1. Cellular and Wireless Networks Broadband Modulation-I
Fundamentals of
Cellular and Wireless Networks
Lecture ID: ET- IDA-113/114
, v11
Prof. W. Adi
Lecture-11
Broadband Modulation-I

2. Broadband Modulation Techniques
This is a summary of the most common modern broadband digital modulation techniques for wireless systems
Spread Spectrum Modulation Techniques (SS-Techniques)
- Frequency Hopped Spread Spectrum FH-SS
- Direct Sequence Spread Spectrum DS-SS
Orthogonal Frequency Division Multiplexing OFDM
Combines:
- Multicarrier (MCM) Modulation
- Multisymbole Modulation
- Multirate Transmission

3. Spread Spectrum (SS) Modulation Techniqies
In SS modulation the transmitted signal occupies much larger bandwidth than that of traditional radio modems
Key Idea of SS: Spread the user signal over a large bandwidth by using a unique spreading function (as a PN (Pseudo-Noise) sequence) for each user signal. The same frequency band is used for many other user signals at the same time.
Bandwidth efficency of SS:
SS is not bandwidth efficient for one user signal, it is bandwidth efficient for multiple-users
Main Advantages of SS Modulation:
User interference rejection capability
Resistance to multipath fading
Can exploit the multipath signal components to improve the performance by using a RAKE receiver
Low sensitivity to narrow band radio jamming

4. General Model of Spread Spectrum Techniques
Noise
User 1
signal S1
Modulator
On frequency band W
De-Modulator
On frequency band W S1 .
Channel
User 1 sequence C1 . C1 .
Mixture of
all signals
and noise
User N
signal SN
Modulator
On frequency band W
De-Modulator
On frequency band W SN CN
Two types of Transmission:
Frequency Hopped SS
Direct Sequence SS
User N sequence CN

5. 1. Frequency Hopped Spread Spectrum FH-SS
Si(t) = A cos (2 fi t )
User
Data D1
fi= fd + fh
fh = Hopper carrier frequency (C1 dependant)
fd = FSK modulatorf frequency (D1 dependant)
Modulator
Usually FSK X
00 fm
01 2fm
10 3fm
11 4fm
1 kHz
2kHz
Example:
fd = i. fm (1 to 4 kHz)
fh = f1 f f8 (100 to 800 kHz)
C1 = 5, 8, 3, 7, ..
3kHz
Frequency Hopper
4kHz
C1=5, 8, 3, 7 ... fi
Sequence
Generator C1 8 7 5 3
FH-SS Transmitter
kHz
PN= Pseudo-Noise
Source: W. Stallings

6. Slow Frequency Hopped Spread Spectrum FH-SS
Example:
Slow Frequency Hopped Spread Spectrum FH-SS
Slow Hopping : Many symbols sent in one hop
User’s sequence 11 10 01 11
4 different combinations 00 01
1 1
1 0
0 1
0 0
Source: W. Stallings

7. Fast Frequency Hopped Spread Spectrum FH-SS
Example:
Fast Frequency Hopped Spread Spectrum FH-SS
Fast Hopping: Many hops during one symbole
User’s sequence 11 10 01 00 11
2 bits/symbol 01 11 10 00 11 01 00 01 00
Source: W. Stallings

8. 2. Direct Sequence Spread Spectrum DS-SS
Key idea in spreading: A special sequence of k chips is EX-OR ed with every input message bit
EX-OR
Message
bits
Ts seconds each
Spreading factor = bit time/chip time = TS/Tc = k
Spreading factor: 10 to 100 in civil applications
up to in military applications A
Modulator
as PSK + C B
PN-Sequence of K chips
Tc seconds each
Noise
PN*-Generator +
Channel
Wide- Band
Filter
Message A
PN-Generator
Linear Feedback Shift Register
With Primitive Connection Polynomial x
PSK
De-Modulator
Same
sequence
Transmitter B
PN-Generator
Synchronization
Receiver
*) PN= Pseudo-Noise

