Wireless WANS and MANS
Chapter 3
Cellular Network Concept
Use multiple low-power transmitters (100 W or less) Areas divided into cells
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Each served by its own antenna Served by base station consisting of transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern)
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Frequency Reuse
Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells
N: reuse factor
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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 N= I2 + J2 + (I x J) J. P. Sheu
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Frequency Reuse
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Capacity Enhancement
Adding new channels 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 sectors, each with their own set of channels Microcells – antennas move to buildings, hills, and lamp posts
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Micro-cell Macro-cell
Pico-cell
Figure 3.2. Cell-splitting 2008/10/24
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Sectorization
Space Division Multiple Access (SDMA) 4 3 2
4
6
3
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(a)Omnidirectional
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(b)Sectorized
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Channel Allocation Algorithms
Fixed channel allocation
Dynamic channel allocation
Allowed to borrow some channels from neighbors Require a centralized arbitrator to allocate channels
Hybrid channel allocation
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A set of local channels and another set of borrowable channels
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Cellular System Overview
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Cellular Systems Terms
Base Station (BS) – includes an antenna, a controller, and a number of transceivers Mobile telecommunications switching office (MTSO) – connects calls between mobile units Two types of channels available between mobile unit and BS
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Control channels – used to exchange information having to do with setting up and maintaining calls Traffic channels – carry voice or data connection between users J. P. Sheu
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Steps in an MTSO Controlled Call Between Mobile Users
Mobile unit initialization
Mobile-originated call
MH sending the called unit on the pre-selected setup channel
Paging
MH scans and select the strongest setup control channel
The MTSO sends a paging message to certain BSs
Call accepted Ongoing call Handoff
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Handoff Performance Metrics
Handoff delay – delay time in the transfer an ongoing call from the current cell to the new cell Duration of interruption – the duration of time during a handoff which is not connected to either BS Handoff success probability – probability that a handoff is successful Probability of unnecessary handoff – probability of the pin-pong effect
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Improved Handoff Strategies
Prioritization: a certain number of channels are reserved for handoff Relative signal Strength: a minimum time for which an MT must be in a cell before it can request a handoff Soft handoffs: a period of time when more than one BS handles a call can be allowed Predictive handoffs: predict the mobility pattern of mobile users Adaptive handoffs: users may have to be shifted across different layers, from micro- to macro- cellular
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Handoff Strategies Used to Determine Instant of Handoff
Relative signal strength (at L1 ) Relative signal strength with threshold (at L2 for Th2) Relative signal strength with hysteresis (at L3 ) Relative signal strength with hysteresis and threshold (at L4 for Th3 ) Prediction techniques: based on the expected future value of the received signal strength
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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 the transmitted signal from the mobile
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Reduce co-channel 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 J. P. Sheu
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Types of Power Control
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
Closed-loop power control
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Adjusts signal strength in reverse channel based on metric of performance BS makes power adjustment decision and communicates to mobile on control channel
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First-Generation Analog
Advanced Mobile Phone Service (AMPS)
In North America, two 25-MHz bands allocated to AMPS
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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
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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 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
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Mobile Wireless TDMA Design Considerations
Global System for Mobile Communications (GSM)
Number of logical channels (number of time slots in TDMA frame): 8 Maximum cell radius (R): 35 km Frequency: region around 900 MHz,1800 MHz, or 1900 MHz Maximum vehicle speed (Vm):250 km/hr Maximum coding delay: approx. 20 ms Maximum delay spread (Δm): 10 μs Bandwidth: Not to exceed 200 kHz (25 kHz per channel)
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Maximum data rate for each channel is 34Kbps J. P. Sheu
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Control Channels
Broadcast Control Channel (BCCH)
A downlink channel that contains the BS’s idendity and channel status. All MTs monitor the BCCH to detect if the have moved to a new cell.
