Second Generation (2G) Cellular
Dr. A. Chockalingam
Assistant Professor
Indian Institute of Science, Bangalore-12
[email protected]
http://www.ece.iisc.ernet.in/~achockal
2G Cellular Systems
 GSM
– European Digital Cellular Standard
– TDMA Access Technology
 IS-54/IS-136
– North American Digital Cellular Standard
– TDMA Access Technology
 IS-95A
– North American Digital Cellular Standard
– CDMA Access Technology
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
2
GSM

Global System for Mobile communications
OMC
BTS
PSTN
BSC
MS
MSC
BTS
ISDN
BTS
BTS
BSC
VLR
AUC
Network and Switching
Subsystem (NSS)
BTS
MS
Base Station Subsystem (BSS)
Dr. A. Chockalingam
HLR
MS: Mobile Station
BTS: Base Transceiver Station
BSC: Base Station Controller
AUC: Authentication Center
WW'99, Dept of ECE, IISc, Bangalore
Data
Networks
Public Networks
MSC: Mobile Switching Center
HLR: Home Location Register
VLR: Visitor Location Register
OMC: Operation Maintenance Center
3
GSM - Interfaces
BTS
MSC
BSC
BTS
PSTN
MS
A Interface
BTS
SS7
(standardized)
Abis Interface
(standardized)
GSM Radio Air
Interface (standardized)
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
4
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 phone
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
5
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)
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
6
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)
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
7
TDMA Frame in GSM
Time
Slot
0
1
2
3
577 microseconds
4
5
6
7
GSM TDMA Frame
(8 time slots = 4.615 milliseconds)
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
8
GSM: Normal Traffic Burst
0
7
1
2
3
4
5
6
Training
Sequence
F
L
A
G
148 BITS
T
A
I
L
DATA
(e.g. encoded voice)
57 BITS
3 BITS
Dr. A. Chockalingam
F
L
A
G
1 BIT
26 BITS
DATA
(e.g. encoded voice)
1 BIT
WW'99, Dept of ECE, IISc, Bangalore
57 BITS
T
A
I
L
3 BITS
9
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
10
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.

Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
11
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
12
IS-54 Radio Interface
Parameters
IS-54 Specifications
Multiple Access
Modulation
Channel bandwidth
Rev Channel band
Fvd channel band
Fvd & Rev channel data rates
TDMA/FDD
Pi/4 DQPSK
30 KHz
824 – 849 MHz
869 – 894 MHz
48.6 Kbps
Spectrum efficiency
1.62 bps/Hz
Channel Coding
7 bit CRC and rate ½ convol.
coding of K=6
3 (full-rate speech coder of 7.95
kbps/user)
6 (with half-rate speech coder of
3.975 kbps/user)
Users per channel
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
13
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
14
GSM / IS-54 Summary
GSM
IS-54
Year of introduction
Multiple Access
1990
TDMA/FDMA/FDD
1991
TDMA/FDMA/FDD
Frequencies
890 - 915 MHz (R)
935 – 960 MHz (F)
GMSK (BT=0.3)
200 KHz
270.833 Kbps
1000
1.35 bps/Hz
RELP-LTP @ 13 Kbps
CRC with r=1/2; K=5
Conv.
Adaptive
Modulation
Carrier separation
Channel data rate
No. of voice channels
Spectrum efficiency
Speech coding
Channel coding
Equalizers
Dr. A. Chockalingam
824 – 849 MHz (R)
869 – 894 MHz (F)
/4 - DQPSK
30 KHz
48.6 Kbps
2500
1.62 bps/Hz
VSELP @7.95 Kbps
7 bit CRC with r=1/2; L=6
Conv
Adaptive
WW'99, Dept of ECE, IISc, Bangalore
15
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

Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
16
CDMA Channels & Frequencies


CDMA frequencies assigned through a 11-bit CDMA
Channel number, N
At Mobile
1  N  777
1013  N  1023
0.030 N  825MHz
0.030( N  1023)  825MHz

