Mobile Digital Services

1
Mobile Digital Services

• Basic Concepts
• FDM, TDMA, CDMA
• Digital Mobil Telephony systems
• TDMA, CDMA, GSM
• Wireless LAN
• 3rd Generation Systems

2
Some Design Imperatives

• Mobile devices must be small and lightweight.
• Limited battery size
• Limited size for displays
• Limited antennas
• Need to avoid wasting power on filters or extra
  transmission
• Consumers want long battery life
• Power must be carefully managed
• Radio Spectrum is a key resource to be conserved
• Low bit rate voice coding
• Special high efficiency data protocols.
• Devices receive signals via multiple reflected
  paths.
• Must compensate for interference and timing
  changes.
• Environment is noisy (multipath, obstacles)
• Error detection/correction required
• Need adequate SNR margins limits modulation
  choices
• Intermittent transmission requires
  resynchronizing receiver

3
Sharing Bandwidth

• Frequency division (FDM)
• Device is assigned a dedicated subset frequency
  range.
• Supports a fixed bit rate (CWlog2(1SNR))
• Not well suited to bursty or variable rates
  (data).
• Time division (TDMA)
• Device is assigned a frequency range for a
  specific time
• Data rate depends on bandwidth and time assigned.
• Synchronization time is required between uses.
• Data is sent at high speed, suitable for bursty
  rates
• Code division (CDMA) (spread spectrum)
• channels overlap in time and frequency, separated
  by redundancy and orthogonal codes
• high speed, accommodates variable rates, number
  of nodes.

4
Mobile Telephony -- TDMA
40 ms
30khz
A
B
C
A
B
C
A
A
B
B
30khz

• North American Standard compatible with AMPS
  (824-924Mhz)
• each 30Khz radio channel carries 25 1944 bit
  frames/second split into 6 timeslots 48.6kb/s
• Base station uses one channel to send to 3 full
  rate mobiles (2 timeslots each per frame), and
  transmits continuously.
• Mobiles transmit in two timeslots of another
  channel per frame

5
Modulation
Even bit pairs
Odd bit pairs

• Quadrature Modulated 4PSK
• Two alternating phase point sets
• Guarantees transition of ?/4 each time
• 48.6Kb/s in 30Khz 1.6 bits/Hz

6
TDMA Formats
12
130
130
12
12
28
Sync
SACCH
Data
CDVCC
Data
RSVD
28
128
128
12
12
16
3
3
R
G
Data
Sync
Data
SACCH
CDVCC
Data

• SACCH Control information
• CDVCC Continuity and identity confirmation
• G Guard time (no transmission, to allow
  alignment)
• R Ramp time (time for mobile to get to assigned
  power level
• Sync synchronization (timeslot and power
  measurements)

7
Voice Coding and Transmission

• Vector Sum LPC Coding (VSELP)
• Raw code takes 159 bits per VSELP frame.
• 2 VSELP frames (20ms) in each TDMA frame gives
  2?159?257950bps
• Add error correction and control
• 77 bits of redundant code for each VSELP frame
• 7 bits of CRC on key bits CRC error on critical
  parameters will cause re-use of old data or
  muting
• Other info for sequencing and interleaving.
• Total of 13kbits/second 260 bits/timeslot

8
Speech Interleaving
260
260
159
VSELP Coder
Error Control
Timeslot
Timeslot

• Speech coding and error control produces 260 bits
  every 20ms.
• Speech from frames is interleaved in each
  timeslot.
• Reduces the probability of burst errors

9
Some design questions

• Assuming that guard time allows each mobile to
  be off by 3 bit times from its assigned
  transmission time, how far can it move before it
  will exceed this?
• How much delay is introduced into a 2-way call by
  TDMA cellular voice coding and packing?

10
Mobile Telephony -- GSM
120/26 ms
7
6
5
4
3
2
1
0
0
1

• World (ITU) standard usually at 890-915 and
  935-960Mhz
• Each 200Khz channel carries 120/26 frames of 8
  156.25 bit bursts/second (270.833kbps)
• Base and mobiles both transmit in bursts only
  when they need to (saves power and interference)

11
Modulation Gaussian MSK

• Close to 4PSK (2 bits per signal)
• Compact spectrum means less filtering and more
  constant power
• More efficient, longer battery life
• Easier to train receivers
• Lower bits/Hz
• 270K/200Khz 1.35, versus 48.6/30 1.62
• No guaranteed transitions in modulation
• Data format must guarantee transitions for clock
  recovery

