Admissions in india 2014

Admissions in India 2015
By:
admission.edhole.com

Second Generation (2G) Cellular
Dr. A. Chockalingam
Assistant Professor
Indian Institute of Science, Bangalore-12
achockal@ece.ucsd.edu
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 3

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 Center
BTS: Base Transceiver Station HLR: Home Location Register
BSC: Base Station Controller VLR: Visitor Location Register
AUC: Authentication Center OMC: Operation Maintenance Center admission.edhole.com

GSM - Interfaces
BTS
BTS
BTS
MS
BSC MSC PSTN
A Interface SS7
(standardized)
Abis Interface
(standardized)
GSM Radio Air
Interface (standardized)

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 apdhmonisesion.edhole.com

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 acdampaiscsitiyo)n.edhole.com

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)

TDMA Frame in GSM
577 microseconds
Time
Slot
0 1 2 3
4 5 6 7
GSM TDMA Frame
(8 time slots = 4.615 milliseconds)

GSM: Normal Traffic Burst
0 1 2 3 4 5 6
7
F
L
A
G
F
L
A
G
148 BITS
DATA DATA
Training
Sequence
(e.g. encoded voice) (e.g. encoded voice)
26 BITS
57 BITS 57 BITS
T
A
I
L
T
A
I
L
3 BITS 1 BIT 1 BIT 3 BITS

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

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 inavdomlviesds iionn a.e cdahllo. le.com

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

IS-54 Radio Interface
Parameters IS-54 Specifications
Multiple Access TDMA/FDD
Modulation Pi/4 DQPSK
Channel bandwidth 30 KHz
Rev Channel band 824 – 849 MHz
Fvd channel band 869 – 894 MHz
Fvd & 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 7.95
kbps/user)
6 (with half-rate speech coder of
3.975 kbps/user)

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

GSM / IS-54 Summary
GSM IS-54
Year of introduction 1990 1991
Multiple Access TDMA/FDMA/FDD TDMA/FDMA/FDD
Frequencies 890 - 915 MHz (R)
935 – 960 MHz (F)
824 – 849 MHz (R)
869 – 894 MHz (F)
Modulation GMSK (BT=0.3) /4 - DQPSK
Carrier separation 200 KHz 30 KHz
Channel data rate 270.833 Kbps 48.6 Kbps
No. of voice channels 1000 2500
Spectrum efficiency 1.35 bps/Hz 1.62 bps/Hz
Speech coding RELP-LTP @ 13 Kbps VSELP @7.95 Kbps
Channel coding CRC with r=1/2; K=5
Conv.
7 bit CRC with r=1/2; L=6
Conv
Equalizers Adaptive Adaptive

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

CDMA Channels & Frequencies
 CDMA frequencies assigned through a 11-bit CDMA
Channel number, N
 At Mobile
N
1  
777
N
1013 1023
MHz
 
N
0.030 
825
N
0.030( 1023) 825
MHz
 At Base Station
 
N
1  
777
N
1013  
1023
N
0.030 870
MHz

N
0.030( 1023) 870
MHz
 

CDMA Channel & Frequency
Reverse
CDMA Channel
Forward
CDMA Channel
1.25MHz 1.25MHz
CDMA Frequency
Channel
Frequency 847.74 MHz
45 MHz
892.74 MHz

Frequency Reuse CDMA
B
D
E
F
E
G C
B
D
A
E
F
F
C
D
A
E
G
F
C
D
B
C
A G
B
C
B
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
7 cell Freq Reuse Plan Freq Reuse Plan in CDMA

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 i j i j  ( ) ( )  0, 
» used to separate one user from another
– PN codes are used on both forward and
reverse (mobile-to-base) links

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

Forward Link Channel Structure
Forward CDMA Channel
(1.25 MHz Chl. Tx by Base Stn
Pilot
Chl
W0
Sync
Chl
W32
Paging
Chl. 1
W1
Paging
Chl. 7
Traffic
Chl. 1
Traffic
Chl. 2
Traffic
Chl. 55
W7 W8 W9 W63
Traffic data
Power Control
Sub channel

FL Modulation Structure
W0
1.2288 Mcps
W32
Pilot Chl: all 0’s
Sync Chl
1200 bps 4800 bps
Convol. Encoder/
Repetition
Block
Interleaver
1.2288 Mcps
Convol. Encoder/
Paging Chl
9600 bps Repetition
Interleaver
4800 bps
2400 bps
Block
19.2 Kbps
Wp
To Quadrature
Spreading
To Quadrature
Spreading
1.2288 Mcps
Paging Chl p
Long code Mask
Long Code
Generator
1.2288 Mcps
Decimator
19.2 Kbps
To Quadrature
Spreading

FL Modulation Structure
Convolutional
Encoder
and Repetition
r=1/2, K = 9
User data
Block
Interleaver
Power Control
M
U
X
Bit
Walsh
Code
19.2 k 1.2288 Mcps
800 Hz
Scrambling
9600 bps
4800 bps
2400 bps
1200 bps
Long Code
generator Decimator Decimator
I-Chl Pilot
PN Seq
Baseband
filter
Quadrature
Spreader
Q-Chl Pilot
PN Seq
Baseband
filter
Long code
for nth user
1.2288 Mcps
Symbol
cover
Forward CDMA Traffic Channel Structure
admission.edholeNo.cteo: Pmilot PN Offset identifies the base station

FL Modulation Parameters
Sync Channel
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

Paging Channel
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

Forward Traffic Channel
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 19,200
PN 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

IS-95 CDMA Reverse Link
Reverse CDMA Channel
(1.25 MHz Chl. Rx by Base Stn
Access
Chl. 1
Access
Chl. 2
Access
Chl. n
Traffic
Chl. 1
Traffic
Chl. 2
Traffic
Chl. m
Traffic
Chl. 3
Addressed by long code PNs

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

RL Modulation Structure
Convolutional
Encoder
and Repetition
r=1/3, K = 9
Information
Long Code
generator
bit
Block
Interleaver
Baseband
filter
Baseband
filter
Long code Mask
for user n 1.2288 Mcps
Zero offset
Pilot PN
Seq I Chl
Reverse CDMA Traffic Channel Structure
9600 bps
4800 bps
2400 bps
1200 bps
Code
symbol
28.8 Ksps
64-1ry
Orthogonal
Modulator
Zero offset
Pilot PN
Seq Q Chl
Data
burst
randomizer
Code
symbol
Walsh
chip
307.2 Kcps
PN chip
D
1/2 PN chiip
delay=406.9 ns

RL Modulation Parameters
Reverse Traffic Channel
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
208.33 208.33 208.33 208.33
(microsec)
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

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

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

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

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%

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

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

Soft Handoff Architecture
MSC
To other switch
Switch Diversity:
MSC selects the bit stream with
lower error rate
R
BSC BSC
R R
BTS BTS BTS BTS
R - Handoff request sent to the
old cell on the degrading link Energy measurements are made at
Mobile
New Link
Old Link
the mobile

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 admission.edhole.com

Handoff Example
Time
Pilot
Strength
(1)
T_ADD
T_DROP
(2) (3) (4) (5) (6) (7)
Neighbor
Set
Candidate
Set Active Set
T_TDROP
Neighbor
Set

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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