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Second Generation (2G) Cellular
Dr. A. ChockalingamAssistant Professor
Indian Institute of Science, Bangalore-12
http://www.ece.iisc.ernet.in/~achockal
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 3
2G Cellular Systems
GSM GSM – European Digital Cellular StandardEuropean Digital Cellular Standard
– TDMA Access TechnologyTDMA Access Technology
IS-54/IS-136 IS-54/IS-136 – North American Digital Cellular StandardNorth American Digital Cellular Standard– TDMA Access Technology TDMA Access Technology
IS-95AIS-95A
– North American Digital Cellular StandardNorth American Digital Cellular Standard– CDMA Access TechnologyCDMA Access Technology
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 4
GSM
Global System for Mobile communications
BTS
BTS
BTS
BTS
BTS
MS
MS
BSC
BSC
MSC
PSTN
ISDN
Data Networks
OMC
HLR VLR AUC
Base Station Subsystem (BSS)
Network and Switching Subsystem (NSS)
Public Networks
MS: Mobile Station MSC: Mobile Switching CenterBTS: Base Transceiver Station HLR: Home Location RegisterBSC: Base Station Controller VLR: Visitor Location RegisterAUC: Authentication Center OMC: Operation Maintenance Centeradmission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 5
GSM - Interfaces
BTS
BTS
BTS
MS
BSC MSC PSTN
SS7A Interface(standardized)
Abis Interface(standardized)
GSM Radio Air Interface (standardized)
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 6
GSM: Services & Features
Teleservices– standard mobile telephony, both mobile originated
and mobile terminated Data Services
– computer to computer traffic (e.g., Async Data)– Digital Fax
Supplementary Services– Caller ID, Short Messaging Service (SMS)
Subscriber Identity Module (SIM)– user memory device to activate service from any
GSM phoneadmission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 7
GSM: Air Interface
TDMA access technology 25 MHz BW spectrum on both directions
– 890 to 915 MHz Reverse link (mobile-to-base)– 935 to 960 MHz Forward link (base-to-mobile)
200 KHz RF carriers 8 TDMA slots on each carrier (i.e., each 200
KHz carrier can support 8 simultaneous calls) (25 MHz / 200 KHz) * 8 = 1000 traffic channels 13 Kbps vocoder rate (half rate vocoder can
double capacity)admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 8
GSM: Air Interface
Slow frequency hopping to mitigate fading effects Channel data rate : 270.833 Kbps GMSK modulation with 0.3 BT product Channel types
– Traffic Channels (TCH) - carry traffic signals» Full Rate TCH, Half Rate TCH
– Control Channels (CCH) - carry call control signals» Broadcast Channel (BCH)
» Common Control Channel (CCH): Paging and Random Access Channels
» Dedicated Control Channel (DCCH)admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 9
TDMA Frame in GSM
GSM TDMA Frame
577 microseconds
(8 time slots = 4.615 milliseconds)
0 1 2 4 5 6 7
Time Slot
3
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 10
GSM: Normal Traffic Burst
0 1 2 3 4 5 6 7
DATA DATA
57 BITS 57 BITS
(e.g. encoded voice) (e.g. encoded voice)
TrainingSequence
FLAG
FLAG
TAIL
TAIL
3 BITS 1 BIT 1 BIT 3 BITS
148 BITS
26 BITS
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 11
GSM: Air Interface
Downlink Control Signaling Frequency Correction: Sends a frequency
reference. Synchronization: Sends a timing reference. Broadcasting: Broadcasts general information
about the Base Station. Paging: Notifies the mobile of incoming calls.
Assigns a traffic channel to a mobile. Uplink Signaling
Random Access Channel: used by mobiles to request a traffic channel admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 12
GSM: Channel Interaction
Mobile - Base Station Initialization Mobile identifies a Frequency Correction burst on the
Frequency Correction Channel. Mobile synchronizes timing using a synchronization
burst on the Synchronization Channel. Mobile obtains general system information over the
Broadcast channel.
