Transcript Slide 1
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 1. 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 2. 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 1- Max delay spread: 𝜏𝑑 Frequency Domain 2- Coherence BW: 𝐵𝑐 1 𝐵𝑐 = 5𝜏𝑑 4- Doppler Shift:𝑓𝑑 3- Coherence Time :𝑇𝑐 𝑇𝑐 = 9 16𝜋𝑓𝑑 2 Doppler Shift S Phase change due to path length difference 2𝜋∆𝐿 2𝜋𝜈Δ𝑡 ∆𝜑 = = 𝑐𝑜𝑠𝜃 𝜆 𝜆 Doppler shift (apparent change in freq.) Δ𝐿 1 Δ𝜑 𝜈 𝑓𝑑 = . = 𝑐𝑜𝑠𝜃 2𝜋 Δ𝑡 𝜆 𝜃 𝜃 X Y d 𝜈 Types of fading 𝜏𝑑 𝜏𝑑 At High Data Rate •High data rate transmission short symbol time compared to the delay spread. 𝑇𝑠𝑦𝑚𝑏𝑜𝑙 < 𝑇𝑑𝑒𝑙𝑎𝑦 𝑇𝐷 = Delay spread = Symbol period 𝑇𝑠 Problems 1ISI = signal BW 𝐵𝑐 = coherence BW 2- 𝐵𝑠 Orthogonal frequency division multiplexing (OFDM) • 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. OFDM Orthogonal Frequency Division Multiplexing •Frequency Division Multiplexing -Divide the information over several carriers Instead of using one big truck Use several small trucks When The truck is lost… All is lost! 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 Conventional multicarrier techniques Ch.10 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 𝑇𝑠 . . . . . . 𝑁𝑇𝑠 Solution to Frequency Selective Fading When the data rate is lower = Delay spread = Symbol period 𝐵𝑠 = signal BW 𝐵𝑐 Frequency Selective => Flat Fading In flat fading, the amplitude varies but there is no ISI = coherence BW 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 magnitude channel subcarrier subchannel frequency Subchannels are 312 kHz wide in 802.11a and HyperLAN II OFDM Frequency Spectrum Use many carriers that are equally spaced: 1 fk f0 k Ts 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 1 f Ts f s 4312.5 Hz Ts 232s N 4096 or 8192 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 Delayed Path Symbol n-1 Symbol n Symbol n-1 Symbol n ISI ISI = Inter Symbol Interference Symbol n+1 Symbol n+1 ISI OFDM Avoid ISI and preserve Orthogonality => Guard Interval Total Symbol length Useful Symbol length Guard Symbol n Direct Path Delayed Path Guard Symbol n-1 Guard Symbol n-1 Guard Symbol n Guard Guard Symbol n+1 Guard Symbol n Integration Period Sym bol Time 232μs Total Sym bol Tim e Guard Guard 58 s 4 Total Sym bol Tim e 232 58 290 s Guard Symbol n+1 Symbol n is added constructively or destructively according to phase Avoid ICI and preserve Orthogonality copy copy CP s y m b o l v samples => cyclic prefix i N samples CP s y m b o l ( i+1) CP: Cyclic Prefix Discrete versus Fast Fourier Transform • Discrete (DFT): N 1 X k xn e j 2 . n. k N n 0 – 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 Example: N 4096 Discrete 4096* 4096 16.777.216 multiplications Fast 4096 * 12 49.152 FFT : 12 0,3% with respect toDFT 4096 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 nonlinearity 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