ICOM 6505: Wireless Networks Medium Access Control

1
ICOM 6505 Wireless Networks- Medium Access
Control -

• By Dr. Kejie Lu
• Department of Electronic and Computer Engineering
• Spring 2008

2
Outline

• Basics
• Duplexing
• Coordinated MAC schemes
• Random MAC schemes
• Spread spectrum and spread spectrum access

3
Spread Spectrum Techniques

• With spread spectrum, signals can be transmitted
  with a bandwidth that is larger than the minimum
  required RF bandwidth
• Spread Spectrum is very useful in coping with
  multipath channel
• Techniques
• Frequency Hopped Spread Spectrum (FHSS)
• Direct Sequence Spread Spectrum (DSSS)
• OFDM

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Spread Spectrum Multiple Access

• Spread spectrum techniques can be extended for
  multiple access
• FHSS ? FHMA
• DSSS ? DSMA
• Also called Code Division Multiple Access CDMA
• OFDM ? OFDMA

5
Frequency Hopping Spread Spectrum

• Each channel employs rapid, discrete changes in
  frequency which are tracked by the intended
  receiver
• For a lightly loaded system, FH (Frequency
  Hopping) system can greatly reduce interference
• For a high channel loadings, the performance of
  FH system is no better than that of a non-hopped
  system

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Hopping Rate

• Rate of hopping versus Symbol rate
• Fast Frequency Hopping
• Hopping rate is greather
• One bit transmitted in multiple hops
• Slow Frequency Hopping
• Symbol rate is greater
• Multiple bits are transmitted in a hopping period
• GSM and Bluetooth are example systems

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Bluetooth

• Frequency band 2.4 GHz
• Uses FHSS in a cell (piconet)
• Wideband spectrum width 79 MHz
• Channel spacing is 1 MHz (narrowband channel
  bandwidth)
• There are 79 narrowband channels (carrier
  frequencies) to hop through
• Freq (f) 2402k MHz, k 0,...,78
• Hopping Rate 1600 Hops/Second
• Hopping sequence is determined by Bluetooth
  Hardware address and Clocks that are synchronized
  between sender and receiver

79 MHZ
0
1
2
3
77
78
.....
79-Hop System
1 MHZ
A hop sequence could be 7,1,78,67,0,
56,39,.......
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Piconet

• The basic network in Bluetooth is Piconet
• Two types of node Master and Slave
• Master
• Master coordinates access to the the media
• All traffic has to go over master
• Slave
• Maximum 7 slaves can be connected to a master
• Slaves can not talk to each-other directly

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Piconet

• Piconet is wireless personal area network (WPAN)
• Range 10m
• Raw Data-rate 1 Mbps/piconet
• FHSS
• All slaves and the master hops according to the
  same hopping sequence
• The hopping sequence is determined by the clock
  and BT_address of the master
• On average the frequency-hopping sequence visits
  each hop carrier with an equal probability

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An Example Piconet
S
FHSS
M
S
S
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Time Slot

• The time between each hop is called a slot
• The slot interval is 625 microseconds
• Each slot uses a different frequency
• 625 1/1600
• Odd slots will be used as uplink (slave ?
  master)
• Rate 800 slots/second
• Even slots will be used as downlink (master ?
  slave)

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Frame

• A frame can utilize 1, 3, or 5 slots
• Frame overhead
• 250260 per slot for frequency hopping settling
  time
• 72 bits for access code
• 54 bits for header

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Example of Time Slot
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FHMA

• A sender and a receiver change frequency (calling
  hopping) using the same pseudo-random sequence
• Timing synchronization is needed
• Multiple sender-receiver pairs use different
  sequence
• Provides a level of security

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FHMA in Bluetooth

• Piconet can be combined into scatternets
• Different piconets use different hopping
  sequences (masters are different)
• This prevents interference between piconets

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Scatternet

• Red slave acts as a bridge between two piconets

Piconet
S
S
M2
Piconet
S
FHSS
S
FHSS
M1
S
S
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Direct Sequence Spread Spectrum

• A binary chip sequence will be used to spread the
  spectrum
• All channel frequency band will be used at the
  same time
• Different to FHSS
• Both FHSS and DSSS are used in IEEE 802.11

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Direct Sequence MA (DSMA)

• Each user shares the same frequency and time but
  is distinguished by a unique numeric binary
  sequence
• DSMA system greatly resists to interference
  effects in a frequency reuse situation
• Theoretically, there may be no hard limit on the
  number of mobile users who can simultaneously
  gain access
• DSMA is also known as CDMA
• Code Division Multiple Access