Mobile - Base Station Communication Mobile and Base communicate over Random Access
and Paging channels when not involved in a call. Mobile and Base communicate over Traffic channels
while involved in a call.admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 13
IS-54
IS-54 – also known as “Digital AMPS” (D-AMPS)– upgrade AMPS analog technology to a digital
technology– same spectrum and frequency spacing (30 KHz)
like AMPS– supports 3 or 6 users on a single single 30 KHz
carrier using TDMA scheme with 6 slots– control channels are identical to analog AMPS
control channels, but twice as many control channels as AMPS
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 14
IS-54 Radio Interface
Parameters IS-54 Specifications
Multiple Access TDMA/FDDModulation Pi/4 DQPSKChannel bandwidth 30 KHzRev Channel band 824 – 849 MHzFvd channel band 869 – 894 MHzFvd & Rev channel data rates 48.6 Kbps
Spectrum efficiency 1.62 bps/Hz
Channel Coding 7 bit CRC and rate ½ convol.coding of K=6
Users per channel 3 (full-rate speech coder of 7.95kbps/user)6 (with half-rate speech coder of3.975 kbps/user)
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 15
IS-54 vs IS-136
IS-54 vs IS-136– IS-54 uses 10 Kbps FSK modulated control channels – IS-136 uses 48.6 Kbps digital modulated control
channels» IS-136 does not support 10 Kbps FSK control channel. » So IS-136 user terminals are not compatible with IS-54
– IS-136 provides a host of new features and services, including
» short messaging capabilities» private user group features (suited for wireless PBX and
paging applications)» “Sleep Mode” to conserve battery power
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 16
GSM / IS-54 Summary
G S M IS -5 4Y e a r o f in tro d u c tio n 1 9 9 0 1 9 9 1M u ltip le A cc e ss T D M A /F D M A /F D D T D M A /F D M A /F D D
F re q u e n c ie s 8 9 0 - 9 1 5 M H z (R )9 3 5 – 9 6 0 M H z (F )
8 2 4 – 8 4 9 M H z (R )8 6 9 – 8 9 4 M H z (F )
M o d u la tio n G M S K (B T = 0 .3 ) 4/ - D Q P S KC a rrie r s e p a ra tio n 2 0 0 K H z 3 0 K H z
C h a n n e l d a ta ra te 2 7 0 .8 3 3 K b ps 4 8 .6 K b ps
N o . o f vo ic e c h a n n e ls 1 0 0 0 2 5 0 0
S p e c trum e ffic ie nc y 1 .3 5 b p s /H z 1 .6 2 b p s /H z
S p e e c h c o d in g R E L P -L T P @ 1 3 K b p s V S E L P @ 7 .9 5 K b ps
C h a n n e l c o d in g C R C w ith r= 1 /2 ; K = 5C o n v.
7 b it C R C w ith r= 1 /2 ; L = 6C o n v
E q u a lize rs A d a p tive A d a p tive
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 17
IS-95 CDMA
Direct sequence spread spectrum signaling on reverse & forward links
Each channel occupies 1.25 MHz Fixed chip rate 1.2288 Mcps Variable user data rate - depends on voice activity Universal frequency reuse fast power control to overcome near-far problem RAKE receiver to take advantage of multipath Soft handoffs
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 18
CDMA Channels & Frequencies
CDMA frequencies assigned through a 11-bit CDMA Channel number, N
At Mobile
MHz
MHz
At Base Station
MHz
MHz
825)1023(030.0
825030.0
10231013
7771
N
N
N
N
870)1023(030.0
870030.0
10231013
7771
N
N
N
N
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 19
CDMA Channel & Frequency
1.25MHz1.25MHz 1.25MHz1.25MHz
ReverseReverseCDMA ChannelCDMA Channel
ForwardForwardCDMA ChannelCDMA Channel
45 MHz45 MHz
FrequencyFrequencyCDMACDMAChannelChannelFrequencyFrequency 847.74 MHz847.74 MHz
892.74 MHz892.74 MHz
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 20
Frequency Reuse CDMA
DDEE
FF
GG BBCC
DDEE
FF
BBCC
GG
DDEE
FFCC
DD
EE
BBCC
GGAABB
CC
FF
BB
AAAA
7 cell Freq Reuse Plan Freq Reuse Plan in CDMA
AAAA
AAAA
AA
AAAA
AA
AAAA
AAAA
AAAA
AAAA
AA
AA
AA
AA
AA
AA
AA
AAAA
AA
AA
AA
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 21
Spreading Codes in IS-95 CDMA
Two types of spreading codes are used in IS-95– Walsh codes of length 64 are used on the
forward link (base-to-mobile link)e.g., c1 = 0 0 0 0
c2 = 0 1 0 1
c3 = 0 0 1 1
c4 = 0 1 1 0
» used to separate one user from another
– PN codes are used on both forward and reverse (mobile-to-base) links
jikckc ji ,0)()(
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 22
IS-95 CDMA Forward Link
Pilot Channel (Code Channel 0)
– provides phase reference for coherent demodulation – pilot strength measurement for handoffs
Paging Channel (up to 7 channels - Code Channels 1 to 7)
– sends control messages and page messages – Walsh Code Channels 1 through 7
Sync Channel (Code Channel 32)
