蜂窝无线通信 与技术要点 - cs.nju.edu.cn ·...
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移动通信
•无线移动通信• 无线信道 – 信道特性，调制解调，复用• 移动通信 – 动态寻址
•移动通信发展
• GPRS(114Kbps) < EGDE(368Kbps) < 3G(3.1Mbps) < HSDPA(14Mbps) < HSPA+(168Mbps) < 4G/LTE(299.6Mbps)
蜂窝无线网络• 第一代模拟系统
– 采用模拟蜂窝网技术
– 1980s，模拟移动通信系统• 北美AMPS，英国TACS，日本JTAC，北欧NMT
• 爱立信与摩托罗拉
– 支持语音传输
– 无线传输采用FM调制， FDMA接入
摩托罗拉DynaTAC 8000X
蜂窝无线网络• 第二代数字系统
– 1990s，数字移动通信系统
– 支持语音与数据业务，增值业务如短信• 9.6K-14.4Kbps
– 主流网络制式– GSM（TDMA），CDMA• 欧洲GSM/GPRS; 基于CDMA的北美PCS(IS-95) (2.5G)
– EDGE：基于GSM/GPRS网络的数据增强型移动通信技术 (2.75G)
– 均衡，交织，RAKE接收，功率控制
Moto 8900 NOKIA 3210 Moto V70
蜂窝无线网络• 第三代宽带系统
– 2000s， IMT-2000
– 2Mbps室内稳定环境，384Kbps高速移动环境
– 四种标准制式• CDMA 2000，美，加，澳，韩，日，中国（电信）：北美根据IS-95演进
• WCDMA， 欧洲，日本，中国（联通）：根据GSM/GPRS演进
• TS-SCDMA，中国（移动）：中国综合根据2G多种标准提出
– 码分多址, 多媒体数据业务
– 多用户检测，智能天线，Turbo编码
蜂窝无线网络• 第四代无线蜂窝电话通信协议
– 2010s， 3GPP LTE-advance， IEEE 802.16m WiMAX
– 传输高质量视频图像
– 1Gbps低移动性，20-100Mbps高移动性
– TD-LTE， FDD-LTE（3.9G， LTE-Advanced）• FDD和TDD两种模式用于成对频谱和非成对频谱
– OFDM，MIMO
– 自适应调制编码，混合自动重传
蜂窝无线网络• 第五代
– IMT-2020 《5G网络技术架构白皮书》
– 关键技术指标（KPI）：10Gbps下载速度
– 应用场景：移动互联网与物联网• 海量无线通信需求，车联网，工业互联网，传输呈现低时延，高可靠，低功耗
蜂窝网标准
https://en.wikipedia.org/wiki/Cellular_network
Radio Access Network Design
• An operator wants usually to make most efficient use of their frequency range
– Full coverage in all places
– Sufficient capacity for all customers
• The basic solution is to make the neighboring cells to use different frequencies
– Frequencies can be re-used in cells further away
• The cells can be shaped using directional antennas
• Transmitter power can be used to regulate the size of a cell
– Pico cells inside buildings
– Micro cells for streets
– Macro cells for larger areas
Principles of Cellular Networks
• Multiplexing
– FDM， TDM，CDM，SDM
– Address： FDMA，TDMA，CDMA，SDMA， OFDMA
• Modulation
• Source coding
• Channel coding
• Interference control
• Underlying technology for mobile phones, personal communication systems, wireless networking etc.
Radio Link Coding Methods
• FDMA, Frequency Division Multiple Access
– One frequency per user
– e.g. NMT，AMPS
• TDMA, Time Division Multiple Access
– Timeslots on the same frequency
– e.g. GSM
• CDMA, Code Division Multiple Access
– Sender and receiver use a code to identify the radio transmission from the background noise
– Multiple senders on same frequency and timeslot
– e.g. UMTS
Wireless Challenges
• Path loss and fading
– Buildings, trees etc. block or weaken signals
• Multipath
– Multiple copies of a signal with small timing differences
• Flash crowds
– The customer's traffic patterns are difficult to predict
阴影效应，远近效应，多径效应，多普勒效应
Fading in the Mobile Environment
• Term fading• Time variation of received signal
• Caused by changes in transmission path(s)
• Fixed Environment• Changes in atmospheric conditions (e.g. rain)
• Mobile Environment• Movement of (mobile unit) antenna
Effects of Multipath Propagation
• Multiple copies of a signal（信号的多个副本）
• Signals may cancel out due to phase differences
• Intersymbol Interference (ISI)（码间干扰）
• Sending narrow pulse at given frequency between fixed antenna and mobile unit
• Channel may deliver multiple copies at different times
• Delayed pulses act as noise making recovery of bit information difficult
• Timing changes as mobile unit moves• Harder to design signal processing to filter out multipath effects
Effects of Multipath Propagation
• Multiple copies of a signal
• Intersymbol Interference (ISI)
Types of Fading
• rapid changes in strength over half wavelength distances fast fading
• slower changes due to user passing different height buildings, gaps in buildings, etc.
