1. Chapter 7 Multiple Division and Signal Encoding Techniques
(Modulation)
Techniques

2. Outline Introduction Concepts and Models for Multiple Divisions
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Orthogonal Frequency Division Multiplexing (OFDM)
Space Division Multiple Access (SDMA)
Comparison of FDMA, TDMA, and CDMA
Modulation Techniques
Amplitude Modulation (AM)
Frequency Modulation (FM)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature Phase Shift Keying (QPSK)
p/4QPSK
Quadrature Amplitude Modulation (QAM)
16QAM

3. Signaling Criteria
RF communications with binary data has been a very tough problem, especially for wireless
What determines how successful a receiver will be in interpreting an incoming signal?
Signal-to-noise ratio (or Eb/No)
Data rate (R)
Bandwidth (B)
Encoding scheme
An increase in data rate increases bit error rate
An increase in SNR decreases bit error rate
An increase in bandwidth allows an increase in data rate

4. Concepts and Models for Multiple Divisions of Signals
Multiple access techniques are based on orthogonalization of signals
A radio signal is a function of frequency, time and code as: s(f,t,c) = s(f,t) c(t)
where s(f,t) is the function of frequency and time and c(t) is the function of code
Use of different frequencies to transmit a signal: FDMA
Distinct time slot: TDMA
Different codes: CDMA
Multiple simultaneous channels: OFDM
Specially separable sectors: SDMA

5. Frequency Division Multiple Access (FDMA)
User 1
User 2
User n …
Time
Single channel per carrier
All first generation systems use FDMA
Frequency

6. Basic Structure of FDMABS f1 f2 fn …
Forward channels (Downlink)
f1’
MS #1
MS #2
MS #n …
f2’ …
fn’
Reverse channels (Uplink)

7. Forward and Reverse channels in FDMA and Guard Band…
f1’
f2’
fn’ f1 f2 fn
Reverse channels
Forward channels
Protecting bandwidth
Guard Band Wg 1 2 3 … N
Frequency
Total Bandwidth W = NWc 4
Sub Band Wc

8. Time Division Multiple Access (TDMA)
Frequency
User 1
User 2
User n …
Time
Multiple channels per carrier
Most of second generation systems use TDMA

9. The Concept of TDMA BS Reverse channels (Uplink) Forward channels
Frequency f ’
Slot BS
Forward channels
(Downlink) … #1
Frame t
Frequency f #2 #n #1 … #1 … …
MS #1 t #2 … #2 … …
MS #2 t … … #n … #n …
MS #n t
Frame
Frame
Reverse channels (Uplink)

10. TDMA/FDD: Channel Structure
Frame
Frame
Frame #1 #2 … #n #1 #2 … #n #1 #2 … #n t
(a). Forward channel … t
f ’ #1 #2 #n
(b). Reverse channel
Frame

11. Forward and Reverse Channels in TDMA
Frame
Frame
Frequency f = f ’ #1 #2 … #n #1 #2 … #n #1 #2 … #n #1 #2 … #n
Time
Reverse channel
Forward channel
Forward channel
Reverse channel
Channels in TDMA/TDD

12. Frame Structure of TDMA
Frequency #1 #2 … #n #1 #2 … #n #1 #2 … #n
Time
Head
Data
Guard time

13. Code Division Multiple Access (CDMA)
Frequency
Users share bandwidth by using code sequences that are orthogonal to each other
Some second generation systems use CDMA
Most of third generation systems use CDMA
User 1
Time
User 2
User n
Code .

14. Structure of a CDMA System
Frequency f ’
Frequency f
C1’ C1
MS #1
C2’ C2
MS #2 … … …
Cn’ Cn
MS #n BS
Reverse channels
(Uplink)
Forward channels
(Downlink)
Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes, Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes

15. Code-Division Multiple Access (CDMA)
Basic Principles of CDMA (a multiplexing scheme used with spread spectrum)
D = rate of data signal
Break each bit into k chips
Chips are a user-specific fixed pattern
This pattern is called the User’s Code
The codes are orthogonal (limited set)
Chip data rate of new channel = kD chips/sec

16. CDMA If k = 6 and code is a sequence of 1’s and -1’s
For each ‘1’ bit, A sends user code as a chip pattern
<c1, c2, c3, c4, c5, c6>
For each ‘0’ bit (-1), A sends complement of user code
<-c1, -c2, -c3, -c4, -c5, -c6>
Receiver knows sender’s code and performs decode function (assume synchronized so that the receiver knows when to apply the user code )
< d1, d2, d3, d4, d5, d6 > = received chip pattern
< c1, c2, c3, c4, c5, c6 > = sender’s code