12
GSM Data layout
15/26 0.577 ms
3
57
57
3
1
8.25
1
26
T
TCH
F
TCH
T
Guard
F
Train

• Tail (T) bits are used to train equalizers
• Flag (F) bit indicates whether content (TCH) is
  voice data or FACCH
• Training bits are a fixed pattern
• Guard time is idle to avoid collisions
• 114 bits maximum voice data, 148 bits of
  information, 8.25 bit times of guard 156.25

13
Voice Coding
456
456
456
Vocoder
Convolutional Coder

• Raw code is RPE-LTP using 260 bits per 20ms voice
  frame
• 50 class 1A most error sensitive
• 132 class 1B moderately sensitive
• 78 Class 2 least sensitive
• Class 1A protected by a CRC, and CRC, Class1A and
  Class 1B bits are coded for error correction,
  total of 456 bits.
• Bits broken into 8 57 bit blocks and interleaved
  to avoid wiping out more than 57 in each burst
• Bit rate is 456/0.20 22.8kb/s.
• data rate available 114?(1000/(120/26))
  24.7kb/s

14
Slow Frequency Hopping

• Carrier frequencies are changed for each frame
• Fading is dependent on frequency and changing
  frequency will change fading (i.e. fading at one
  frequency wont cause all speech to be lost)
• Frequency changing is different in different
  cells, so the set of phones you might interfere
  with changes with each frequency change.
• Result is better overall quality for same power
  and cell spacing, or more phones in same spectrum
  with less interference.

15
Code Division Multiple Access (CDMA)
Broad Channel

• Basic technique is spread spectrum transmission
• Signal is broken into pieces and spread across
  multiple channels
• Redundancy in signal allows recovery even when
  some is lost
• equalizes and reduces interference more users
  per channel
• Provides security
• Two types
• Frequency hopping each piece in narrow
  frequency band
• Direct coding each piece in the whole band but
  coded (CDMA)
• Original Patent holder is a WWII era movie
  actress! (Heddy Lamarr)

16
Direct Spreading Coding (CDMA)
chip clock
Clock Multiplier
Code Sequence Generator
Carrier
Clock
Data

Modulator
Filter
Buffer

• Each bit of signal is exclusive exclusive orred
  with a code sequence to form a sequence of
  chips
• Resulting signal is modulated on a broadband
  carrier and broadcast
• Senders share the same frequency but each uses a
  different code sequence.
• Signals from different senders add and interfere
  on individual chips
• Receiver takes in the entire sequence and
  correlates it with its chip code, determining
  received value based on correlation

17
CDMA Coding Example
Coding Process

• Each Channel has a unique Chip code sequence
• Each channels bit is exclusive orred with
  each chip to produce a sequence of values
• Transmitting the values causes them to be
  added together to produce a multi-level result

18
CDMA Coding/Decoding
Decoding Process

• To decode channel N, multiply each chip of the
  incoming signal by the corresponding chip
  code.
• Sum the result of all chips to produce a
  correlation value
• Perfect correlation will produce a value of
  or n for N chips
• Interference will result in values that are
  higher or lower
• Signal can be received as long as interference N

19
CDMA Code Sequences

• For the code/decode trick to work, other signals
  must look like random noise with respect to the
  desired signal
• Key property is that the correlation signal (sum
  of products of codes) for other codes is near
  zero.
• Different approaches possible
• PN codes Pseudo-random codes with long repeat
  times
• Useful any time but not guaranteed not to
  correlate
• Walsh codes Orthogonal code sequences that
  have no correlation normally
• Guaranteed non-correlation (sum is always zero)
  but must be tightly synchronized
• Each channel is a shift of another, so
  transmission must be synchronized to prevent
  correlating interference.

20
Why it works

• If there are N chips
• Desired signal will contribute N to the sum
• Other signals will contribute 1 or 1 to the sum
  (non correlating)
• If the chips are summed before level comparison
• Power of the desired signal is N2
• Average power of an interfering signal is 12
• Signal to Interference (noise) ratio for M
  interfering signals is 10log10(N2/M).
• For IS95 (North American) CDMA, N64,M63,
  SIR18dB, which is greater than 13.6 needed for
  decoding 2PSK and 4PSK.