slow fading
• affects all frequencies in same proportion simultaneouslyflat fading
• different frequency components affected differently
selective fading
Error Compensation Mechanisms
➢forward error correction• applicable in digital transmission applications
• typically, ratio of total bits to data bits is 2-3
• Big overhead• Capacity one-half or one-third
• Reflects difficulty or mobile wireless environment
➢adaptive equalization• applied to transmissions
that carry analog or digital information
• used to combat intersymbol interference
• involves gathering the dispersed symbol energy back together into its original time interval
Error Compensation Mechanisms
• diversity– based on fact that individual channels experience
independent fading events
– use multiple logical channels between transmitter and receiver
– send part of signal over each channel
– doesn’t eliminate errors, but reduces
– space diversity involves physical transmission paths
– more commonly refers to frequency or time diversity
– most important example of frequency diversity is spread spectrum
Error Compensation Mechanisms• 分集两字含义:分散得到几个合成信号，而后对其集中（合并）处理
• 分集接收的基本思想：快衰落信道中接收的信号是到达接收机的各径分量的合成。如果能在接收端同时获得几个不同的合成信号，并将其适当合并成总的接收信号，只要被分集的几个合成信号之间是统计独立的，那么经适当的合并后就将可能大大减小衰落的影响。
• 分集方式
• 空间分集。在接收端架设几副天线，天线间要求有足够的距离（一般在100个信号波长以上），以保证各天线上获得的信号基本相互独立。
• 频率分集。用多个不同载频传送同一个消息，如果各载频的频差相隔比较远（例如，频差选成多径时延差的倒数），则各分散信号也基本互不相关。
• 角度分集。这是利用天线波束不同指向上的信号互不相关的原理形成的一种分集方法，例如在微波面天线上设置若干个反射器，产生相关性很小的几个波束。
• 极化分集。这是分别接收水平极化和垂直极化波而构成的一种分集方法。一般说，这两种波是相关性极小的（在短波电离层反射信道中）。
• 时间分集。如RAKE接收器的方法，如果一个信号的多个副本两两相距多个码片时隙到来，接收器通过这些码片序列与接收到的主信号之间的联系来恢复信号。
Principles of Cellular Networks
• Area divided into cells
– Each with own antenna
– Each with own range of frequencies
– Served by base station
• Transmitter, receiver, control unit
– Adjacent cells on different frequencies to avoid crosstalk
• cells sufficiently distant can use same frequency band
• Underlying technology for mobile phones, personal communication systems, wireless networking etc.
Shape of Cells
• Square– Width d cell has four neighbours at distance d and four at distance d
– Better if all adjacent antennas equidistant
• Simplifies choosing and switching to new antenna
• Hexagon– Provides equidistant antennas
– Radius defined as radius of circum-circle
• Distance from center to vertex equals length of side
– Distance between centers of cells radius R is R
– Not always precise hexagons
• Topographical limitations
• Local signal propagation conditions
• Location of antennas
Shape of Cells
Frequency Reuse Patterns
Increasing Capacity• Add new channels
– Not all channels used to start with
• Frequency borrowing
– Taken from adjacent cells by congested cells
– Or assign frequencies dynamically
• Cell splitting
– Non-uniform distribution of topography and traffic
– Smaller cells in high use areas
• - Original cells 6.5 – 13 km
• - 1.5 km limit in general
• - More frequent handoff
• - More base stations
Increasing Capacity• Cell Sectoring
– Cell divided into wedge shaped sectors (3 – 6 sectors per cell)
– Each with own channel set
• Subsets of cell’s channels
– Directional antennas at base station
• Microcells– Move antennas from tops of hills and large buildings to tops of small
buildings and sides of large buildings to cover a much smaller area
• Even lamp posts
– Good for city streets, along roads and inside large buildings
– Reduced power
Frequency Reuse Example
26
▪ Haas,Z. “Wireless And Mobile Networks”▪ (a) 32 cells, R=1.6km, K=336, N=7
▪ (b) 128 cells, R=0.8km, K=336, N=7
▪ What geographic area is covered, how many channels are there per cell, what is total number of concurrent calls that can be handled?
