Spread Spectrum Multiple Access

1. SSMA
Spread Spectrum Multiple Access
AJAL.A.J
Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM
MAIL: ec2reach@gmail.com

2. SSMA
 Spread spectrum systems : The desired
signal is transmitted over a bandwidth
which is much larger than the Nyquist
bandwidth. It is first developed for military
applications for
– Security
– Undetectability: minimum probability of being
detected
– Robust against intentional jammers

3. Applications
 Security
 Robust against unintentional interference
 It is not bandwidth efficient when used by a single user
but has the capability to overcome narrowband jamming
signals (cannot overcome AWGN or wideband jamming
signal) and multi-path.
 Providing multiple access
 If many users can share the same spread spectrum
bandwidth without interfering with one another,
bandwidth efficient improved but will affect the capability
to overcome jamming.

4. Spread Spectrum Access
 Two techniques
– Frequency Hopped Multiple Access (FHMA)
– Direct Sequence Multiple Access (DSMA)
 Also called Code Division Multiple Access – CDMA

5. Frequency Hopping (FHMA)
 Digital muliple access technique
 A wideband radio channel is used.
– Same wideband spectrum is used
 The carrier frequency of users are varied in a
pseudo-random fashion.
– Each user is using a narrowband channel
(spectrum) at a specific instance of time.
– The random change in frequency make the
change of using the same narrowband channel
very low.

6. Frequency Hopping (FHMA)
 The sender receiver change frequency
(calling hopping) using the same pseudo-
random sequence, hence they are
synchronized.
 Rate of hopping versus Symbol rate
– If hopping rate is greather: Called Fast
Frequency Hopping
 One bit transmitted in multiple hops.
– If symbol rate is greater: Called Slow Frequency
Hopping
 Multiple bits are transmitted in a hopping period
 GSM and Bluetooth are example systems

9. Code Division Multiple Access
(CDMA)
 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. This is called spreading.
 CDMA is also called DSSS (Direct Sequence
Spread Spectrum). DSSS is a more general term.
 Message consists of symbols
– Has symbol period and hence, symbol rate

10. Code Division Multiple Access
(CDMA)
 Spreading signal (code) consists of chips
– Has Chip period and and hence, chip rate
– Spreading signal use a pseudo-noise (PN) sequence (a pseudo-
random sequence)
– PN sequence is called a codeword
– Each user has its own cordword
– Codewords are orthogonal. (low autocorrelation)
– Chip rate is oder of magnitude larger than the symbol rate.
 The receiver correlator distinguishes the senders signal
by examining the wideband signal with the same time-
synchronized spreading code
 The sent signal is recovered by despreading process at
the receiver.

11. CDMA Advantages
 Low power spectral density.
– Signal is spread over a larger frequency band
– Other systems suffer less from the transmitter
 Interference limited operation
– All frequency spectrum is used
 Privacy
– The codeword is known only between the sender
and receiver. Hence other users can not decode
the messages that are in transit
 Reduction of multipath affects by using a larger
spectrum

12. CDMA Advantages
 Random access possible
– Users can start their transmission at any time
 Cell capacity is not concerete fixed like in TDMA
or FDMA systems. Has soft capacity
 Higher capacity than TDMA and FDMA
 No frequency management
 No equalizers needed
 No guard time needed
 Enables soft handoff

13. 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 1 1 1 0 1 0 1 1
Coded
Signal
Chip period
Input to the modulator (phase modulation)

14. Processing Gain
• Main parameter of CDMA is the processing gain
that is defined as:
Bspread Bchip
Gp = =
R R
Gp: processing gain
Bspread: PN code rate
Bchip: Chip rate
R: Data rate
• IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between
10 and 100.
– 1.228 Mhz chipping rate
– 1.25 MHz spread bandwidth

15. Near Far Problem and Power Control
• At a receiver, the signals
may come from various B pr(M)
(multiple sources.
– The strongest signal
usually captures the M
modulator. The other
signals are considered M
as noise
– Each source may have
different distances to M
the base station
M

16. Near Far Problem and Power
Control
 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 eacother.
– This requires power control at the mobiles.
 Power Control : Base station monitors the
RSSI values from different mobiles and then
sends power change commands to the
mobiles over a forward channel. The mobiles
then adjust their transmit power.

17. DSSS Transmitter
Message Baseband sss(t)
+
m(t) BPF Transmitted
p(t) Signal
PN Code
Generator Oscillator
fc
Chip Clock
2 Es
sss (t ) = m(t ) p (t ) cos(2πf c t + θ )
Ts

18. DSSS Receiver
s1 (t ) m(t )
IF Wideband Phase Shift Keying
Filter Demodulator Received
Data
sss (t ) p (t )
Received PN Code Synchronization
DSSS Signal Generator System
at IF
2 Es
s1 (t ) = m(t ) cos(2πf c t + θ )
Ts

19. Spectra of Received Signal
Spectral Interference Spectral
Density Density Signal
Interference
Signal
Frequency Frequency
Output of Wideband filter Output of Correlator after
dispreading,
Input to Demodulator

20. CDMA Example
R Receiver (a base station)
Data=1011… Data=0010…
A B
Transmitter (a mobile) Transmitter
Codeword=010011 Codeword=101010
Data transmitted from A and B is multiplexed using CDMA and codeword.
The Receiver de-multiplexes the data using dispreading.

