Wireless Networks

1
Wireless Networks

• David Watkins
• November 7, 2001

2
Introduction

• Last Time
• Challenges of wireless networks
• Multipath fading
• Propagation delays
• Digital communication (modulation)
• Sharing the capacity of the channel (FDMA, TDMA,
  CDMA)
• High bit error rates (channel coding, puncturing,
  interleaving)
• Infrastructure (infrastructure based vs. ad hoc)
• Cellular networks

3
Introduction

• This Time
• Introduction to TDMA (IS-136)
• Logical Channels
• Framing Structure
• TDMA Network Elements

4
Cellular Telephony

• 1st generation
• Analog circuit switched systems (AMPS)
• 2nd generation
• Digital circuit switched systems (GSM, IS-136,
  IS-95)
• 2.5 generation
• Digital packet switched systems (GPRS/EGPRS)
• 3rd generation
• Digital packet switched systems (UMTS, cdma2000)

5
TDMA (IS-136)

• Uses FDMA and TDMA
• Channels that are each 30 kHz wide
• Cellular (850 band) uplink/downlink channels
  separated by 45 MHz
• PCS (1900 band) uplink/downlink channels
  separated by 80 MHz
• Each channel is further divided using TDMA into 6
  time slots
• Each time slot lasts 6.66 ms and contains 324
  bits
• Voice call uses 2 times slots in every frame
• 20 ms speech sample interleaved over two
  consecutive bursts

A
B
C
A
B
C
30 kHz
frame 40 ms
timeslot 6.66 ms
6
Logical Channels

• Analog
• Analog Control Channels (ACCH)
• Analog Voice Channels (AVCH)
• Digital
• Digital Control Channels (DCCH)
• Digital Traffic Channels (DTCH)

7
Analog Control Channels (ACCH)

• Forward Analog Control Channel (FOCC)
• Paging (20 pages/second)
• Initial voice or digital channel designation
• Overhead message
• Gives information necessary for mobile to operate
  in system
• Reverse Analog Control Channel (RECC)
• Page response message
• Origination message

8
Analog Voice Channels (AVCH)

• Analog Voice Information
• Supervisory Audio Tone (SAT)
• Ensures maintenance of a reliable transmission
  path
• Loss of SAT implies channel conditions have
  become impaired and may cause call to be
  terminated
• Used to identify co-channel interference
• 3 possible SAT values
• Signaling Tone (ST) Control
• Used to indicate status change
• Confirms messages sent from the base station
• Blank-And-Burst Control
• Audio signals are inhibited and replaced by
  signaling messages

9
Digital Control Channels (DCCH)

• Uplink (mobile ? base station)
• Random Access (RACH)
• Used by mobile to request access to the system
• Respond to message sent by base station
• Downlink (base station ? mobile)
• Broadcast Control (BCCH)
• Give DCCH structure
• Neighbor cell information
• Short Message, Paging, Access Response (SPACH)
• 111 pages/second 6 slots
• Response to mobile access attempt
• Shared Channel Feedback (SCF)
• Gives information about the RACH (ie. whether
  its busy)

10
Digital Control Channels (DCCH)
Reverse (Uplink) Digital Control Channel (324
bits, 6.66ms)
GT 6
Pream 16
Sync 28
Data 122
Sync 24
Data 122
RT 6
GT Guard Time RT Ramp Time Pream Preamable
Sync Synchronization and Training Data Data
Bits of DCCH Messages Sync - Additional
Synchronization
Forward (Downlink) Digital Control Channel (324
bits, 6.66ms)
SCF 12
Sync 28
Data 130
CSFP 12
Data 130
SCF 10
R 2
CSFP Coded Superframe Phase (distinguishes DCCH
from DTCH) R Reserved Bits
Sync Synchronization and Training SCF Shared
Channel Feedback Data Data Bits of DCCH
Messages
11
Typical DCCH Search

• Last DCCH used or Coded Digital Control Channel
  Locator (CDL)
• Manufacturer defined search
• Scan for ACCH, look for DCCH pointer
• Search RF channels in order of signal strength
• If a traffic channel is found check for CDL
• Search alternate frequency band

12
Digital Traffic Channels (DTCH)

• User Information
• Speech data
• 20 ms speech sample interleaved over consecutive
  bursts
• Digital Verification Color Code (DVCC)
• Basestation sends code to mobile when channel is
  established
• Mobile echoes code back to basestation
• Identifies current channel from co-channels
• Slow Associated Control Channel (SACCH)
• Slow control channel that is part of every speech
  frame
• Takes 22 time slots (11 frames, 440 ms) to
  complete message
• Fast Associated Control Channel (FACCH)
• Fast control channel that replaces a 20 ms speech
  frame
• Used to control radio in poor radio conditions

