Wireless Communications and Networks

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Wireless Communications Engineering
Lecture 11 Spread Spectrum and CDMA Prof.
Mingbo Xiao Dec. 16, 2004
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Spread Spectrum

• Input is fed into a channel encoder
• Produces analog signal with narrow bandwidth
• Signal is further modulated using sequence of
  digits
• Spreading code or spreading sequence
• Generated by pseudonoise, or pseudo-random number
  generator
• Effect of modulation is to increase bandwidth of
  signal to be transmitted

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Spread Spectrum

• On receiving end, digital sequence is used to
  demodulate the spread spectrum signal
• Signal is fed into a channel decoder to recover
  data

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Spread Spectrum System Model
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Spread Spectrum

• What can be gained from apparent waste of
  spectrum?
• Immunity from various kinds of noise and
  multipath distortion
• Can be used for hiding and encrypting signals
• Several users can independently use the same
  higher bandwidth with very little interference

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Frequency Hoping Spread Spectrum (FHSS)

• Signal is broadcast over seemingly random series
  of radio frequencies
• A number of channels allocated for the FH signal
• Width of each channel corresponds to bandwidth of
  input signal
• Signal hops from frequency to frequency at fixed
  intervals
• Transmitter operates in one channel at a time
• Bits are transmitted using some encoding scheme
• At each successive interval, a new carrier
  frequency is selected

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Frequency Hoping Spread Spectrum

• Channel sequence dictated by spreading code
• Receiver, hopping between frequencies in
  synchronization with transmitter, picks up
  message
• Advantages
• Eavesdroppers hear only unintelligible blips
• Attempts to jam signal on one frequency succeed
  only at knocking out a few bits

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Frequency Hoping Spread Spectrum
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FHSS Using MFSK

• MFSK signal is translated to a new frequency
  every Tc seconds by modulating the MFSK signal
  with the FHSS carrier signal
• For data rate of R
• duration of a bit T 1/R seconds
• duration of signal element Ts LT seconds
• Tc ? Ts - slow-frequency-hop spread spectrum
• Tc lt Ts - fast-frequency-hop spread spectrum

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FHSS Performance Considerations

• Large number of frequencies used
• Results in a system that is quite resistant to
  jamming
• Jammer must jam all frequencies
• With fixed power, this reduces the jamming power
  in any one frequency band

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Direct Sequence Spread Spectrum (DSSS)

• Each bit in original signal is represented by
  multiple bits in the transmitted signal
• Spreading code spreads signal across a wider
  frequency band
• Spread is in direct proportion to number of bits
  used
• One technique combines digital information stream
  with the spreading code bit stream using
  exclusive-OR (Figure 7.6)

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Direct Sequence Spread Spectrum (DSSS)
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DSSS Using BPSK

• Multiply BPSK signal,
• sd(t) A d(t) cos(2? fct)
• by c(t) takes values 1, -1 to get
• s(t) A d(t)c(t) cos(2? fct)
• A amplitude of signal
• fc carrier frequency
• d(t) discrete function 1, -1
• At receiver, incoming signal multiplied by c(t)
• Since, c(t) x c(t) 1, incoming signal is
  recovered

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DSSS Using BPSK
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Spread spectrum signal
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PN Sequences

• PN generator produces periodic sequence that
  appears to be random
• PN Sequences
• Generated by an algorithm using initial seed
• Sequence isnt statistically random but will pass
  many test of randomness
• Sequences referred to as pseudorandom numbers or
  pseudonoise sequences
• Unless algorithm and seed are known, the sequence
  is impractical to predict

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Important PN Properties

• Randomness
• Uniform distribution
• Balance property
• Run property
• Independence
• Correlation property
• Unpredictability

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Linear Feedback Shift Register Implementation
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Properties of M-Sequences

• Property 1
• Has 2n-1 ones and 2n-1-1 zeros
• Property 2
• For a window of length n slid along output for N
  (2n-1) shifts, each n-tuple appears once, except
  for the all zeros sequence
• Property 3
• Sequence contains one run of ones, length n
• One run of zeros, length n-1
• One run of ones and one run of zeros, length n-2
• Two runs of ones and two runs of zeros, length
  n-3
• 2n-3 runs of ones and 2n-3 runs of zeros, length 1

