Second Generation (2G) Cellular

1
Second Generation (2G) Cellular

• Dr. A. Chockalingam
• Assistant Professor
• Indian Institute of Science, Bangalore-12
• achockal_at_ece.ucsd.edu
• http//www.ece.iisc.ernet.in/achockal

2
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

3
GSM

• Global System for Mobile communications

OMC
BTS
BSC
PSTN
MSC
BTS
MS
ISDN
BTS
Data Networks
BSC
HLR
AUC
VLR
BTS
Public Networks
Network and Switching Subsystem (NSS)
BTS
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
MS
Base Station Subsystem (BSS)
4
GSM - Interfaces
BTS
MSC
BSC
PSTN
BTS
MS
A Interface (standardized)
BTS
SS7
Abis Interface (standardized)
GSM Radio Air Interface (standardized)
5
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

6
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)

7
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)

8
TDMA Frame in GSM
Time Slot
577 microseconds
0
1
2
4
5
6
7
3
GSM TDMA Frame
(8 time slots 4.615 milliseconds)
9
GSM Normal Traffic Burst
0 1 2 3 4 5 6
7
148 BITS
T A I L
F L A G
T A I L
F L A G
DATA
DATA
Training Sequence
(e.g. encoded voice)
(e.g. encoded voice)
26 BITS
57 BITS
57 BITS
3 BITS
1 BIT
1 BIT
3 BITS
10
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

11
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.

12
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

13
IS-54 Radio Interface
14
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

15
GSM / IS-54 Summary
16
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

17
CDMA Channels Frequencies

• CDMA frequencies assigned through a 11-bit CDMA
  Channel number, N
• At Mobile
• 
  MHz
• 
  MHz
• At Base Station
• 
  MHz
• 
  MHz

18
CDMA Channel Frequency
Reverse CDMA Channel
Forward CDMA Channel
1.25MHz
1.25MHz
Frequency
CDMA Channel Frequency
847.74 MHz
45 MHz
892.74 MHz
19
Frequency Reuse CDMA
Freq Reuse Plan in CDMA
7 cell Freq Reuse Plan
20
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

21
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

22
Forward Link Channel Structure
Forward CDMA Channel (1.25 MHz Chl. Tx by Base Stn
Paging Chl. 7
Paging Chl. 1
Traffic Chl. 1
Traffic Chl. 55
Traffic Chl. 2
Sync Chl
Pilot Chl
W0
W1
W7
W8
W63
W32
W9
Power Control Sub channel
Traffic data
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FL Modulation Structure
W0
1.2288 Mcps
To Quadrature Spreading
Pilot Chl all 0s
W32
Sync Chl
To Quadrature Spreading
1.2288 Mcps
1200 bps
4800 bps
Convol. Encoder/ Repetition
Block Interleaver
Wp
1.2288 Mcps
To Quadrature Spreading
Paging Chl
19.2 Kbps
Block Interleaver
Convol. Encoder/ Repetition
9600 bps 4800 bps 2400 bps
1.2288 Mcps
19.2 Kbps
Paging Chl p Long code Mask
Long Code Generator
Decimator
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FL Modulation Structure
Power Control Bit
Walsh Code
M U X
Convolutional Encoder and Repetition r1/2, K
9
User data
19.2 k
1.2288 Mcps
Block Interleaver
9600 bps 4800 bps 2400 bps 1200 bps
Symbol cover
Scrambling
Quadrature Spreader
800 Hz
Long Code generator
Decimator
Decimator
Q-Chl Pilot PN Seq
I-Chl Pilot PN Seq
1.2288 Mcps
Baseband filter
Baseband filter
Long code for nth user
Forward CDMA Traffic Channel Structure
Note Pilot PN Offset identifies the base station
25
FL Modulation Parameters
Sync Channel
26
FL Modulation Parameters
Paging Channel
27
FL Modulation Parameters
Forward Traffic Channel
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IS-95 CDMA Reverse Link
Reverse CDMA Channel (1.25 MHz Chl. Rx by Base Stn
Traffic Chl. m
Access Chl. 1
Access Chl. 2
Traffic Chl. 1
Access Chl. n
Traffic Chl. 2
Traffic Chl. 3
Addressed by long code PNs
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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

30
RL Modulation Structure
Long code Mask for user n
Long Code generator
1.2288 Mcps
PN chip
Code symbol
Walsh chip
Code symbol
Information bit
Zero offset Pilot PN Seq Q Chl
Convolutional Encoder and Repetition r1/3, K
9
64-1ry Orthogonal Modulator
Data burst randomizer
Block Interleaver
9600 bps 4800 bps 2400 bps 1200 bps
Zero offset Pilot PN Seq I Chl
28.8 Ksps
307.2 Kcps
1/2 PN chiip delay406.9 ns
D
Baseband filter
Baseband filter
Reverse CDMA Traffic Channel Structure
31
RL Modulation Parameters
Reverse Traffic Channel
32
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

33
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

34
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

35
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

36
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

37
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

38
Soft Handoff Architecture
Switch Diversity MSC selects the bit stream with
lower error rate
To other switch
MSC
R
BSC
BSC
R
R
BTS
BTS
BTS
BTS
Old Link
New Link
R - Handoff request sent to the old cell on the
degrading link
Energy measurements are made at the mobile
Mobile
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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

40
Handoff Example
Pilot Strength
T_ADD
T_DROP
(1)
(5)
(6)
(7)
(2)
(4)
(3)
Time
Neighbor Set
Neighbor Set
Candidate Set
Active Set
T_TDROP
41
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