同时与两个（或以上）基站连接

• 发送：两个基站同时接收该移动单元信号，并传给mobile switching center, 后者从中选择信号较强的一个

• 接收：mobile switching center同时向两个基站发送数据，移动单元合并这两个信号以恢复出原始信息

• Pilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference (for demodulation) and provides means for signal strength comparison (for handoff decision)

• Synchronization (channel 32) - used by mobile station to obtain identification information about cellular system (system time, protocol revision, etc)

• Paging (channels 1 to 7) - contain messages for one or more mobile stations

• Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels (now support date rate up to 14400 bps)

• Speech is encoded at a rate of 8550 bps

• Data transmitted in 20-ms blocks with forward error correction provided by a convolutional encoder (with rate of ½)….19.2 kbps

• Data interleaved in blocks to reduce effects of errors

• Data bits are scrambled (加扰的）, serving as a privacy mask

• Power control information inserted into traffic channel

• DS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mbps using one row of 64 x 64 Walsh matrix

• Digital bit stream modulated onto the carrier using QPSK modulation scheme

• Voice quality comparable to the public switched telephone network

• 144 kbps data rate available to users in high-speed motor vehicles over large areas

• 384 kbps available to pedestrians standing or moving slowly over small areas

• Support for 2.048 Mbps for office use

• Symmetrical / asymmetrical data transmission rates

• Support for both packet switched and circuit switched data services

• An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic

• More efficient use of the available spectrum in general

• Support for a wide variety of mobile equipment

• Flexibility to allow the introduction of new services and technologies

• Bandwidth – limit channel usage to 5 MHz

• 5 MHz可以有效地对抗多径效应

• 5 MHz是3G能获得的带宽的上限

• 5 MHz也可以提供３G所需要的144和384kbps数据速率

• Chip rate – depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps or more is reasonable

• Multirate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service

p.220, 倒数第6行，”接收” =》 “信号接收”

p. 221, line 13, 传递 =》“切换而产生的”

P.223, 倒数第16行，后一句为“或者说，为了使系统的阻塞率保持在某一上限之下，需要多大系统容量？”。这里，“另外”应为“或者说”，下句同。

p. 225, 本页上公式等号左边变量应为P而不是A