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Properties of M-Sequences

• Property 4
• The periodic autocorrelation of a 1
  m-sequence is

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Definitions

• Correlation
• The concept of determining how much similarity
  one set of data has with another
• Range between 1 and 1
• 1 The second sequence matches the first sequence
• 0 There is no relation at all between the two
  sequences
• -1 The two sequences are mirror images
• Cross correlation
• The comparison between two sequences from
  different sources rather than a shifted copy of a
  sequence with itself

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Advantages of Cross Correlation

• The cross correlation between an m-sequence and
  noise is low
• This property is useful to the receiver in
  filtering out noise
• The cross correlation between two different
  m-sequences is low
• This property is useful for CDMA applications
• Enables a receiver to discriminate among spread
  spectrum signals generated by different
  m-sequences

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Gold Sequences

• Gold sequences constructed by the XOR of two
  m-sequences with the same clocking
• Codes have well-defined cross correlation
  properties
• Only simple circuitry needed to generate large
  number of unique codes
• In following example (Figure 7.16a) two shift
  registers generate the two m-sequences and these
  are then bitwise XORed

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Gold Sequences
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Orthogonal Codes

• Orthogonal codes
• All pairwise cross correlations are zero
• Fixed- and variable-length codes used in CDMA
  systems
• For CDMA application, each mobile user uses one
  sequence in the set as a spreading code
• Provides zero cross correlation among all users
• Types
• Welsh codes
• Variable-Length Orthogonal codes

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Walsh Codes

• Set of Walsh codes of length n consists of the n
  rows of an n n Walsh matrix
• W1 (0)
• n dimension of the matrix
• Every row is orthogonal to every other row and to
  the logical not of every other row
• Requires tight synchronization
• Cross correlation between different shifts of
  Walsh sequences is not zero

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Introduction to CDMA system

• CDMA is both an access method and air-interface
• CDMA systems have a similar network architecture
  to GSM network, consisting of MS, BTS, BSC, MSC,
  OMC, relevant database (EIR, AUC, HLR, VLR)
• The major differences between GSM and CDMA system
  are BTS, MS, air interface between BTS and MS
• Power control and handoffs are different
  Frequency reuse factor is 1

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CDMA Network Reference Model
IWF Internetworking Function SME Short Message
Entity MC Message Center AC Authentication
Center A, C, , Um Interfaces
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Multiple Access Techniques
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CDMA for Direct Sequence Spread Spectrum
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CDMA in DSSS
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Advantages of CDMA Cellular

• Higher capacity
• Improves voice quality (new coder)
• Soft-handoffs decrease the call dropping
  probability
• Less power consumption (6-7 mW)
• Choice for 3G systems

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Advantages (Contd)

• Frequency diversity frequency-dependent
  transmission impairments have less effect on
  signal
• Multipath resistance chipping codes used for
  CDMA exhibit low cross correlation and low
  autocorrelation
• Privacy privacy is inherent since spread
  spectrum is obtained by use of noise-like signals
  
• Graceful degradation system only gradually
  degrades as more users access the system

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Drawbacks of CDMA Cellular

• Self-jamming arriving transmissions from
  multiple users not aligned on chip boundaries
  unless users are perfectly synchronized
• Near-far problem signals closer to the receiver
  are received with less attenuation than signals
  farther away
• Soft handoff requires that the mobile acquires
  the new cell before it relinquishes the old this
  is more complex than hard handoff used in FDMA
  and TDMA schemes

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Drawbacks (Contd)

• Air-interface is the most complex
• Not symmetrical (unlike TDMA)
• Forward and reverse channels are different
• Forward channel (1 to Many) synchronized
• Forward channel uses orthogonal spreading codes
• Reverse channel transmissions are not
  synchronized
• Orthogonal codes are used for orthogonal waveform
  coding

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IS-95 Air interface parameters
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IS-95 CDMA Forward Channel

• The forward link uses the same frequency spectrum
  as AMPS (824-849 Mhz)
• Each carrier 1.25MHz
• 4 types of logical channel A pilot, a
  synchronization, 7 paging, and 55 traffic
  channels
• Channels are separated using different spreading
  codes
• QPSK is the modulation scheme
• Orthogonal Walsh codes are used (64 total)
• After orthogonal codes, they are further spread
  by short PN spreading codes
• Short PN spreading codes are M sequences
  generated by LFSRs of length 15 with a period of
  32768 chips.