17. CDMA Example User A code = <1, –1, –1, 1, –1, 1>
To send a 1 bit = <1, –1, –1, 1, –1, 1>
To send a 0 bit = <–1, 1, 1, –1, 1, –1>
User B code = <1, 1, –1, – 1, 1, 1>
To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code
(A’s code) x (received chip pattern)
User A ‘1’ bit decoded results: + 6 which translates into 1
User A ‘0’ bit:  binary 0
User B ‘1’ or ‘0’ bit decoded result: 0  signal ignored, SA signal decode results in a value of 0 which is different from a decode value of +/- 6 for transmitted bits ‘1’ or ‘0’ from User A

18. CDMA Example Continued
B Sends (data bit = 1) 1 -1
Receiver codeword A (decode)
Multiplication
= 0
SA(1,1,-1,-1,1,1) = 1 X X (-1) + (-1) X (-1) + (-1) X X (-1) X 1 = 0
B Sends (data bit = 0) -1 1
Receiver codeword A
Multiplication
= 0
SA(-1,-1,1,1,-1,-1) = (-1) X (-1) X (-1) X (-1) X (-1) X (-1) + (-1) X 1 = 0

19. B (data bit = 0) -1 1 C (data bit = 1) Combined signal 2 -22 -2
Receiver codeword B
Multiplication
= -4
Top Case B sends a 1  SB = Bottom Case B sends a 0  SB = - 4

20. Transmissions from B and C, receiver attempts recovery using A’s codeword (an error situation)
B (data bit = 0) -1 1
C (data bit = 1)
Combined signal 2 -2
Receiver codeword A
Multiplication (SA)
= 0
Decode result: SA = 0 for this case where B and C have sent data and we attempt to recover a transmission from A . Different receiver codeword can result in Sx that is non-zero but much less than the correct orthogonal result.

21. Spread Spectrum
Spreading of data signal s(t) by the signal c(t) results in message signal m(t) as:
Two Types of m(t) DSSS and FHSS
Digital signal s(t)
Code c(t)
Spreading signal m(t)
Spreading
Frequency
Power
Frequency
Power
c(t) is a pseudo-noise (PN) sequence or a pseudo-noise code. 1-bit of c(t) is a chip

22. Spread Spectrum Input is fed into a channel encoder
Produces analog signal with narrow bandwidth
Signal is then further modulated using a spreading scheme of binary digits
Spread by orthogonal codes or a spreading sequence
Spreading sequences are generated by PN (pseudonoise/pseudo-random number) generators
Both spreading types (codes & PN sequences) can be used in the same system as is the case in cell systems
Effect of modulation is to increase the bandwidth of the signal to be transmitted (as shown in the previous slide)

23. Spread Spectrum (continued)
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 bandwidth with very little interference (local separation)
Provides dynamic system load flexibility
Produces isolation of cell users from each other (separation), both locally (cell) and globally (cluster)

24. Spread Spectrum (continued)
On receiving end, the same spreading scheme is used to demodulate the spread spectrum signal
Has the effect of narrowing the bandwidth of the desired signal (thus reducing the noise and improving the SNR)
The signal is then fed into a channel decoder to recover the data.
Originally used by the military for secure/secret comm while lowering the probability of jamming.
The original idea (FHSS) was conceived by a Hollywood star (Hedy Lamarr) in 1940; patent granted in 1942; realized in 1957 by Sylvania. First operational use during the Cuban missile crisis in 1962 after the patent had expired.

25. Processing Gain of Spread Spectrum

26. Direct Sequence Spread Spectrum (DSSS)
Power
Digital Message s(t)
Frequency
DSSS Receiver Output
Receiver not shown
Transmitted Signal Sss(t)
Frequency
Power
Frequency
Power

27. Direct Sequence Spread Spectrum (DSSS)
Each bit in the original signal is represented by multiple bits in the transmitted signal
Spreading code spreads the signal across a wider frequency band
The amount of spreading is in direct proportion to number of pseudonoise (PN) bits used or in other words related to the PN bit rate  bandwidth
One technique combines digital information stream with the spreading code bit stream (PN bit stream) using exclusive-OR

