802.11 Wireless Networks (PHY) Kate Ching-Ju Lin ( 林靖茹 ) Academia Sinica 2016.03.18 CSIE, NTU
Reference 1. OFDM Tutorial online: http://home.iitj.ac.in/~ramana/ofdmtutorial.pdf 2. OFDM Wireless LWNs: A Theoretical and Practical Guide By John Terry, Juha Heiskala 3. Next Generation Wireless LANs: 802.11n and 802.11ac By Eldad Perahia
We will cover Medium Access Control Infrastructure mode vs. Ad Hoc mode DCF vs. PCF CSMA/CA with exponential backoff Hidden terminal Physical Layer Basics Packet Detection OFDM Synchronization
Packet Detection Packet Packet Packet A n B n Power ratio M n =A n /B n threshold Double sliding window packet detection Optimal threshold depends on the receiving power
Packet Detection preamble preamble A n B n threshold Use cross-correlation to detect the preamble
We will cover Medium Access Control Infrastructure mode vs. Ad Hoc mode DCF vs. PCF CSMA/CA with exponential backoff Hidden terminal Physical Layer Basics Packet Detection OFDM (orthogonal frequency-division multiplexing) Synchronization
Why OFDM? Signal over wireless channels y[n] = Hx[n] Work only for narrow-band channels, but not for wide-band channels e.g., 20 MHz for 802.11 Capacity = BW * log(1+snr) 20MHz frequency 2.45GHz (Central frequency)
Basic Concept of OFDM Wide-band channel Multiple narrow-band channels Send a sample using the entire band Send samples concurrently using multiple orthogonal sub-channels
Why OFDM is Better? t t f Wide-band 0 1 1 0 0 0 1 0 1 1 0 0 0 1... Narrow-band f Multiple sub-channels (sub-carriers) carry samples sent at a lower rate Almost same bandwidth with wide-band channel Only some of the sub-channels are affected by interferers or multi-path effect
Importance of Orthogonality Why not just use FDM (frequency division multiplexing) Not orthogonal Leakage interference from adjacent sub-channels Individual sub-channel f Need guard bands between adjacent frequency bands à extra overhead and lower throughput Guard bands protect leakage interference guard band f
Difference between FDM and OFDM guard band Frequency division multiplexing f Orthogonal sub-carriers in OFDM Don t need guard bands f
Orthogonal Frequency Division Modulation * x[1] f f Data coded in frequency domain IFFT * x[2] * x[3] TransformaMon to Mme domain: each frequency is a sine wave In Mme, all added up t transmit receive t FFT Decode each subcarrier separately f Time domain signal Frequency domain signal
OFDM Transmitter and Receiver
Orthogonality of Sub-carriers Time-domain signals: x(t) Frequency-domain signals: X[k] IFFT Encode: frequency-domain samples à time-domain sample x(t) = N 2 1 k= N 2 X[k]e j2πkt N FFT Decode: time-domain samples à frequency-domain sample X[k] = 1 N N 2 1 t= N 2 x(t)e j2πkt N Orthogonality of any two bins : N 2 1 t= N 2 e j2πkt N j2π pt N e = 0, p k
Serial to Parallel Conversion Say we use BPSK and 4 sub-carriers to transmit a stream of samples 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, -1, 1, 1, -1, -1, -1, 1, 1 Serial to parallel conversion of samples Frequency-domain signal c1 c2 c3 c4 symbol1 1 1-1 -1 symbol2 1 1 1-1 symbol3 1-1 -1-1 symbol4-1 1-1 -1 symbol5-1 1 1-1 symbol6-1 -1 1 1 IFFT Time-domain signal 0 2-2i 0 2 + 2i 2 0-2i 2 0 + 2i -2 2 2 2-2 0-2i -2 0 + 2i 0-2 - 2i 0-2 + 2i 0-2 + 2i 0-2 - 2i Parallel to serial conversion, and transmit timedomain samples 0, 2-2i, 0, 2 + 2i, 2, 0-2i, 2, 0 + 2i, -2, 2, 2, 2, -2, 0-2i, -2, 0 + 2i, 0, -2-2i, 0, -2 + 2i, 0, -2 + 2i, 0, -2-2i,
t1 t2 t3 t4 t5 t6 subcarrier1 symbol1 1 1-1 -1 symbol2 1 1 1-1 symbol3 1-1 -1-1 symbol4-1 1-1 -1 symbol5-1 1 1-1 symbol6-1 -1 1 1 subcarrier2 subcarrier3 subcarrier4
t1-4 t5-8 t9-12 t13-16 t17-20 t21-24 bin1 symbol1 1 1-1 -1 symbol2 1 1 1-1 symbol3 1-1 -1-1 symbol4-1 1-1 -1 symbol5-1 1 1-1 symbol6-1 -1 1 1 bin2 bin3 bin4
t1-4 t5-8 t9-12 t13-16 t17-20 t21-24 bin1 symbol1 1 1-1 -1 symbol2 1 1 1-1 symbol3 1-1 -1-1 symbol4-1 1-1 -1 symbol5-1 1 1-1 symbol6-1 -1 1 1 bin2 bin3 bin4 Send the combined signal as the Mme-domain signal
Multi-Path Effect y(t) = h(0)x(t)+ h(1)x(t 1)+ h(2)x(t 2)+ = h( Δ)x(t Δ) = h(t) x(t) Δ time-domain Y ( f ) = H( f )X( f ) frequency-domain
Current symbol + delayed-version symbol à Signals are deconstructive in only certain frequencies
Frequency Selective Fading Frequency selective fading: Only some sub-carriers get affected Can be recovered by proper coding!