21
It works with numbers, what about waves?
-1
1 4 -2 1
Carrier Waveform
1
Phase detector output

• In 2PSK, the two signals sent are 180 degrees out
  of phase and cancel each other, just like 1 and
  1.
• 4PSK is two channels (I and Q), each of which is
  2PSK modulated.
• Result of interference will be a multi level wave
  for each of I and Q channels.

22
Another way to think about CDMA, TDMA, FDMA
Capacity
Shannons law
Log2(SNR)
Bandwidth

• Frequency division splits the bandwidth into
  sub-bands
• Time division allocates the entire channel to
  different signals at different times
• Code division in effect splits the SNR among
  competing signals that share the channel Other
  signals look like noise and reduce SNR, but
  reception occurs as long as the competing noise
  is kept sufficiently small.

23
Coding for IS95 CDMA
Data
Short Seq
Walsh Code
Long Seq

• Short Sequence pseudo-random sequence unique to
  cell
• Insures interference between cells using same
  channel (Walsh) code is random
• Walsh Code One of 64 orthogonal codes
• Insures no correlation between signals in one
  cell (defines a channel)
• Long Sequence pseudo-random sequence unique to
  phone
• Used for Privacy to prevent others from listening

24
Multipath signals and CDMA
1100101
1110010

• Channel codes are cyclic shifts of eachother
• Many, if not most links involve reflections
• Different Multipath signals will be delayed by
  one or more chip times.
• Inerfering Multipath signal looks like a signal
  on another channel (e.g. uncorrelated noise)
• In TDMA and GSM a reflected signal may cancel the
  primary signal and create dropouts.

25
Power Control

• Key to making CDMA work is maintaining constant
  received power
• Mobile transmit power must be controlled so
  power level at the transmitter is equal
• Two Strategies
• Open loop - Mobile measures received signal and
  assumes the same loss on the return path
• Closed loop - Base station tells mobiles to
  increase or decrease power

26
Soft Handoff

• If the same frequency is used in an adjacent
  cell, the mobile can receive signal from two
  cells.
• Strengthens signal in weak areas
• Allows mobile to move and pick up the new base.
• Both cells listen to the mobile and the MSC
  Integrates the received signal
• MSC must coordinate the distribution of signal
  and reconstruction

27
Coding and Bandwidth

• For IS95 CDMA
• Channel bandwidth is 1.25Mhz
• 64 chips per bit
• Voice is coded using QCELP (9.6Kbps)
• Redundant coding raises rate to 19.2Kbps base to
  mobile and 28.8Kbps mobile to base
• Chip rate is 1.2288 Mbps
• Modulation is QPSK (4PSK)

28
Click to Connect Service (Nextel)
Control
ATM Cells
ATM (Data) Switch

• User clicks to open an instant connection to
  another user of the service
• Nextel implementation based on unusual standards
• GSM radio interface with non-standard control
• Voice is carried as ATM data in the access
  network and never connected to the Telephone
  network (hence why it only works to other Nextel
  customers and why it is fast)
• Many companies working on more standard
  implementation
• Fast connection setup is a significant problem

29
802.11 Physical Layer structure
LLC PDU
LLC
MAC HDR
MAC SDU
CRC
MAC Layer
Physical Layer Convergence Procedure
PLCP PRMBL
PLCP HDR
PLCP PDU
Physical Layer
Physical Medium Dependent
Figure 6.74
30
Alternative Physical Levels

• Frequency Hopping
• 79 channels of 1Mhz or 2Mhz bandwidth
• 26 channels in each of 3 groups defined by
  patterns of changes every 224 microseconds
• 1 or 2Mhz net data rate
• Direct Spreading (802.11b)
• 1or 2Mhz using 11 chip Barker patern
• 5.5 or 11Mhz using 64 chip pattern
• Most interfaces adapt dynamically.
• OFDM (802.11a)

31
Frequency Hopping PLCP
80 bits
16
12
4
16
Variable length
Sync
Start Frame Delimiter
Length
Signaling
CRC
Payload data
PLCP preamble
PLCP header

• Sync establishes reception and timing
• Length up to 216

Figure 6.75
32
Direct Sequence PLCP
128 bits
16
8
8
16
Variable length
16
Sync
Start frame delimiter
Signal
Length
CRC
Payload data
Service
PLCP preamble
PLCP header

• Each bit is converted into an 11 bit Barker
  sequence
• 1Mbps signal is converted to 11Mbps and
  transmitted in 11Mhz
• 11 overlapping channels in 83Mhz bandwidth

Figure 6.77
33
11 chip Barker sequence
1
1 1
1 11
-1
-1
-1 -1-1
11 symbol times
To transmit 1, send
1
1 11
1 1
-1
-1 -1 -1
-1
11 symbol times
To transmit -1, send
1
1 11
1
-1
-1 -1
-1 -1 -1
11 symbol times
Figure 6.76
34
802.11b higher rates

• Data is coded using 64 8 chip code words
• Each code word codes up to 6 bits (some
  redundancy)
• Same 11 overlapping channels.
• Net rate is 5.5Mbits/second with BPSK or
  11Mbits/second with QPSK.