9. Example: Direct Sequence Spread Spectrum DS-SS
k=4 chips per bit B
For each message bit, a sequence of 4 bits are sent
PN= Pseudo-Noise
Source: W. Stallings

10. Code Division Multiple Access CDMA
For Direct Sequence Spread Spectrum DS-SS
User 1
Perfect sequence properties:
Ci(t) X Cj(t)  1 for i = j
Ci(t) X Cj(t)  0 for i  j
User 1
User 2 . .
User n
dn(t)
User n
3rd Generation Mobile
System uses CDMA !
Cn(t)
Source: W. Stallings

11. Sample PN-Sequence Generator
autocorrelation function:
R(t) = 1/N  Bk Bk-t N
K=1
R(t) = 1 for t = 0, N, 2N ..
R(t) = - 1/N otherwise
For 0  -1
1  1
Other sequence properties:
2n-1 ones and 2n-1 –1 zeros
Sliding a window of n-bits over the sequence results with all 2n-1 nonzero patterns of n-bits
Source: W. Stallings

12. PN-Sequence Generating Polynomials
Source: W. Stallings

13. known Good Sequences + + +
Barker Code: or
Has good autocorrelation. Used in IEEE wireless network
Other sequences: Walsh Code, Orthogonal sequences, Gold sequences
Gold Sequences:
Have well defined cross correlation properties Ci(t) X Cj(t)  0 for i  j
to differentiate between users, every user is assigned a different sequence + D5 D2
Gold sequences S1, S2 .. S33
# of sequences = +
C(D) =100101= D5 + D2 + 1, period=31 + D5 D2
C(D) = D5 + D4 + D3 + D2 + 1, period=31

14. Spread Spectrum in GSM (2nd Generation Mobile) Slow frequency hopping
Digital Signal
270 kbps
2-GMSK
Modulator FH
Modulator
270 ksps
217 changes/sec PN
Sequence
Code Generator ~
Carrier
MHz
- Frequency hopping rate 217 changes per second (TDM frame rate)
FH is optional
Activated by base station

15. Spread Spectrum in UMTS (3rd Generation Mobile)
Two-stage spreading
Digital Signal
60 kbps
7.68 Mcps
7.68 Mcps
QPSK
Modulator
3.84 Msps + +
OVSF
SF= 4, 8, 16,
Gold
Sequences ~
Carrier
1965 MHz
OVSF: Orthogonal Variable Spreading Factor
Spreading in Two stages:
1st stage: OVSF: Orthogonal Variable Spreading Factor
Provides mutual orthogonality among all users in the same cell
2nd stage: Gold Sequences: Provide mutual randomness between users in different cells
Cluster size in UMTS is =1

16. Orthogonal Frequency Division Multiplexing OFDM Modulation
Combines three transmission principles:
1. Multicarrier Modulation MCM: uses N carriers instead of one carrier. Every carrier transmits Rs/N symbols per second.
Offers more immunity for Frequency selective fading channels as usually not all carriers are disturbed at the same time (frequency diversity).
Example: HIPERLAN-2 standard, total allocated bandwidth is 20 MHz having 64 subcarriers (sub-channels) => 20 MHz/64=312.5 kHz/sub-channel. Every sub-channel carries 250 k symbol per second => BW-Efficiency=250 ksps/312.5 kHz= 0.8 symbol/sec/Hz. Only 48 subchannels are used for data.
Multi-symbol Modulation: like QPSK or QAM, as one symbol represents several bits.
Example: 64-QAM used for HIPERLAN-2 standard. Data rate = 250 ksps x 6 (64-QAM) = 1.5 Mbps. Coded using rate ¾ convolutional Code => net data rate= ¾ 1.5 Mbps= Mbps. Total data rate for 48 channels => = 54Mbps.
Multi-rate Transmission: Data transmission rate is reduced in case of bad S/N ratio and increased for good S/N ratio.
Example: HIPERLAN-2 data rate ranges accodingly from 54 Mbps to 6 Mbps.

