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Wireless Communications:Systems and Services
Li-Hsing YenAssistant Prof.
Dept. of CSIE, Chung Hua Univ.
Design Issues in WirelessSystems
• Packet Switching vs. Circuit Switching• Duplex Method• Multiple Access Methods• Signalling
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Circuit Switching
• A dedicated channel must be created beforethe transmission starts
• Guaranteed transmission rate• Inefficient utilization of the communication
channels for many types of sessions• Example:
– Telephone network
Packet Switching
• Need not allocate channel beforetransmission
• Uses communication links on demand basis• Variable transmission rate• Efficient utilization of communication links• Example
– Internet
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Duplex Operations
• Simplex• Half-Duplex
– Time-Division Duplex
• Full-Duplex– Frequency-Division Duplex
Access to Trunking System
• access– the method of sharing channels
• trunked radio system– the number of channels available for all users is
less than the number of all possible users
• methods– FDMA, TDMA, SDMA
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Multiple Access Methods inWireless Networks
• Frequency Division Multiple Access (FDMA)• Time Division Multiple Access (TDMA)• Code Division Multiple Access (CDMA)• CSMA/CA
– IEEE 802.11 Wireless LAN
Frequency-Division MultipleAccess (FDMA)
• Each user is assigned a different radiofrequency (channel)
• Channel information may be transmitted ina dedicated frequency
• Simple but inefficient
. . .
guard band
Frequency
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Time-Division Multiple Access(TDMA)
• Users are assigned time slots to share aphysical channel
• Cellular systems always overlays TDMA ontop of an FDMA structure
. . . . . . . . .Time
guard timeframe
Frequency
Time
. . . . . . . . .
. . . .
. . . .
. . . .
TDMA Over FDMA
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Code Division Multiple Access(CDMA)
• Frequency-hopping CDMA (FH/CDMA)– each station transmits at a particular frequency for a
short duration, then hops to another frequency
• Direct-sequence CDMA– each bit of the transmitter is replaced by a codeword,
i.e., a sequence of bits (chips)– orthogonal codewords must be chosen to prevent too
much confusion at a receiver– spread spectrum: uses a wider spectrum than is strictly
necessary for carrying the transmitter’s signal
CDMA: Basic Principle
• Break each bit into k chips– according to a fixed pattern specific to each user– Bit data rate D ⇒ kD chips per second
• Example: the code for A: cA=⟨1,-1,-1,1,-1,1⟩
User A
1
0
⟨1,-1,-1,1,-1,1⟩
⟨-1,1,1,-1,1,-1⟩
1 -1 -1 1 -1 1 1 -1 -1 1 -1 1× 1 1 1 1 1 1
-1 1 1 -1 1 -1 1-1 -1 1 -1 1× -1-1 -1-1 -1 -1
Decodingfunction
k =6
(complement)
Σ=6
Σ=-6
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CDMA: Multiple UsersUser AUser BUser C
1 -1 1 1
1 11 1
-1 -1
-1 -1
1 1 -1 1 1 -1
User B ⟨1, 1,-1,-1, 1, 1⟩ 1 1 -1 -1 1 1 1 -1 -1 1 -1 1× 1 -1 1 -1 -1 1 Σ=0
cA
cB
cC
cADecoding with cA
User A ⟨-1, 1, 1,-1, 1,-1⟩ -1 1 1 -1 1 -1 1 1 -1 -1 1 1×-1 1 -1 1 1 -1 Σ=0
cBDecoding with cB
cA and cB are orthogonal
IEEE 802.11 Wireless LANCarrier Sense Multiple Access withCollision Avoidance (CSMA/CA)
source
destination
busy
DIFS
ACK
SIFS
contention window
data frame
DIFS
SIFS
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Signaling
• A function to organize the interworking ofnetwork entities in a telecommunicationnetwork– is required to establish, maintain, and terminate
connections or communication links
• Out-of-band signaling– signaling channel is separated from traffic
channel
Types of Wireless Networks
• Infrastructureless networks(Ad hoc networks)
• Infrastructurednetworks
to wired networks
Example: Wireless LAN Example: Cellular Phone System
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Wireless LAN
Mobile Ad HocNetwork
MS
MS
MS
MS
Agent
Internet
Agent
FH
MS
Cellular Phone Systems
Cell #1 Cell #2
Switches andProcessor
Handoff (Handover):switches cells
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Why Cellular Mobile TelephoneSystems?
