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Wideband Communications

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

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Wideband Communications

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  1. Wideband Communications Lecture 8-9: DMT Aliazam Abbasfar

  2. Outline • DMT examples • Channel estimation • Noise estimation

  3. 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)

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

  5. 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

  6. 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

  7. Channel estimation • Channel model (n=0, 1,.., N-1) • yn = xn * pn + un • Yn = PnXn + Un • P = Q p • Time/Freq. domain estimates and errors • pn , Pn • en = yn – xn * pn • En = Yn - PnXn • 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]

  8. 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

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

  10. Noise estimation • Noise variance estimator • 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

  11. Windowing • DMT/OFDM has a windowing for each symbol • 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

  12. Reading • Cioffi Ch. 4.7, 4.9

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