17. Properties of Mobile Radio Path
Fast Fading (Rayleigh fading):
Caused by multipath propagation. Signal arriving at the receiver is the vector sum of the transmitted signal and its reflected paths with varying phase shifts and amplitudes due to the varying path lengths. Divided into 2 groups according to the delay differences: selective & flat fading.
Slow Fading (log-normal fading) “shadowing”
Slow fading caused by topology (hills) and obstacles (buildings). The signal is attenuated.
In GSM, this type of attenuation can be overcome in part by adaptive power control, that is to say changing the output power of BTS or MS, so the Rx strength is measured and analyzed.

18. Properties of Mobile Radio Path
Fast Fading:
Selective fading
Reflected signal comes from objects that are away from the receiver (GSM, 1 to 5 Km). Bit rate in GSM/DCS 1800 is 270 Kbit/s, this means time between bits is 3.7 uS which correspond to 1.1 Km.
Flat Fading
Caused by vector summation of signals from near objects. Resultant value may be beneficial, but also the result could be zero or close to it, causing severe fading dip (GSM900/DCS1800 dips occur at half wavelength 17 cm, frequency dependent)

19. Diversity Techniques, Time, Frequency and Space
1. Time Diversity Techniques:
In wireless medium we have multipath arrival due to reflection, diffraction
and scattering. This causes Inter Symbol Interference ISI.
If multipath signals can be isolated, then these signals offer a source
of diversity. The receiver can make use of these different copies of the
signal to improve the system performance.
RAKE Receiver:
RAKE receiver invented 1950s in MIT Lincoln Laboratory provides
robust performance by making use of time diversity
RAKE receiver is commonly used in DS-SS receiver in CDMA cellular
telephones
RAKE receiver equalizes the effects of mutipath fading channel

20. RAKE Receiver Note that Rake is not an acronym. The name Rake was
Signal demodulation and despreading is done in a so-called Rake receiver which constructively utilizes multipath propagation over the radio interface. Several, preferably the strongest, multipath replicas of the original signal are processed in separate ”fingers” in the receiver. The finger outputs are then combined and the original binary information is obtained.
Note that Rake is not an acronym. The name Rake was
coined during the 1950s since the receiver structure
resembles a garden rake
Channel Estimator

21. Time Diversity and the RAKE Receiver
Correlators similar to those of DS-SS correlators

22. Diversity Techniques 2. Frequency Diversity Techniques:
In wireless medium frequency selective response. As a result different
signal frequencies are received with significant power fluctuation.
(Typically dB under average)
The width of the fades is proportional to the delay spread of the multipath
arrival of the signal.
Making use of frequency diversity:
Send the same signal at different frequencies as in FH-SS
Example: GSM standard recommends slow frequency hopping to enable
frequency diversity
Muticarrier systems can increase the power sent on faded carriers

23. Diversity Techniques 3. Space Diversity Techniques:
Received signal to the antenna is composed of number of signals arriving
through different paths from different spatial angles.
The location of the antenna gives different receiving quality due to multipath.
Space diversity schemes:
Multiple antennas
Polarization diversity
Sectored antennas and angle diversity
Adaptive angle diversity
Example: Smart antennas and CDMA by focusing the signals in one
direction to limit interference and increase system capacity

24. Antenna/Receiver Diversity
By using 2 receiver aerials we have much better chance that one of them is not influenced by a fading dip at an instant in time. By combining the signals at each antenna fading dips are minimized.
Distance between antennas is relative to the frequency (GSM/DCS 1800 a separation of about 6 to 3 m, distance gives gain in signal strength of ~6 dB)

25. Space Diversity Schemes
Multiple antennas
Polarization diversity
Sectored antennas and angle diversity
Adaptive angle diversity
Source: Pahlavan and Krishnamurthy