• Operational limitations of conventionalmobile telephone systems– limited service capability– poor service performance– inefficient frequency spectrum utilization
Duplexing in Cellular Systems
• Up link– from MS to BS– multiple access methods
• Down link– from BS to MS– scheduling by BS
• Duplex methods– Time-Division Duplex (TDD)– Frequency-Division Duplex (FDD)
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Advanced Mobile Phone Service(AMPS)
• Analog cellular system (1983)• Frequency Division Multiple Access (FDMA)• 50 channels per cell
BS
MTSOLE
MS
PSTN
Digital Cellular Phone SystemsStandard
Frequency band
Multiple access
Duplex method
No. of channels
Channel spacing
Modulation
Channel bit rate
GSM 900 GSM1800 IS-54/-136 IS-95
270.83 kb/s 270.83 kb/s 48.6 kb/s 1.2288 Mb/s
GMSK GMSK π/4 DQPSK QPSK/DQPSK
200 kHz 200 kHz 30 kHz 1250 kHz
1248 users/channel
3748 users/channel
8323 users/channel
20798 users/channel
FDD FDD FDD FDD
CDMA/FDMATDMA/FDMATDMA/FDMATDMA/FDMA
down 935-960 up 880-915
down 1805-1880 up 1710-1785
down 869-894 up 824-849
down 869-894 up 824-849
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Digital Cellular Phone Systems(Cont.)
System IS-54 IS-95(DS) GSM 900/1800Portable txmit powerMax/avg.
600 mW/200mW
600 mW/600 mW
1W/125 mW
Speech coding VSELP QCELP RPE-LTPSpeech rate (Kbps) 7.95 8 (var.) 13Ch. coding 1/2 rate conv. 1/2 rate fwd
1/3 rate rev.1/2 rate conv.
Frame (ms) 40 20 4.615
GSM Data Services
• GSM provides four types of services– Voice service– Short message– Fax– Data
• Transparent data (no error correction, constant delay)• Non-transparent data (with error correction, variable delay)• Max 9600 bps• Access to modems in PSTN
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GSM Data Limitation
• Uplink and downlink channels allocated for a user forentire call period
• User pays based on connection time, not based on volume– Bad connections makes more money to operator
• Connection setup takes 20 .. 25 seconds• Limited capacity (9.6 Kbps)• GSM is designed for speech, not data
– Typically 50% of the radio capacity is wasted– Not optimal channel coding for data
Cordless Phone SystemsSystem CT2 CT2+ DECT PHS PACSDuplexing TDD TDD TDD FDDFrequency band(MHz)
864-868944-948
1880-1900
1895-1918
1850-1910/1930-1990
Carrier spacing(KHz)
100 1728 300 300/300
Number ofcarriers
40 10 77 16 pairs/10 MHz
Bearer channel/carrier
1 12 4 8/pair
Channel bit rate(kbps)
72 1152 384 384
Modulation GFSK GFSK π/4 QPSK π/4 QPSKSpeech coding 32 kbps 32 kbps 32 kbps 32 kbpsAverage handsetTX power (mW)
5 10 10 25
Peak handsetTX power (mW)
10 250 80 200
Frame duration(ms)
2 10 5 2.5
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Data Services in DECT
• DECT data link layer is designed for circuitand packer mode services– in the packet mode, it is possible to allocate
multiple time slots to SUs
Mobile Data Systems
• DataTAC/Ardis: IBM, Motorola -1983• MobiTex/RAM: Ericsson, Bellsouth -1989• CDPD: Open System (IBM, AT&T) -1993• GPRS: ETSI
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Mobile Data Systems (cont.)