Operation of Cellular System
Two types of
channels between
mobile unit
and base
station (BS)
• Control Channels
• set up and maintain calls
• establish relationship between mobile unit and nearest BS
• Traffic Channels
• carry voice and data
Call Stages
• Mobile unit initialization• Scan and select strongest set up control channel
• Automatically selected BS antenna of cell• Usually but not always
nearest (propagation anomalies)
• Handshake to identify user and register location
• Scan repeated to allow for movement• Change of cell
• Mobile unit monitors for pages (see below)
Call Stages
• Mobile unit initialization• Mobile originated call
• Check set up channel is free• Monitor forward channel
(from BS) and wait for idle
• Send number on pre-selected channel• Mobile transmit on reverse
(to BS) channel, BS send request to MTSO
Call Stages
• Mobile unit initialization• Mobile originated call• Paging
• MTSO attempts to connect to mobile unit
• Paging message sent to BSs depending on called mobile number
• Paging signal transmitted on set up channel
Call Stages
• Mobile unit initialization• Mobile originated call• Paging
• Call accepted
• Mobile unit recognizes number on set up channel
• Responds to BS which sends response to MTSO
• MTSO sets up circuit between calling and called BSs
• MTSO selects available traffic channel within cells and notifies BSs
• BSs notify mobile unit of channel
Call Stages
• Mobile unit initialization• Mobile originated call• Paging
• Call accepted
• Ongoing call
• Voice/data exchanged through respective BSs and MTSO
• Handoff
• Mobile unit moves out of range of cell into range of another cell
• Traffic channel changes to one assigned to new BS
• - Without interruption of service to user
Other Functions
➢ call blocking
⚫ after repeated attempts, if all traffic channels are busy, a busy tone is returned
➢ call termination
⚫when a user hangs up channels at the BS are released
➢ call drop
⚫when BS cannot maintain required signal strength
➢ calls to/from fixed and remote mobile subscriber
⚫MTSO connects to the PSTN
GSM
GSM - Roaming
• When a MS enters another operators network, it can be allowed to use the services of this operator
• Operator to operator agreements and contracts
• Higher billing
• The MS is identified by the information in the SIM card and the identification request is forwarded to the home operator
• The home HLR is updated to reflect the MS's location
GSM – How to call a mobile phone
• A call is made from the PSTN to a mobile subscriber's number
• The call enters the GMSC
• The GMSC queries the HLR for information about which MSC serves the MS
• The call is forwarded to the MSC in charge
• The MSC queries the VLR about which BSC is currently in charge of the MS
• BSC sends a paging call to all cells to find the exact location of the mobile
• The mobile signals that it answers
• The call is completed
GSM – Voice services
VAD － Voice Activity Detection （话音激活检测）
DTX － Discontinuous Transmission （不连续发送）
DRX － Discontinuous Reception （不连续接收）
TP － Timing Program （时序调整）
GSM – data services
• The basic GSM data service transmits data instead of voice, using a time slot like a voice call• 9.6 or 14.4 kbps rate• Time based billing
• The data connection from the MS is usually connected to an traditional analog modem, which is connected to the PSTN• The operator can also provide direct data connections
• Long setup time 5-30 seconds
• High Speed Circuit Switched Data (HSCSD) uses multiple time slots to increase the data rate• Up to 57.6 kbps
• GSM data appears to be an evolutionary dead end and packet based data transmission is going to overtake it
GSM – data services
• High Speed Circuit-Switched Data (HSCSD)
• General Packet Radio Service (GPRS)
• Aimed at interfacing the Internet (e.g., for Web browsing)
• Rates up to 170kb/s
• Short Message Service (SMS)
• Similar to advanced paging systems
• Makes use of the control channel
GSM – SMSBSS MSC/VLR HLR SMS-MSC
Message transferRouting info req.
Routing info
Forward messagePaging
Assumption: before being paged, the terminal is idle
Channel setup
Authentication and ciphering
Message
Message ACKMessage ACK
Message tr. report
Release of the radio channel
GSM – Mobile Management
IDLE
READYSTAND-BY
Attachment
to the network
Detachment
Time out
Sending or reception of data
Detachment
or time out
Idle: no active GPRS session
Ready: session established; ongoing data exchange; precise mobile location (which cell)
Stand-by: session established, with no ongoing data exchange; approximate mobile location,
the mobile has to be tracked in its routing area
During a GPRS session (Ready or Stand-by states), the session itself is identified by a TLLI
(Temporary Logical Link Identity)
总结
问题？