Wireless communication channel

Effects on Radio Communication Signal degradation can be classified by type : Path Loss happen during distance covered by the radio signal, it is called “Free space path loss “, it can be calculated by LFS = 32.44 + 20 log F (MHz) +20 log d (Km) Signal attenuation Resulting from shadowing effects introduced by the obstacles between transmitter and receiver Fading of the signal Caused by numerous effects all of which are related to the Radio propagation phenomenon

Wireless Multipath Channel One of the most problem in communication channel is fading

Fading Problems Shadowing (Normal fading): The reason for shadowing is the presence of obstacles like large hills or buildings in the path between the site and the mobile. The signal strength received fluctuates around a mean value while changing the mobile position resulting in undesirable beats in the speech signal.

Fading Problems Raleigh Fading (Multi-path Fading): The received signal is coming from different paths due to a series of reflection on many obstacles. The difference in paths leads to a difference in paths of the received components.

Parameters of multi-path channel Time Domain Frequency Domain 1- Max delay spread: 2- Coherence BW: 3- Coherence Time : 4- Doppler Shift:

Doppler Shift Phase change due to path length difference Doppler shift (apparent change in freq.)

Types of fading

At High Data Rate 1- 2- High data rate transmission Problems short symbol time compared to the delay spread. = Delay spread = Symbol period Problems 1- ISI = signal BW = coherence BW 2-

Orthogonal frequency division multiplexing (OFDM)

History OFDM was introduced in 1950 but was only completed in 1960’s Originally grew from Multi-Carrier Modulation used in High Frequency military radio. Patent was granted in 1970’s Earlier OFDM wasn’t popular Large arrays of sinusoidal generators and coherence demodulator Too expensive and complex. Later when DFT and IDFT became a known solution to the arrays of generators and demodulators. It was still not popular as there is no efficient method to perform the IFFT and FFT operation. Advances in VLSI technology allows implementation of fast and cheap FFT and IFFT operation drive OFDM popularity.

Applications

OFDM Orthogonal Frequency Division Multiplexing -Divide the information over several carriers Instead of using one big truck When The truck is lost… All is lost! Use several small trucks When one truck is lost… Only a portion of the shipment is lost!

Concept of an OFDM signal Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Conventional multicarrier techniques frequency Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Ch.3 Ch.5 Ch.7 Ch.9 Saving of bandwidth 50% bandwidth saving Orthogonal multicarrier techniques frequency

. . OFDM changes Frequency Selective Fading to Flat Fading Channel N number of subcarrier . .

Solution to Frequency Selective Fading When the data rate is lower = Delay spread = Symbol period = signal BW = coherence BW Frequency Selective => Flat Fading In flat fading, the amplitude varies but there is no ISI

Multicarrier Modulation Divide broadband channel into narrowband subchannels No ISI in subchannels if constant gain in every subchannel and if ideal sampling Orthogonal Frequency Division Multiplexing Based on the fast Fourier transform Standardized for DAB, DVB-T, IEEE 802.11a, 802.16a, HyperLAN II Considered for fourth-generation mobile communication systems channel subcarrier magnitude subchannel frequency Subchannels are 312 kHz wide in 802.11a and HyperLAN II

OFDM Frequency Spectrum Use many carriers that are equally spaced: k = 0, 1, … , N-1 Ts = Symbol Time Carrier 1 has a maximum where all other carriers are 0 1 2 3 4 5 frequency

OFDM Many carriers with small spacing => Long symbol time But many carriers carry a lot of information! Long symbol time is an advantage! Delay Spread (Multipath) Direct Path Symbol n-1 Symbol n Symbol n+1 Delayed Path Symbol n-1 Symbol n Symbol n+1 ISI ISI ISI = Inter Symbol Interference

OFDM Avoid ISI and preserve Orthogonality => Guard Interval Total Symbol length Useful Symbol length Guard Symbol n Guard Direct Path Guard Symbol n-1 Guard Symbol n Guard Symbol n+1 Delayed Path Guard Symbol n-1 Guard Symbol n Guard Symbol n+1 Symbol n is added constructively or destructively according to phase Integration Period

Avoid ICI and preserve Orthogonality => cyclic prefix N samples v samples CP: Cyclic Prefix CP s y m b o l i s y m b o l ( i+1) copy

Discrete versus Fast Fourier Transform Discrete (DFT): For each frequency sample ‘k’ (0 to N-1) loop ‘n’ (over 0 to N-1) => N2 complex multiplications Fast (FFT, Cooley-Tukey algorithm): “An efficient algorithm to calculate a DFT” N.log(N) complex multiplications

OFDM Block Diagram

Main advantages • High spectral efficiency. And high data rate. • Efficient in multipath environments. • Simple digital realization by using the FFT operation. • Low complex receivers due to avoidance of ISI. •Different modulation schemes can be used on individual sub-carriers.

x Drawbacks Large Peak to Average Ratio (PAR). Added sinusoid cause large PAR and issue of amplifier non-linearity arises. Accurate frequency and time synchronization is required. More sensitive to Doppler spreads than single-carrier schemes. Sensitive to frequency offset and phase noise caused by imperfections in the transmitter and the receiver oscillators. Guard interval causes loss in spectral efficiency