System DataTAC MobiTex CDPDFreq. Band (MHz) 800 900, 400 800Channel spacing (KHz) 12.5/25 12.5/25 30Protocol RD-LAP MPAKS TCP/IPData rate (Kbps) 4.8/19.2 8/16 19.2Throughput (Kbps) 2.2/12.0 4-5 9.6-14.4Specification Closed Closed Open
General Packet Radio Service(GPRS)
• Based on GSM• the transmission medium is used on demand only:
master-slave dynamic rate access protocol• provides connectionless and connection-oriented
point-to-point, multicast and group call point-to-multipoint services
• charging depends on the amount of datatransmitted and the quality of service negotiated
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Personal CommunicationSystems (PCS)
• Goals– to provide the timely exchange of various kinds
of information (voice/data/video/image) withanyone, anywhere, at anytime, at low costthrough portable handsets
PCS Features
• multiple environments• multimedia services with high quality• multiple user type• global roaming capability• single personal telecommunication number• very high capacity• universal handset• service security
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Some Design Features of GSM
• System Architecture• Speech Channel Coding• Authentication and Encryption• Call Setup Procedure• DTMF Support
GSM System Architecture
BSC HLR VLR EIRBTS
VLR
MSC MS
(ME/SIM)
MS(ME/SIM)
A-bis
A-bis
Um
Um
C B
G
D
F
MSC PSTN, ISDN, PSPDN,CSPDN
E
A
NSS
BSS
AuC
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GSM Physical Layer (MS Side)voice voicesignaling
speechcoding
channel coding
interleaving
burst formatting
ciphering
modulation
speechdecoding
channel decoding
de-interleaving
burst de-formatting
deciphering
demodulation
signaling
R/F R/F
20 ms
260 bits
speech encoding (RPE-LTP)
456 bits
channel encoding
0 57 114 171 228 285 342 39964 121 178 235 292 349 406 7 : : : : : : : :392 449 50 107 164 221278 335
57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 5757 57 57 57 57 57 57 57
57 rows
frameburst
GSM Speech Transmission
interleaving
burstformatting
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GSM Speech Channel Coding
Class 1a50 bits
Class 1b132 bits
Class 278 bits
378 bits 78 bits
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Convolutional Coding
Parity bitsprotecting 1a
91 bits 91 bits
260 bits
456 bits
reordering TailBits
Tailing Bits and Reordering
d(0)d(1)d(2)d(3)
…
d(179)d(180)d(181)
p(0)p(1)p(2)
d(0)d(2)d(4)
d(180)d(178)
:
d(181)d(179)d(177)
d(1)d(3)
:
u(0)u(1)u(2)
u(90)u(89)
::
p(1)p(2)
p(0) u(91)u(92)
u(95)u(94)u(93)
u(96)
u(183):
u(184)
u(185)u(186)u(187)u(188)
0000
Tailing Bitsreorder
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Convolutional Encoder for GSMSpeech (Rate=1/2, K=5)
ak ak-4ak-1 ak-2 ak-3
U0 … U188
GSM Normal Burst Formatting
57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 5757 57 57 57 57 57 57 57
frame
B A B A …
burst
A B C
A B A B …
3 tail bits
… B A B … A B A
3 tail bits57 bits 57 bits
Training sequence Stealing flag
8.25guardperiod
28 bits
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Ki KiRAND
A3 A3 A8A8
SRES
SRES
Home System
VisitedSystem
=?Y
Nreject
accept
Mobile Station
authentication
encryption
Kc
Kc Kcframe number
A5
plain textciphered data
SRES
S1 S2
A5
S1 S2
plain text
Authentication & Encryption/Decryption in GSM
SIM
Call Setup Procedure: MobileTerminated Call
MSC
MS
VLRHLRGMSC(INTX)
otherswitches
otherswitches
1 1
1 1
22
3
33
request roaming number
dial MSISDN
allocate MSRN
routing
INTerrogating eXchange (INTX)Mobile Station ISDN Number (MSISDN) (Country Code, see E.164)Mobile Station Roaming Number (MSRN) (Mobile Country Code, see E.212)
+886935...
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Dual Tone Multiple Frequency(DTMF) in PSTN
DTMFSwitch
Dialing
PBXSwitch
Connected
DTMF in GSM
SETUP
MSC
START_DTMF
STOP_DTMF
MSC PBX
Dialing
Connected
