蜂窝无线通信 与技术要点 - nanjing...
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数据通信
蜂窝无线通信与技术要点
http://cs.nju.edu.cn/yafeng/
Room 301, Building of CS
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
2
Shape of Cells
• 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
3
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
9
▪ 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
Mobile Radio Propagation Effects
• Signal strength
• Strength of signal between BS and mobile unit needs to be strong enough to maintain signal quality
• Not too strong so as to create co-channel interference
• Must handle variationsin noise
• Fading
• Time variation of received signal
• Caused by changes in transmission path(s)
• Even if signal strength is in effective range, signal propagation effects may disrupt the signal
Figure 10.7 Sketch of Three Important Propagation Mechanisms:
Reflection (R), Scattering (S), Diffraction (D)
R
R
D
S
lamp
post
Fading in Mobile Environemtns
Received
LOS pulse
Received
multipath
pulses
Time
Time
Transmitted
pulse
Transmitted
pulse
Received
LOS pulse
Received
multipath
pulses
Figure 10.8 Two Pulses in Time-Variant Multipath
Multipath Effect
Types of Fading
• Rapid variations in signal strength occur over distances of about one-half a wavelengthFast fading
• Change in the average received power level due to user passing different height buildings, vacant lots, intersections, etc.
Slow fading
• All frequency components of the received signal fluctuate in the same proportions simultaneouslyFlat fading
• Attenuation occurring over a portion of the bandwidth of the signal
Selective fading
Error Compensation Mechanisms
• Forward error correction
• Applicable in digitaltransmission applications
• The ratio of total bits sent to data bits sent is between 2-3
• 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• 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。
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
第三代和第四代蜂窝网络比较
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
LTE - Advanced
➢ Based on use of orthogonal frequency division multiple access (OFDMA)
Two candidates have emerged for
4G standardization:
Long Term Evolution (LTE)
Developed by the Third Generation Partnership
Project (3GPP), a consortium of North American, Asian, and
European telecommunications
standards organizations
WiMax
(from the IEEE 802.16 committee)
Figure 10.10 LTE-Advanced Configuration Elements
Donor
eNodeB
Evolved
Packet
Core MME
HSSSGW
PGW
UE
eNodeB = evolved NodeB
HSS = Home subscriber server
MME = Mobility Management Entity
PGW = Packet data network (PDN) gateway
RN = relay node
SGW = serving gateway
UE = user equipmentcontrol traffic
data traffic
RNUE
Internet
LTE - Advanced
Femtocells
Figure 10.11 The Role of Femtocells
DSL/FTTH line
Base station
(radius: several km)
Femtocell
gateway
Femtocell
access point
(radius: several m)
Operator
macrocell
system
Internet
Characteristics of TDD and FDD for LTE-Advance
Figure 10.12 Spectrum Allocation for FDD and TDD
(a) FDD
(b) TDD
U1
WU
D1 D2 D3 D4U2 U3 U4
Channel 1 Channel 2 Channel 3 Channel 4
WU + WD
Guard band WGUplink band Downlink band
WD
Carrier Aggregation
Figure 10.13 Carrier Aggregation
Carrier
component
frequency
3G station
Carrier
component
Carrier
component
Carrier
component
Carrier
component
3G station
(a) Logical view of carrier aggregation
(b) Types of carrier aggregation
Band A
4G station
3G station
Carrier
component
Carrier
component
Band A
Intra-band
contiguous
Intra-band
non-contiguous
Inter-band
non-contiguousBand A Band B
Carrier
component
Carrier
component
蜂窝网标准
https://en.wikipedia.org/wiki/Cellular_network
总结
问题？
http://cs.nju.edu.cn/yafeng/
Room 301, Building of CS