28. Direct Sequence Spread Spectrum (DSSS)

29. CDMA for Direct Sequence Spread Spectrum
BPSK
Demodulator
CDMA User #1’s Code
Signal frequency
Spreading Code of User 1
BPSK
The use of the code at the receiving end has the effect of narrowing the bandwidth for the specific user.
User n’s Code
Including noise

30. Frequency Hopping Spread Spectrum (FHSS)
Power
Digital Data
Frequency
FHSS Receiver Output
Receiver Not Shown
Frequency-hopping Signal
Frequency
Power
Frequency
Power

31. 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 (IEEE standard uses 300 mS intervals)

32. Frequency Hoping Spread Spectrum
Channel sequence dictated by spreading code
Receiver, hopping between frequencies in synchronization with transmitter using the same spreading code, receives the message using the same (known to user) encoding scheme as the transmitter
Spreading code = c(t) also known as chipping code
Frequency changes can be slow (equal or greater than the signal element time) or fast (changes within the signal element itself)
Advantages
Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at knocking out a few bits

33. Frequency Hopping Pattern
An Example of
Frequency Hopping Pattern
Frequency
Time

34. Categories of Spreading Sequences
Spreading Sequence Categories
PN sequences (pseudonoise)
Orthogonal codes
For FHSS systems
PN sequences most common
For DSSS systems not employing CDMA
For DSSS CDMA systems
PN sequences
Spreading codes result in a higher transmitted data rate  increased bandwidth; increased system redundancy (jamming resilient); the spreading codes are noise like in their appearance

35. Received signal strength
Near-far Problem
MS2 BS
MS1
Received signal strength
Distance
Distance d2
MS2 BS d1
MS1

36. Received Signals at BS Power Frequency MS1 MS2 Solution ? f1 f2
Reception of CDMA signals at BS
(reverse link) requires equal power
levels from all MSs in the cell.
MS1 is causing adjacent channel
interference to MS2
Power
MS1
MS2
Solution ?
Frequency f1 f2

37. Power Control in CDMA
Controlling transmitted (effective) power affects the CIR
Pr = Received power in free space (Units of Pr and Pt must be the same)
Pt = Transmitted power (usually dB or dBm)
d = Distance between receiver and transmitter
f = Frequency of transmission
c = Speed of light (3 X 108 m/s if d is in meters and f is in Hz)
a = Attenuation constant (2 to 4)

38. CDMA Advantages for a Cellular Network
Transmission is in the form of a Direct Sequence Spread Spectrum (DSSS) which provides
Frequency diversity – frequency-dependent transmission impairments (e.g. fading) have less effect on signal which is spread over a large bandwidth
Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation thus a signal delayed by more than 1 chip interval does not interfere with it’s own stronger/direct signal.
Privacy – privacy is inherent. For DSSS, each user has a unique code resulting in spread spectrum/noise-like signals
Graceful degradation – system only gradually degrades (SNR  error rate increase) as more users access the system up to the point of an unacceptable error rate

39. Drawbacks of CDMA Cellular
Self-jamming – unless all of the MS are perfectly synchronized, the arriving transmissions will not be perfectly aligned on chip boundaries  interference. Requires very accurate timing sources (GPS receiver disciplined clock). No time or frequency guard bands as in TDMA and FDMA.
Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away and given lack of complete orthogonality, distant stations more difficult to recover – power control very important
Soft handoff – requires that the mobile acquire the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

40. Comparison of various Multiple Division Techniques
FDMA
TDMA
CDMA
SDMA
Concept
Divide the frequency band into disjoint subbands
Divide the time into non-overlapping time slots
Spread the signal with orthogonal codes
Divide the space in to sectors
Active terminals
All terminals active on their specified frequencies
Terminals are active in their specified slot on same frequency
All terminals active on same frequency
Number of terminals per beam depends on FDMA/ TDMA/CDMA
Signal separation
Filtering in frequency
Synchronization in time
Code separation
Spatial separation using smart antennas
Handoff
Hard handoff
Soft handoff
Hard and soft handoffs
Advantages
Simple and robust
Flexible
Very simple, increases system capacity
Disadvantages
Inflexible, available frequencies are fixed, requires guard bands
Requires guard space, synchronization problem
Complex receivers, requires power control to avoid near-far problem
Inflexible, requires network monitoring to avoid intracell handoffs
Current applications
Radio, TV and analog cellular
GSM and PDC
2.5G and 3G
Satellite systems, other being explored