21. CDMA Example – transmission from two sources
A Data
1 0 1 1
A 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1
Codeword
Data ⊕ Code 1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0 1 0 1 1 0 0
A Signal
B Data 0 0 1 0
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
B
Codeword
Data ⊕ Code 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 1 0 1 0 1 0
B Signal
Transmitted
A+B
Signal

22. CDMA Example – recovering signal A at the receiver
A+B
Signal
received
A
Codeword
at
receiver
(A + B) ∗ Code
Integrator
Output
Comparator
Output 0 1 0 0
Take the inverse of this to obtain A

23. CDMA Example – recovering signal B at the
receiver
A+B
Signal
received
B
Codeword
at
receiver
(A + B) ∗ Code
Integrator
Output
Comparator
Output
1 1 0 1
Take the inverse of this to obtain B

24. CDMA Example – using wrong codeword at the receiver
A+B
Signal
received
Wrong
Codeword
Used at
receiver
Integrator
Output
Comparator
Output X 0 1 1
Noise
Wrong codeword will not be able to decode the original data!

25. Hybrid Spread Spectrum
Techniques
 FDMA/CDMA
– Available wideband spectrum is frequency
divided into number narrowband radio
channels. CDMA is employed inside each
channel.
 DS/FHMA
– The signals are spread using spreading codes (direct
sequence signals are obtained), but these signal are
not transmitted over a constant carrier frequency;
they are transmitted over a frequency hopping carrier
frequency.

26. Hybrid Spread Spectrum Techniques
 Time Division CDMA (TCDMA)
– Each cell is using a different spreading code (CDMA
employed between cells) that is conveyed to the
mobiles in its range.
– Inside each cell (inside a CDMA channel), TDMA is
employed to multiplex multiple users.
 Time Division Frequency Hopping
– At each time slot, the user is hopped to a new
frequency according to a pseudo-random hopping
sequence.
– Employed in severe co-interference and multi-path
environments.
 Bluetooth and GSM are using this technique.

27. Capacity of CDMA Systems
• Uplink Single-cell System Model
User 2 Assumptions
• Total active users Ku
• The intra-cell MAI can be
...
modeled as AWGN
User 1 User k • Perfect power control is
assumed
. . • Random sequences
. .
. .
... User n
User Ku

28. Capacity of CDMA Systems
 Coarse estimate of the reverse link (uplink) capacity
 Assumptions:
 Single Cell.
 The interference caused by other users in the cell can be
modeled as AWGN.
 Perfect power control is used, i.e. the received power of
each user at the base station is the same.
 If the received power of each user is Ps watts, and the background
noise can be ignored (ex: micro-cells), then the total interference
power (MAI) at the output of the desired user’s detector is
where Ku is the total number of equal energy users in the cell.
Suppose each user can operate against Gaussian noise at a bit-
energy-to-noise density level of Eb/Io. Let W be the entire spread
bandwidth, then the interference spectral density can be expressed
as:
I ≅ ( K u − 1) Ps I0 =
I
Watts / Hz (one − sided )
W

29. Capacity of CDMA Systems
{
Ps
Also, the bit energy Eb is Eb =
Interference Rb
limited I I ⋅W W Rb
Thus, K u −1 = = 0 =
Ps E b ⋅ R b E b I 0
★Now, if we consider the factors of voice activity (G v), sectorized
antenna gain (GA), and other-cell interface factor (f), where
Gv ≈ 1/v = 2.67
GA (three sectors) ≅ 2.4
f = (Interference form other cells)/(Interference from given cell) ≅ 0.6

30. Capacity of CDMA Systems
W R b Gv ⋅ GA
In this case, Ku u ≅ be approximated by
K can ⋅
E b I 0 (1 + f )
4 ⋅ (W R )
Ex: If Gv ≅ 2.67, GA ≅ 2.4, f ≅ b0.6
⇒ Ku ≅
( Eb Io )
If (Eb/Io) required is 6 dB (i.e. Eb/Io = 4)
W
⇒ Ku ≅
Rb
which will be larger than the TDMA or FDMA systems in the cellular
environment.

31. SDMA
• Use spot beam antennas
• The different beam area can use TDMA, FDMA, CDMA
• Sectorized antenna can be thought of as a SDMA
• Adaptive antennas can be used in the future
(simultaneously steer energy
in the direction of many users)
spot beam
antenna

32. Features:
 A large number of independently steered
high-gain beams can be formed without
any resulting degradation in SNR ratio.
 Beams can be assigned to individual
users, thereby assuring that all links
operate with maximum gain.
 Adaptive beam forming can be easily
implemented to improve the system
capacity by suppressing co channel
interference.