Cellular and Wireless Networks Broadband Modulation-I Fundamentals of Cellular and Wireless Networks Lecture ID: ET- IDA-113/114 27.07.2010 , v11 Prof. W. Adi Lecture-11 Broadband Modulation-I

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

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

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

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 f2 ... 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 100 800 kHz PN= Pseudo-Noise Source: W. Stallings

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 1 1 11 10 01 11 4 different combinations 00 01 1 1 1 0 0 1 0 0 Source: W. Stallings

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

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 10 000 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

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

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

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

PN-Sequence Generating Polynomials Source: W. Stallings

known Good Sequences + + + Barker Code: 10110111000 or +1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1 Has good autocorrelation. Used in IEEE 802.11 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 = 25-1 + 2 + C(D) =100101= D5 + D2 + 1, period=31 + D5 D2 C(D) 111101= D5 + D4 + D3 + D2 + 1, period=31

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 900-1800 MHz - Frequency hopping rate 217 changes per second (TDM frame rate) FH is optional Activated by base station

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, 32 .. 256 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

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= 1.125 Mbps. Total data rate for 48 channels => 48 . 1.125 = 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.

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.

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)

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

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

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

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 30-40 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

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

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)

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