Wideband Communications Lecture 8-9: DMT Aliazam Abbasfar

Outline DMT examples Channel estimation Noise estimation

ADSL/VDSL ADSL : The most popular broadband Internet service Over telephone lines ITU .G992.1 DMT : T = 250 usec Down stream 256 tones, 4.3125 KHz spacing, real baseband (ADSL2+ /VDSL -> 512/4096 tones) 1/T’ = 2.208 MHz ( BW = 1.104 MHz) N + v = 512 + 40 (Hermitian symmetry ) 2-3 tones are not used (phone line) Tone 64 is pilot ( known QPSK data), Tone 256 not used Pmax = 20.5 dBm Up stream Upstream transmission uses 32 tones to frequency 138 KHz 1/T’ = 276 KHz ( BW = 138 KHz) N + v = 64 + 5 (Hermitian symmetry ) 1st tone not used (phone line) Pmax = 14.5 dBm Upto 12/1.5 Mbps down/upstream Bit loading to optimize data rate bmax = 15 (per tone)

Isaksson’s Zipper Bidirectional communication (full duplex) N = 10, v=2, D = 3 Suffix = 2 D Suffix = D Time advance at TX

WiFi Wireless LAN 802.11a/g @ 5/2.4 GHz COFDM : T = 4 usec 64 tones, 312.5 KHz spacing, complex baseband 1/T’ = 20 MHz BW = 20 MHz (15.56) N + v = 64 + 16 Tones : -31 to 31 (48 tones for data ) -31 to -27, 0, 27 to 32 are not used -27, -7, 7, and 21 are pilot Data rate = k * 48 * 250 KHz = 12k Mbps k = bn : bits/tone Upto 54 Mbps Variable coding No bit loading bn is constant for all tones Pmax = 16/23/29 dBm

Digital Video Broadcast (DVB) Digital TV broadcast Single frequency network (SFN) Improves coverage Creates ISI COFDM 2048 or 8192 tones, 4.464/1.116 KHz spacing complex baseband 1/T’ = 9.142 MHz BW = 20 MHz (15.56) N T’ = 8192 T’ = 896 usec (1/1.116 KHz) (N+v) T’ = 924/952/1008/1120 usec N T’ = 2046 T’ = 224 usec (1/4.464 KHz) (N+v) T’ = 231/238/252/280 usec 4/16/64 QAM Coding : 172/204 * 1/2, 2/3, 3/4, 5/6, or 7/8 Data rates : 4.98  31.67 Mbps Carries 2-8 TV channels

Channel estimation Channel model (n=0, 1,.., N-1) yn = xn * pn + un Yn = Pn Xn + Un P = Q p Time/Freq. domain estimates and errors pn , Pn en = yn – xn * pn En = Yn - Pn Xn MSE = E[|e|2] /N = E[|E|2] /N MSE = E[|u|2] /N + E[|p-p|2] Ex MSE = E[|U|2] /N + E[|P-P|2] Ex MSEch = E[|p-p|2] Ex = E[|P-P|2] Ex SNR = E[|x|2 ] /N |p|2 / E[|u|2] /N = Ex |p|2 / s2u SNRn = |Pn|2 E[|Xn|2] / E[|Un|2] = |Pn|2 E[|Xn|2]/ s2n SNRch,n >> SNRn  E[|Un|2] >> E[|Pn - Pn|2]

Channel estimation (2) Estimation method Average over L trials Estimation error E[ |En|2] = E[|Un|2] + E[|P-P|2] /N = E[|Un|2] (1+ 1/L) SNRch,n = L SNRn L = 40  1/L = 0.1 dB  0.1 dB accuracy Time–domain constraint improves the estimation

FEQ Frequency domain equalizer Adaptive methods Yn x Wn = Xn Wn = 1/Pn Zero Forcing MMSE

Noise estimation Noise variance estimator 0.1 dB accuracy  L=3200 Mean = true estimate Variance 0.1 dB accuracy  L=3200 %10 chance of more deviation %1  12,800 %0.1  28,800 Noise tracking First order update L = 6400  m’ = 6x10-4 Note : 1/gn = k s2

Windowing DMT/OFDM has a windowing for each symbol Windowing in time = Duration = T Rectangular/Raised cosine Needs extra guard time excess bandwidth Windowing in time = Convolution in frequency At TX Reduces out of band emission At RX Reduces narrowband noise Lower sidelobes improves gn

Reading Cioffi Ch. 4.7, 4.9