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Example of DSMA/CDMA
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Spreading

• Spreading procedure
• In CDMA, the narrowband message signal is
  multiplied by a very large bandwidth signal
  called spreading signal (code) before modulation
  and transmission over the air
• Message consists of symbols
• Has symbol period and hence, symbol rate
• Spreading signal (code) consists of chips
• Has Chip period and and hence, chip rate
• Chip rate is order of magnitude larger than the
  symbol rate
• Spreading signal uses a pseudo-noise (PN)
  sequence (a pseudo-random sequence)
• PN sequence is called a codeword
• Each user has its own codeword
• Codewords are (nearly) orthogonal (low
  autocorrelation)

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Despreading

• The sent signal is recovered by despreading
  process at the receiver
• The receiver correlator distinguishes the senders
  signal by examining the wideband signal with the
  same time-synchronized spreading code

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Advantages of CDMA

• Improving capacity
• The frequency reuse factor is 1
• The same frequency band is used in all cells
• Note Cell capacity is not fixed like in TDMA or
  FDMA systems
• Depending on the interference
• Low power spectral density
• Signal is spread over a larger frequency band
• Other systems suffer less interference from the
  transmitter
• Reduction of multipath affects by using a larger
  spectrum

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Advantages of CDMA

• Random access possible
• Users can start their transmission at any time
• Privacy
• The codeword is known only between the sender and
  receiver
• Hence other users can not decode the messages
  that are in transit without the code
• Others
• No frequency management
• No equalizers needed
• No guard time needed
• Enables soft handoff

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Handoff

• Handoff
• In cellular system, a handoff will happen when a
  mobile phone move from the coverage area of one
  cell to another
• Hard handoff the old connection is terminated
  before a new connection is activated
• Soft handoff a new connection is made before
  breaking the old connection

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Hard Handoff

• In TDMA or FDMA, due to spectrum reuse, a given
  slot on a given frequency channel cannot be used
  by neighboring cells
• When a phone which is in a call moves from one
  cell to another, at a certain point it has to
  switch between cells
• In FDMA and TDMA, it will be commanded by the
  system to change frequencies, all at once

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Soft Handoff

• In CDMA, on the other hand, all the cells operate
  on the same frequency, the single RF receiver
  picks up all of those which are within range
• When the phone is about halfway between two cells
  while in a call, the phone is not only handling
  its transport of data back and forth to the cell,
  but it is also actively looking for other cells
• When it finds one whose signal strength is good,
  it will inform the cell system of this
• The cell system might decide at that point to
  route the call through both cells simultaneously
• The specification actually permits a phone to
  talk to six cells at once, though no phone
  currently in existence has this capability

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Soft Handoff

• When a CDMA phone in a call moves from one cell
  to another, the handoff process happens in
  multiple steps
• First the phone notices the second cell, and it
  begins to carry the call on both cells
• As the phone continues to move, eventually the
  signal strength from the one the phone is moving
  away from will drop to the point where it isn't
  useful any longer
• The system will drop the original cell

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CDMA Principle
Represent bit 1 with 1 Represent bit 0 with -1
One bit period (symbol period)
1
1
Data
0
1 1 1 0 1 0 1 1
PN-Code (codeword)
1 1 1 0 1 0 1 1
Coded Signal
Chip period
Input to the modulator (phase modulation)
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Example
(a) Binary chip sequences for four stations (b)
Bipolar chip sequences (c) Six examples of
transmissions (d) Recovery of station Cs signal
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Processing Gain

• Main parameter of CDMA is the processing gain
  that is defined as
• Example
• IS-95 System (Narrowband CDMA) has a gain of 64
• 1.228 Mhz chipping rate
• 1.25 MHz spread bandwidth
• Other systems have gain between 10 and 100

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Near Far Problem and Power Control

• At a receiver, the signals may come from various
  (multiple) sources
• Each source may have different distances to the
  base station
• The strongest signal usually captures the
  modulator
• The other signals are considered as noise
• In CDMA, we want a base station to receive CDMA
  coded signals from various mobile users at the
  same time
• Therefore the receiver power at the base station
  for all mobile users should be close to each
  other
• This requires power control at the mobiles
• Power Control
• Base station monitors the power levels from
  different mobiles and then sends power change
  commands to the mobiles over a forward channel
• The mobiles then adjust their transmit power

B
pr(M)
M
M
M
M
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Variable Spreading Factor

• OVSF code
• Orthogonal Variable Spreading Factor
• Two codes can be used simultaneously if they are
  in different branches