13
Digital Traffic Channels (DTCH)
Reverse (Uplink) Digital Traffic Channel (324
bits, 6.66ms)
GT 6
Data 16
Sync 28
Data 122
SACCH 12
Data 122
RT 6
CDVCC 12
GT Guard Time RT Ramp Time Data Data Bits
of DCCH Messages
Sync Synchronization and Training SACCH Slow
Associated Control Channel
Forward (Downlink) Digital Traffic Channel (324
bits, 6.66ms)
Sync 28
Data 130
Data 130
CDL 11
R 1
CDVCC 12
SACCH 12
CDVCC Coded Digital Verification Color Code R
Reserved Bit CDL Coded Digital Channel Locator
Sync Synchronization and Training SCF SACCH
Slow Associated Control Channel Data Data Bits
of DCCH Messages
14
DCCH Framing Structure
TDMA Frame (40 ms)
block
block
Superframe (640 ms)
SFP 0
SFP 1
SFP 2
SFP 3
SFP 12
SFP 13
SFP 14
SFP 15
SFP 16
SFP 17
SFP 28
SFP 29
SFP 30
SFP 31
---
---
Hyperframe (1.28 sec)
Pri SF
Sec SF
Hyperframe Set (15.36 sec)
SF 0
SF 1
SF 2
SF 3
SF 20
SF 21
SF 22
SF 23
SF 11
SF 12
SF 13
SF 14
SF 10
SF 15
---
---
HFC 0
HFC 1
HFC 5
HFC 6
HFC 7
HFC 11
HFC 10
15
Multiplexing Logical Channels in a Superframe
(full-rate)
S-BCCH
E-BCCH
F-BCCH
22 blocks of SPACH
logical channel sequence
F
F
F
F
F
E
E
E
S
S
---
SFP 0
SFP 1
SFP 31
0
1
2
3
4
5
28
29
30
31
---
1
2
3
4
5
6
block
superframe
16
DCCH System AccessRACH Subchannel Random Access
4
1
timeslot number
SCF
FDCCH
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
I - -
B R PE
I R -
P1
P2
P3
P4
P5
P6
P1
P2
P3
P4
P5
P6
P1
P2
P3
P4
choose subchannel
check for burst reception
check for burst reception
RDCCH
subchannel number
1st Burst
Last Burst
I idle B busy R received PE partial echo
P3
P4
P5
P6
P1
P2
P3
P4
P5
P6
P1
P2
P3
P4
17
Importance of Framing Structure

• Paging
• Assigned paging subchannel is determined by the
  mobiles paging channel allocation ID (PAID)
• Hyperframe counter allows mobile to determine
  which hyperframe contains the mobiles paging
  subchannel
• Allows mobile to perform discontinuous reception
• Saves mobiles battery power
• Sleep period determined by Paging Frame Class
  (PFC)
• Min value every hyperframe
• Max value every 96th hyperframe (122.88 sec)
• Mobile requests PFC during registration
• Paging information is the same in both
  superframes (primary and secondary) of a
  hyperframe

18
TDMA System Architecture
MS Mobile Station BTS Base Station AUC
Authentication Center MSC Mobile Switching
Center
HLR Home Location Register VLR Visitor
Location Register PSTN Public Switched
Telephone Network
19
TDMA System Architecture

• Mobile Station (MS)
• User device used to communicate to the network
  (ie. Cell phone, laptop with wireless adaptor)
• Base Station (BTS)
• Network element that communicates with mobile
  stations
• Performs power measurements
• Mobile Switching Center (MSC)
• Determines channel assignments
• Makes handoff decisions based on measurements
  calculated by base stations
• Forwards paging messages to appropriate base
  stations
• Authentication Center (AUC)
• Used during initial access to authenticate user

20
TDMA System Architecture

• Home Location Register (HLR)
• Stores information about users whose home
  location area is within the control of this HLR
• Information includes
• Mobile capabilities
• Mobile unit identification
• Current location area where mobile is located
• Visitor Location Area (VLR)
• Stores information about users who are currently
  authenticated within the VLRs location area
• Information includes
• Current location where mobile is located
• State of mobile (active, standby, etc.)
• Public Switched Telephone Network (PSTN)
• Land-line phone network

21
Summary

• Logical channels and framing structure define a
  wireless protocol
• Network elements define the interfaces, some of
  the network elements may actually be multiple
  processes running on one computer system
• Description of IS-136 protocol, but concepts
  apply to many wireless standards

22
Questions