Inter Symbol Interference (ISI) The delayed version of a symbol overlaps with the adjacent symbol One simple solution to avoid this is to introduce a guard-band Guard band
However, we don t know the delay spread exactly The hardware doesn t allow blank space because it needs to send out signals continuously Solution: Cyclic Prefix Cyclic Prefix (CP) Make the symbol period longer by copying the tail and glue it in the front In 802.11, CP:data = 1:4
Cyclic Prefix (CP) Because of the usage of FFT, the signal is periodic FFT( ) = exp(-2jπ Δ f)*fft( ) delayed version original signal Delay in the time domain corresponds to phase shift in in the frequency domain Can still obtain the correct signal in the frequency domain by compensating this rotation
Cyclic Prefix (CP) w/o mulmpath y(t) à FFT( ) ày[k] = H[k]X[k] original signal w mulmpath y(t) à FFT( original signal + delayed-version signal ) ày[k] = α(1+exp(-2jπ Δ k))*x[k] = H [k]x[k] Lump the phase shift in H
Side Benefit of CP Allow the signal to be decoded even if the packet is detected after some delay undecodable decodable
OFDM Diagram Transmitter Modulation Insert S/P IFFT P/S CP D/A channel + noise De-mod remove P/S FFT S/P CP A/D Receiver
Unoccupied Subcarriers Edge sub-carriers are more vulnerable Frequency might be shifted due to noise or multi-path Leave them unused In 802.11, only 48 of 64 bins are occupied bins Is it really worth to use OFDM when it costs so many overheads (CP, unoccupied bins)?
Modulation Transmitter Insert S/P IFFT P/S CP 20MHz D/A baseband Oscillator passband 2.4GHz channel + noise De-mod remove P/S FFT S/P CP Receiver A/D 20MHz baseband Oscillator passband 2.4GHz
Carrier Frequency Offset (CFO) Oscillator (Tx) f tx f rx Error accumulate! Oscillator (Rx) The oscillators of Tx and Rx are not typically tuned to identical frequencies Up-convert baseband signal s n to passband signal y n =s n *e j2πf txnt s Down-convert passband signal y n back to r n =s n *e j2πf txnt s*e -j2πf rx nt s=sn *e j2πf ΔnT s
Samplig Frequency Offset (SFO) DAC (Tx) ADC (Rx) = T rx T tx T tx T tx = 1 f tx T rx = 1 f rx DAC (at Tx) and ADC (at Rx) never have exactly the same sampling period A slow shift of the symbol timing point, which rotates subcarriers Intercarrier interference (ICI), which causes loss of the orthogonality of the subcarriers
Correct CFO in Time Domain r n =s n *e j2πf ΔnT s r n+n =s n+n *e j2πf Δ(n+N)T s * r n r n+n s n S n+n Symbol 1 Symbol 2 = s n e j2π f ΔnT s * s n+n = e j2π f ΔNT s * s n s n+n = e j2π f ΔNT s s n 2 e j2π f Δ (n+n )T s z = f Δ = L n=1 L n=1 * r n r n+n = e j2π f ΔNT s * s n s n+n L n=1 = e j2π f ΔNT s s n 2 1 2π NT s z
Sampling Frequency Offset (SFO) DAC (Tx) ADC (Rx) Y i =H i X i * e j2πδin s/nm The transmitter and receiver may sample the signal at slightly different timing offset All subcarriers experience the same sampling delay, but have different frequencies Each subcarrier is rotated by a constant phase shift
Sample Rotation due to SFO Q subcarrier 3 x subcarrier 2 x x x x x x θ θ θ x x x x x subcarrier 1 I Ideal BPSK signals (No rotamon) Signals keep rotamng
Correct SFO in Frequency Domain phase 1 2πδN s /N m (SFO) 2πf Δ T s (Residual CFO) Subcarrier index Change in phase between Tx and Rx arer CFO correcmon SFO: slope; residual CFO: intersection of y-axis
Data-aided Phase Tracking regression x x x 2πt Δ N s /N m (SFO) 1 x 2πδfT s (Residual CFO) Change in phase between Tx and Rx arer CFO correcmon Using pilot bits (known samples) to compute H i *e j2πδin s/n fft =Y i/ X i Find the phase change experienced by the pilot bits using regression Update H i = H i *ej2πδin s/n fft for every symbol
After Phase Tracking Q x x x x x x x x x x x I Arer correcmon
OFDM Diagram Transmitter Modulation Insert S/P IFFT P/S CP D/A channel + noise De-mod Phase track remove P/S FFT S/P CP Correct CFO A/D frequency-domain Receiver Mme-domain
Quiz Please explain what is the multipath effect Please explain what is frequency selective fading and what is its root cause