35
57-73 slots
4
3
16
Variable length
32
16
Sync
Start frame delimiter
Data rate
Length
CRC
Payload data
DC level adjust
PLCP preamble
PLCP header
Figure 6.78
36
Physical Layer Specifications of 802.11a
Standard

• To provide wireless connectivity to stations that
  are portable or hand-held or mounted on moving
  vehicles with in local area.
• It is the physical layer standard for the
  Wireless LANS.

From Pavan Inturi
37
Orthogonal Frequency Division Multiplexing (OFDM)

• A type of multi carrier modulation
• Single high-rate bit stream is converted to low
  rate N parallel bit streams
• Each parallel bit stream is modulated on one of N
  sub carriers
• Each sub carrier can be modulated differently.
  For example BPSK, QPSK or QAM
• To achieve high bandwidth efficiency, the
  spectrum of the sub carriers are closely spaced
  and overlapped
• Nulls in each sub carriers spectrum land at the
  center of all other sub carriers
• OFDM symbols are generated using IFFT

From Pavan Inturi
38
Advantages of OFDM

• Robustness in multi-path propagation environment
• More tolerant of delay spread
• Due to the use of many sub carriers, the symbol
  duration on the sub carriers is increased,
  relative to delay spread.
• Inter-symbol interference is avoided through the
  use of guard interval
• More resistant to fading. FEC is used to correct
  for sub-carriers that suffer from deep fade.

From Pavan Inturi
39
802.11a Physical Layer Data Format

• Short Training Sequence
• 10 symbols of 0.8µs each
• Used for AGC
• Long Training Sequence
• 2 symbols of 3.2µs each1.6µs of Guard interval
• Used for symbol timing, channel and frequency
  estimation
• SIGNAL field
• Indicates data rate and length of remaining
  data
• Coded in lowest rate
• DATA symbols
• Coded in one of 8 data rates from 6Mbps to 54
  Mbps.

From Pavan Inturi
40
PPDU Format

PLCP Header
Coded/OFDM
Coded/OFDM
(BPSK, r1/2) (RATE
is included in the SIGNAL)

From Pavan Inturi
41
Other elements of 802.11a

• Convolutional coding Produces redundancy for
  error control
• Scrambling adjacent bits are repositioned
• Adjacent bits in different sub-carriers
• Adjacent bits will not be both be the least
  significant bits in QAM
• Channel estimation
• Receiver must determine timing and frequency
  distortion of the channel.

42
3rd Generation Wireless

• Successor to current Second Generation
  technologies (GSM, TDMA, CDMA)
• Primary driver seen as data and multi-media
  services, but extra voice capacity is also an
  issue
• Greater interoperability is another benefit.

43
3rd Generation Wireless (3G) Topics

• Common air interface, and formats to allow
  terminals to be used worldwide
• Common bands chosen at 2Ghz, not allocated in all
  countries
• Higher voice capacity/channel
• WCDMA and CDMA-1 roughly double capacity
• Higher data rates to support new services
• 2Mb/s stationary, 384Kb/s Mobile promised, dont
  expect that much, but better than 14.4-19.2 kbps
  for 2G
• Evolution from 2G formats
• Very Complicated

44
3G Technology Aspects
Radio (air) Interface
Services and Control
Telephone Network
Internet
Access Network

• Air Interface - Need for compatibility to fit in
  assigned spectrum
• Access network - Desire to re-use existing
  structures and evolve to accommodate voice/DATA
• Services and Control - Reuse plus more
  flexibility
• Migrate towards internet standards for data and
  voice
• Multiple Starting points, principally GSM, TDMA,
  and CDMA
• GSM - driven by 3GPP group evolution plan
• CDMA - Driven by 3GPP2 group plan
• TDMA - Left to evolve towards GSM or CDMA