Dedicated Control Channel (DCCH)
Used for call-setup, locations updates, and all callmanagement related information exchange. Every call has its own allotted DCCH
Common Control Channels (BCCH)
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Consists of the downlink paging channel to page any MT MT to BS for call-initiation, and the access grant chnnel J. P. Sheu
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Speech Coding
Traditional speech coding use pulse code modulation (PCM)
The data rate of PCM: 64 kbps This rate is undesirably high for use in cellular radio
With current technology, 12kbps is enough
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Due to the maximum coding delay is 20 ms it is reasonable to form the encoded speech into blocks of 20 ms duration, or speech samples of 240 bits
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ITU’s View of Third-Generation Capabilities
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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 J. P. Sheu
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ITU’s View of Third-Generation Capabilities (IMT 2000)
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
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Table 3.1. Evolution plan to 3G standards
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Existing 2G Standard
3G Standard
Europe
GSM
W-CDMA (UMTS)
Japan
PDC
W-CDMA (DoCoMo)
USA
IS-95 / cdma one
Cdma2000
USA
IS-136
UWC-136
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Table 3.2. IMT-2000 Service Types Service
Upstream
Downstrea m
Example
Switching
Interactive Multimedia
256 Kbps
256 Kbps
Video conference
Circuit
High Multimedia
20 Kbps
2 Mbps
TV
Packet
Medium Multimedia
19.2 Kbps
768 Kbps
Web surfing
Packet
Switched Data
43.2 Kbps
43.2 Kbps
Fax
Circuit
Simple Messaging
28.8 Kbps
28.8 Kbps
E-mail
Packet
Speech
28.8 Kbps
28.8 Kbps
Telephony
Circuit
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The Problems with 3G Systems
Difficult to find a common slice of spectrum to enable global roaming Disappointing performance of CDMA in practice It is hard to find suitable applications
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Table 3.3 CDMA – The debate Claims
Reality
Capacity of 20 times that of AMPS No more dropped calls No problem of interference Quality if speech promised at 8 Kbps
Only 3-4 times that of AMPS 40 percent dropped calls when loaded Interference from existing AMPS Had to change to 13 Kbps
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Wireless in Local Loop
Local loop: the last hop connectivity between the subscriber and PSTN Advantages:
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Easy deployment, high scalability Low investment cost, cost effective
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FSU
FSU
Telephone
Telephone
BTS FSU
Telephone
Figure 3.6. WLL architecture
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IEEE 802.16 Standard
Metropolitan area networks (MANs) span several km and cover large parts of cities MANs much larger in size than LANs and their functionalities differ from those LANs IEEE 802.16 is called air interface for fixed broadband wireless access systems
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Based on OSI model, specifies air interface including data link layer and physical layer
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Differences between 802.11 and 802.16
IEEE 802.16 was designed for broadband data such as digital video and telephony The number of users and BW usage per user is much higher than IEEE 802.11 IEEE 802.16 is completely connectionoriented and QoS guarantees
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Physical Layer
Uses traditional narrow-band radio (10-66 GHz) with conventional modulation scheme Two new protocols attempt to close to IEEE 802.11
802.16a operates in the 2 – 11 GHz and 802.16b operates in 5 GHz ISM band
Since the signal strength falls off sharply with distance from the BS, the following three modulation scheme are used
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QAM-64 used by subscribers located near the BS QAM-16 used by subscribers located at intermediate distance from the BS QPSK used by subscribers located far away from the BS J. P. Sheu
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Data Link Layer
DLL of IEEE 802.16 can be subdivided into three sublayers
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Security sublayer: only the payloads are encrypted MAC sublayer: deals with channel management and slot allocation to stations Services specific convergence sublayer: interface to the network layer J. P. Sheu
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MAC Sublayer
On the downlink, data to the SS is TDM and on the uplink, the medium is shared by the SSs using TDMA Each uplink connection has four classes of services:
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Constant bit rate service (voice) Real-time variable bit rate service (multimedia) Non-real-time variable bit rate service (file transfer) Best effort service (others) J. P. Sheu
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Table 3.4. A brief comparison among IEEE 802.11b WLANs, IEEE 802.16 WMANs, and GSM WWANs Feature
IEEE 802.11B WLANs
IEEE 802.16 WMANs
GSM WWANs
Range
Few hundred meters
Several Km
Few tens of Km
Frequency
2.4 GHz ISM band
10-66 GHz
900 or 1800 MHz
Physical Layer
CCK, BPSK, QPSK
QAM-64, QAM16, QPSK
GMSK
Maximum Data Rates
11 Mbps
60–180 Mbps
9.6 Kbps/user
Medium Access
CSMA/CA
TDM/TDMA
FDD/TDMA
QoS Support
DCF-No PCF-Yes
Yes
Yes
Connectivity
DCF-connectionless PCF-connection oriented
Connection oriented
Connection oriented
Typical Applications
Web browsing, e-mail
Multimedia, digital TV broadcasting
Voice
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Home Work
2, 4, 7, 9
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