– broadcasts system timing messages
Traffic Channel (up to 63 channels - remaining code channels)
– supports variable data rates at 9600, 4800, 2400, or 1200 bps
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 23
Forward Link Channel Structure
Forward CDMA Channel(1.25 MHz Chl. Tx by Base Stn
PilotChl
W0
SyncChl
W32
PagingChl. 1
W1
PagingChl. 7
W7 W8 W9 W63
TrafficChl. 1
TrafficChl. 2
TrafficChl. 55
Traffic dataPower ControlSub channel
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 24
FL Modulation Structure
W0
W32
Pilot Chl: all 0’s
Convol. Encoder/Repetition
Block Interleaver
1.2288 Mcps
1.2288 Mcps
Sync Chl1200 bps 4800 bps
Convol. Encoder/Repetition
Block Interleaver9600 bps
4800 bps2400 bps
19.2 Kbps1.2288 Mcps
Wp
Paging Chl
Paging Chl pLong code Mask
Long CodeGenerator
Decimator
1.2288 Mcps19.2 Kbps
To QuadratureSpreading
To QuadratureSpreading
To QuadratureSpreading
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 25
FL Modulation Structure
Convolutional Encoder
and Repetitionr=1/2, K = 9
User dataBlock
Interleaver
Long Code generator Decimator Decimator
MUX
Basebandfilter
Basebandfilter
Long code for nth user
1.2288 Mcps
1.2288 Mcps19.2 k
Power ControlBit
800 Hz
Walsh Code
Symbolcover
Scrambling
I-Chl Pilot PN Seq
Q-Chl Pilot PN Seq
Forward CDMA Traffic Channel Structure
9600 bps4800 bps2400 bps1200 bps
Quadrature Spreader
Note: Pilot PN Offset identifies the base stationadmission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 26
FL Modulation Parameters
Parameter Data Rate (bps)
User Data Rate 1200
Coding rate 1/2
Repetition 2
Coded data rate 4800
PN Chips/coded data bit 256
PN chip rate (Mcps) 1.2288
PN Chips/bit 1024
Sync Channel
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 27
FL Modulation Parameters
Parameter Data Rate (bps)
User Data Rate 9600 4800 2400
Coding rate 1/2 1/2 1/2
Repetition 1 2 4
Coded data rate 19,200 19,200 19,200
PN Chips/coded data bit 64 64 64
PN chip rate (Mcps) 1.2288 1.2288 1.2288
PN Chips/bit 128 256 512
Paging Channel
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 28
FL Modulation Parameters
Parameter Data Rate (bps)
User Data Rate 9600 4800 2400 1200
Coding rate 1/2 1/2 1/2 1/2
Repetition 1 2 4 8
Coded data rate 19,200 19,200 19,200 19,200PN Chips/coded data bit 64 64 64 64
PN chip rate (Mcps) 1.2288 1.2288 1.2288 1.2288
PN Chips/bit 128 256 512 1024
Forward Traffic Channel
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 29
IS-95 CDMA Reverse Link
Reverse CDMA Channel(1.25 MHz Chl. Rx by Base Stn
AccessChl. 1
AccessChl. 2
AccessChl. n
TrafficChl. 1
TrafficChl. 2
TrafficChl. m
TrafficChl. 3
Addressed by long code PNs
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 30
IS-95 CDMA Reverse Link
Access Channels– enables mobile to communicate non-traffic information (e.g.,
call request) in random access mode
– fixed data rate at 4.8 kbps – identified by a distinct access channel long code sequence
offset– a paging channel number is associated with access channel
Traffic Channels– identified by long distinct user code offset
– data rate 9.6, 4.8, 2.4, 1.2 Kbps– data is convolutionally encoded, block interleaved, 64-ary
orthogonal modulated, and direct sequence spread before transmission
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 31
RL Modulation Structure
Convolutional Encoder
and Repetitionr=1/3, K = 9
Informationbit
Block Interleaver
Long Code generator
Basebandfilter
Basebandfilter
Long code Mask for user n 1.2288 Mcps
Zero offsetPilot PN Seq I Chl
Reverse CDMA Traffic Channel Structure
9600 bps4800 bps2400 bps1200 bps
Code symbol
28.8 Ksps
64-1ryOrthogonalModulator
Zero offsetPilot PN Seq Q Chl
Data burst
randomizer
Code symbol
Walshchip
307.2 Kcps
PN chip
D
1/2 PN chiipdelay=406.9 ns
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 32
RL Modulation Parameters
Parameter Data Rate (bps)
User Data Rate 9600 4800 2400 1200
Coding rate 1/3 1/3 1/3 1/3
Tx Duty Cycle (%) 100 50 25 12.5
Coded data rate (sps) 28,800 28,800 28,800 28,800
Bits per Walsh symbol 6 6 6 6
Walsh symbol rate 4800 4800 4800 4800
Walsh chip rate (Kcps) 307.2 307.2 307.2 307.2
Walsh symbol duration(microsec)
208.33 208.33 208.33 208.33
PN chips per code symbol 42.67 42.67 42.67 42.6
PN chips per Walsh symbol 256 256 256 256
PN Chips per Walsh chip 4 4 4 4
PN chip rate (Mcps) 1.2288 1.2288 1.2288 1.2288
Reverse Traffic Channel
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 33
Power Control
To combat the effect of fading, shadowing and distance losses