At Base Station
1  N  777
1013  N  1023
0.030 N  870MHz
0.030( N  1023)  870MHz
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
17
CDMA Channel & Frequency
Reverse
CDMA Channel
Forward
CDMA Channel
1.25MHz
1.25MHz
CDMA
Channel
Frequency
Frequency
847.74 MHz
Dr. A. Chockalingam
45 MHz
WW'99, Dept of ECE, IISc, Bangalore
892.74 MHz
18
Frequency Reuse CDMA
F
E
B
C
G
D
A
C
A
C
C
G
A
D
F
E
C
B
7 cell Freq Reuse Plan
Dr. A. Chockalingam
A
A
A
A
A
A
A
E
B
G
F
B
D
F
B
E
D
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
Freq Reuse Plan in CDMA
WW'99, Dept of ECE, IISc, Bangalore
19
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
 c (k )c (k )  0, i  j
i
j
» used to separate one user from another
– PN codes are used on both forward and
reverse (mobile-to-base) links
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
20
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
21
Forward Link Channel Structure
Forward CDMA Channel
(1.25 MHz Chl. Tx by Base Stn
Pilot
Chl
W0
Sync
Chl
W32
Paging
Chl. 1
Paging
Chl. 7
W1
W7
Traffic
Chl. 1
W8
W63
W9
Traffic data
Dr. A. Chockalingam
Traffic
Chl. 55
Traffic
Chl. 2
Power Control
Sub channel
WW'99, Dept of ECE, IISc, Bangalore
22
FL Modulation Structure
W0
1.2288 Mcps
Pilot Chl: all 0’s
To Quadrature
Spreading
W32
Sync Chl
1200 bps
1.2288 Mcps
Convol. Encoder/
Repetition
4800 bps
Block
Interleaver
To Quadrature
Spreading
Wp
Paging Chl Convol. Encoder/
Repetition
9600 bps
Block
Interleaver
4800 bps
2400 bps
1.2288 Mcps To Quadrature
Spreading
19.2 Kbps
1.2288 Mcps
19.2 Kbps
Paging Chl p
Long code Mask
Dr. A. Chockalingam
Long Code
Generator
Decimator
WW'99, Dept of ECE, IISc, Bangalore
23
FL Modulation Structure
Power Control
Bit
User data
9600 bps
4800 bps
2400 bps
1200 bps
Convolutional
Encoder
and Repetition
r=1/2, K = 9
19.2 k
Block
Interleaver
M
U
X
Walsh
Code
1.2288 Mcps
Symbol
cover
Scrambling
Quadrature
Spreader
800 Hz
Long Code
generator
Decimator
Decimator
1.2288 Mcps
I-Chl Pilot
PN Seq
Baseband
filter
Long code
for nth user
Q-Chl Pilot
PN Seq
Baseband
filter
Forward CDMA Traffic Channel Structure
Note: Pilot PN Offset identifies the base station
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
24
FL Modulation Parameters
Sync Channel
Parameter
Data Rate (bps)
User Data Rate
1200
Coding rate
1/2
Repetition
2
Coded data rate
PN Chips/coded data bit
4800
256
PN chip rate (Mcps)
1.2288
PN Chips/bit
1024
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
25
FL Modulation Parameters
Paging Channel
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
PN Chips/coded data bit
19,200
64
19,200
64
19,200
64
PN chip rate (Mcps)
1.2288
1.2288
1.2288
PN Chips/bit
128
256
512
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
26
FL Modulation Parameters
Forward Traffic Channel
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
PN Chips/coded data bit
19,200
64
19,200
64
19,200
64
19,200
64
PN chip rate (Mcps)
1.2288
1.2288
1.2288
1.2288
PN Chips/bit
128
256
512
1024
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
27
IS-95 CDMA Reverse Link
Reverse CDMA Channel
(1.25 MHz Chl. Rx by Base Stn
Access Access
Chl. 1 Chl. 2
Access
Chl. n
Traffic
Chl. 1
Traffic
Chl. 2
Traffic
Chl. 3
Traffic
Chl. m
Addressed by long code PNs
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
28
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
29
RL Modulation Structure
Long code Mask
for user n
Long Code
generator
1.2288 Mcps
PN chip
Information
Convolutional
bit
Encoder
9600 bps
and Repetition
4800 bps
r=1/3, K = 9
2400 bps
1200 bps
Code
symbol
Code
symbol
Block
Interleaver
28.8 Ksps
Walsh
chip
64-1ry
Orthogonal
Modulator
Zero offset
Pilot PN
Seq Q Chl
Data
burst
randomizer
307.2 Kcps
Zero offset
Pilot PN
Seq I Chl
Baseband
filter
Reverse CDMA Traffic Channel Structure
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
1/2 PN chiip
delay=406.9 ns
D
Baseband
filter
30
RL Modulation Parameters
Reverse Traffic Channel
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)
Bits per Walsh symbol
28,800
6
28,800
6
28,800
6
28,800
6
Walsh symbol rate
4800
4800
4800
4800
Walsh chip rate (Kcps)
307.2
307.2
307.2
307.2
Walsh symbol duration
(microsec)
PN chips per code symbol
208.33
208.33
208.33
208.33
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
31
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
32
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
33
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
34
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%
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
35
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
36
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 assignment
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
37
Soft Handoff Architecture
Switch Diversity:
MSC selects the bit stream with
lower error rate
MSC
To other switch
R
BSC
BSC
R
BTS
R
BTS
BTS
Old Link
New Link
R - Handoff request sent to the
old cell on the degrading link
Mobile
Dr. A. Chockalingam
BTS
Energy measurements are made at
the mobile
WW'99, Dept of ECE, IISc, Bangalore
38
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
39
Handoff Example
Pilot
Strength
T_ADD
T_DROP
(1) (2) (3)
Neighbor
Set
Candidate
Set
(4)
(5) (6) (7)
Time
Neighbor
Set
Active Set
T_TDROP
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
40
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
Dr. A. Chockalingam
WW'99, Dept of ECE, IISc, Bangalore
41