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Two Spreading Codes

• The orthogonal codes are used to differentiate
  between the transmissions within a cell
• The PN spreading codes are used to isolate
  different cells (BSs) that are using the same
  frequencies.
• The same PN sequence is used in all BSs.
• The offset for each BS is different. Of course,
  this requires synchronization
• Synchronization is achieved by GPS.

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IS-95 Forward Channel
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Basic Spreading Procedure
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The pilot channel

• Provide a reference signal for all MSs that
  provides the phase reference for COHERENT
  demodulation
• 4-6 dB stronger than all other channels
• Used to lock onto other channels
• Obtained using all zero Walsh code i.e.,
  contains no information except the RF carrier
• Spread using the PN spreading code to identify
  the BS. (512 different BS64 offsets)
• No power control in the pilot channel

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Pilot Channel Processing
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Sync channel

• Used to acquire initial time synchronization
• Synch message includes system ID (SID), network
  ID (NID), the offset of the PN short code, the
  state of the PN-long code, and the paging channel
  data rate (4.8/9.6 Kbps)
• Uses W32 for spreading
• Operates at 1200 bps

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Sync Channel Processing
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Paging channels

• Used to page the MS in case of an incoming call,
  or to carry the control messages for call set up
• Uses W1-W7
• There is no power control
• Additionally scrambled by PN long code, which is
  generated by LFSR of length 42
• The rate 4.8 Kbps or 9.6Kbps

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Paging Channel Processing
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The traffic channels

• Carry user information
• Two possible date rates
• RS19.6, 4.8, 2.4, 1.2 Kbps
• RS214.4, 7.2, 3.6, 1.8 Kbps
• RS1 is mandatory for IS-95, but support for RS2
  is optional
• Also carry power control bits for the reverse
  channel

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Forward Traffic Channel Processing in IS 95
(Rate Set 1)
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Forward Traffic Channel Processing in IS 95
(Rate Set 2)
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IS-95 CDMA Reverse Channel

• Fundamentally different from the forward channels
  
• Uses OQPSK for power efficiency
• QPSK demodulation is easy
• 869-894 MHz range
• No spreading of the data using orthogonal codes
• Same orthogonal codes are used for WAVEFORM
  encoding
• Two types of logical channels The access
  channels and the reverse traffic channels

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IS-95 Reverse Channel
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Mapping data bits to Walsh encoded symbols
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Access Channel Processing
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Mobile Wireless CDMA DesignConsiderations

• RAKE receiver when multiple versions of a
  signal arrive more than one chip interval apart,
  Rake receiver attempts to recover signals from
  multiple paths and combine them
• Soft Handoff mobile station temporarily
  connected to more than one base station
  simultaneously
• Power Control attempts to achieve a constant
  received mean power for each user (i.e., solves
  the near-far problem) in the uplink.

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Principle of Rake Receiver
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Soft Handoff

• - Reduce the interference into other cells
• Improve performance through macro
• diversity
• In the uplink, two or more base stations
• can receive the same signal because of
• the reuse factor of one.
• After demodulation and combining, the
• signal is forwarded to the BSC.

- In the downlink, however, soft handover creates
more interference to the system, since the new
base station now transmits an additional
signal for the mobile station.
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Power Control

• OPEN LOOP POWER CONTROL adjusts the initial
  access channel transmission power of the mobile
  station and compensates large abrupt variations
  in the pathloss attenuation.
• Closed loop power control-- To account for the
  independence of the Rayleigh fading in the uplink
  and downlink, the base station also controls the
  mobile station transmission power. One-bit 800Hz
  control.
• Downlink Slow Power Control The base station
  controls its transmission power to a given mobile
  station according to the path-loss and
  interference situation.

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Open loop power control
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Closed loop power control
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CDMA System Parameters

• N the number of users
• S the signal power of each user
• R baseband information bit rate
• W total RF bandwidth
• ? background thermal noise in the spread
  bandwidth
• Assume perfect power control

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Capacity of cellular CDMA
The number of users that can access the system is
thus given as
where W/R is called the processing gain