45
3rd Generation Evolution Steps
W-CDMA (UMTS)
GSM
GPRS
Edge
2Mb/s
115Kbs
384Kbs
TDMA
Squeezing more data in existing frequency bands
New air interface and frequency bands
IS95 (cdmaOne)
1X-DO 3X-DV
CDMA2000-1X
2.4Mb/s
144Kb/s
2G
2.5G
3G

• Very Complicated multi-step evolution planned
• Intermediate steps called 2.5G
• Still doesnt quite achieve common world standard

46
3G Radio Bandwidth
IS95
-1X Formats
-3X Formats
1.25Mhz
Frequency
1.25Mhz
1.25Mhz
Voice/Data
Voice/Data
Voice only

• CDMA-one and WCDMA both migrate to 5Mhz bands
  (3.75Mhz usable, rest guard)
• Wider bands allow faster data rates and more CDMA
  channels/band (longer chip sequences and more
  codes)
• Intermediate steps use 3 1.25Mhz bands for
  compatibility with IS95 CDMA channels
• Multiple channels may be combined.

47
3G air interface Basic Concept
Code 1
Performance Limit
I0
3.75Mhz Channel
Code 2
Code 3
Time

• Users transmit in bursts using different CDMA
  spreading codes
• Timing and transmission managed to maintain
  acceptable performance (degree of overlap)
• Many different specific schemes for multiplexing

48
Getting Higher Bit Rates
E0
Minimum E0/I0 Required for correct reception
(4PSK)
Power
E0
time

• Faster rate means shorter bits and higher power
  for same E0
• Power output is limited, so reduce attenuation
  and/or noise and interference
• Different rates for different cell sizes (2Mb/s
  only for very small in building cells
• Improve antenna selectivity
• Noise Cancellation
• Coherent Detection

49
Smart antennas
path difference phase shift(n1/2)? cancelled
path difference phase shiftn? (enhanced)

• Multi-element antennas controlled with different
  phase signals to direct signal to a user
• Antenna element chosen to avoid fades (i.e.
  spatial diversity)
• Receive signals integrated to null out
  interfering signals
• Can gain significant signal energy

50
Coherent Demodulation
Pilot (code 1)
Data (code n)
data
p
data
p
data
p

• Signals contain carrier pilot in both
  directions to clearly identify correct phase for
  decoding
• Bit sequence with dedicated CDMA code (base to
  mobiles)
• known bits in each transmitted signal to sync up
  received reference (mobiles to base)
• Good for 3dB improvement in detection performance
• allows ½ the power or twice the interference.

51
Power Control
Shadow Fading

• Signal fades when mobile moves behind a building
• Extra power margin needed to avoid loss
• Can be minimized if mobile can quickly ask the
  base to increase power to compensate
• Good for about 3dB of improvement

52
Convolutional (Turbo) coding
C0

Data in
C1

• Coder produces multiple output bits per input bit
  by combining last k bits of data
• 1/2 coder produces 2 bits for each 1
• 1/4 coder produces 4 for each 1
• Multiple errors and burst errors can be
  recovered from the resulting data.
• Tolerates higher error rate, reduces required SNR
• 3G uses higher order coders (more redundancy)

53
Interference Cancellation
Interferers
Signal

• Approximately 65 of the interference is other
  signals in the same cell/sector. (rest is from
  other cells or real noise)
• Base Station is decoding all incoming signals
  from this cell/sector
• So When you identify the bit sequences of the
  interfering signals, you can compute what their
  waveforms would have been and subtract them out
  of the received signal to isolate the intended
  one
• In theory you get a 2.7 times reduction in
  interference (1/0.35), In practice somewhat less.

54
3G Data Options
From Jack Kozik Lucent Technologies
55
Data Throughput Comparison(Downlink)
From Jack Kozik Lucent Technologies
56
Voice Capacity Comparison
From Jack Kozik Lucent Technologies
57
3G Conclusions

• 3G deployment just starting
• Japan and European first steps entered service
  late 2001 and early 2002
• US first steps (Verizon, ATT July 2002),
  (Sprint, T-Mobil (aka VoiceStream) early 2003)
• Many technology issues and alternatives remain.
• Biggest uncertainty is probably business who
  will pay for it?
• People will pay for data, but 3G competes with
  WiFi and other technologies
• Many carriers giving away data services to create
  business, but this also creates expectations
• Increasing voice capacity is valuable to some
  carriers, not others