Transmit only the minimum required power to achieve a target link performance (e..g, FER)– Minimizes interference– Increases battery life
FL Power Control– To send enough power to reach users at cell edge
RL Power Control– To overcome “near-far” problem in DS-CDMA
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 34
Power Control
Types of Power Control– Open Loop Power Control– Closed loop Power Control
Open Loop Power Control (on FL)– Channel state on the FL is estimated by mobile
– RL Transmit power made proportional to FL channel Loss
– Works well if FL and RL are highly correlated» which is generally true for slowly varying distance and
shadow losses» but not true with fast multipath Rayleigh fading
– So open loop power control can effectively compensate for
distance and shadow losses, and not for multipath fading admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 35
Power Control
Closed Loop Power Control (on RL)
– Base station measures the received power– Compares it with the desired received power (target
Eb/No) – Sends up or down command to mobile asking it to
increase or decrease the transmit power– Must be performed fast enough a rate (approx. 10
times the max. Doppler BW) to track multipath fading
– Propagation and processing delays are critical to loop performance
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 36
Power Control in IS-95
At 900 MHz Carrier frequency and 120 km/h mobile speed, Doppler = 100 Hz
In IS-95A, closed loop power control is operated at 800 Hz update rate
Power control bits are punctured into the traffic data stream Closed loop power control step size is +/- 1 dB Power control bit errors do not affect performance much Coding and interleaving has effect on CLPC performance Both open (outer) and closed (inner) loops drive the
transmit power to ensure a target FER of 1%admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 37
RAKE Receiver
4 RAKE fingers are used in the Mobile Receiver
– 3 fingers for tracking and demodulating multipath components of the FL CDMA channel
– 1 finger is used for searching and estimating the signal strength on different pilots
» used to select the desired (strongest) base station in idle mode
» for generating pilot strength information messages
during traffic mode to enable Handoff
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 38
Handoffs in IS-95 CDMA
Types of Handoff
– Soft Handoff» Mobile commences commun with a new base station without
interrupting commun with old base station» same freq assignment between old and new base station» provides different site selection diversity
– Softer handoff» Handoffs between sectors in a cell
– CDMA-to-CDMA Hard Handoff» Mobile transmits between two base stations with different
frequency assignmentadmission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 39
Soft Handoff Architecture
Mobile
BSC BSC
BTS BTSBTSBTS
New LinkOld Link
RR
MSCTo other switch
R
R - Handoff request sent to the old cell on the degrading link Energy measurements are made at
the mobile
Switch Diversity:MSC selects the bit stream with lower error rate
admission.edhole.com
Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 40
Handoff Procedure
Pilot Sets– Active Set
» Pilot associated with FL traffic channels assigned to the mobile
– Candidate Set» Pilots that are not in Active Set but are received by the mobile
with sufficient strength
– Neighbor Set» Pilots not in Active or Candidate Set but are likely candidates
for handoff
– Remaining Set» Set in the current system on current freq assignment, excluding
the above 3 sets
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 41
Handoff Example
Time
PilotStrength
(1)
T_ADD
T_DROP
(2) (3) (4) (5) (6) (7)
NeighborSet
CandidateSet Active Set
T_TDROP
NeighborSet
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Dr. A. Chockalingam WW'99, Dept of ECE, IISc, Bangalore 42
Handoff Example (..contd)
(1) Pilot strength exceeds T_ADD. Mobile sends a Pilot Strength Measurement Message (PSMM) to base station and transfers pilot to the Candidate Set
(2) Base station sends a Handoff Direction Message (HDM)
(3) Mobile transfers pilot to Active Set and sends s Handoff Completion Message (HCM)
(4) Pilot strength drops below T_DROP. Mobile starts handoff drop timer
(5) Handoff drop timer expires. Mobile sends a PSMM
(6) Base station sends a HDM
(7) Mobile moves pilot from Active Set to Neighbor Set and sends a HCM
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