p. 1 dsp case study adsl modems geert leus fac. eemcs, tudelft [email protected]
Post on 21-Dec-2015
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DSP p. 2
ADSL Modems
Overview
• ADSL (VDSL) modems
Modem technology : voice-band vs. xDSL
Communication impairments (...call for DSP solutions)• Equalization• Echo Cancellation
DSP p. 3
ADSL (VDSL) Modems
References:
• J.A.C. Bingham, `The theory and practice of modem design’, Wiley, 1980
• J.A.C. Bingham, `Multicarrier modulation for data transmission, an idea whose time has come’, IEEE Communications Magazine, May 1990, pp.5-14
• W.Y. Chen, `DSL simulation techniques and standards development for digital subscriber line systems’, Macmillan, 1998
• ...
DSP p. 4
ADSL (VDSL) Modems
Modem Technology :
• …-1970 : development of efficient modulation techniques for transmission over band limited channels, spurred by military applications (-> … 9.6 kbits/sec)
• 1970-1980 : Semiconductor technology (size & power reduction)• 1980-1990 : Introduction of coding techniques
(-> … 19.2 kbits/sec)• 1990-2000 : reaching Shannon limit
(-> 33kbits/sec, asymmetric 56kbits/sec modems)
• 1995-… : ADSL (8Mbits/sec downstream, 60 kbits/sec upstream) • 2000-… : VDSL (52Mbits/sec downstream, 6Mbits/sec upstream), ...
DSP p. 5
ADSL (VDSL) Modems
Performance improvement results from:• improved telephone channel quality (digital switches, etc.) • improved modulation/coding/DSP techniques (QAM, trellis
coded modulation (TCM), echo cancellation (EC), adaptive equalization,…)
• semiconductor technology
19721976198419891996
4.8kbits/sec (V.27) 9.6kbits/sec (V.29) 9.6kbits/sec (V.32)19.2kbits/sec (V.33b)33.6kbits/sec (V.34)
8-PSK 16-QAM 32-QAM/TCM 64-QAM/TCM128-QAM/TCM
simplexsimplexEC-duplexEC-duplexEC-duplex
DSP p. 6
ADSL (VDSL) Modems
PS: duplexing techniques• Frequency division duplexing (FDD)
`downstream’ = central office -> remote user
`upstream’ = remote user -> central office
• Time division duplexing (TDD) :
`ping-pong’
• Full duplex with echo-cancellation (EC)
up down
frequency
up down
time
up & down
time & frequency
DSP p. 7
ADSL (VDSL) Modems
• ADSL : `Asymmetric Digital Subscriber Line’
…-1993: ADSL spurred by interest in video-on-demand (VOD)
1995 : ADSL/VOD interest decline
1996 : ADSL technology trials prove viability.
1997-... : ADSL deployment, reoriented to data applications,
as telco’s reaction to cable operators offering high-
speed internet access with cable modems.
DSP p. 8
ADSL (VDSL) Modems
• ADSL : `Asymmetric Digital Subscriber Line’
• VDSL : `Very High Speed Digital Subscriber Line’
BackboneNetwork
AccessNode max 3.5…5 km
BackboneNetwork
AccessNode
max 0.3…1.5 km
cabinet (ONU)
optical fiber
copper wire
copper wire
DSP p. 9
ADSL (VDSL) Modems
• Distance/bitrate trade-off (upstream)
distance (km)1 2 3 4
10
20
30
40
bitrate(Mbits/
sec)
ADSL
VDSL
DSP p. 10
ADSL (VDSL) Modems
Communication Impairments :• Frequency-dependent cannel attenuation
introduces inter-symbol interference (ISI)• EM coupling between wires in same or
adjacent binders introduces crosstalk Near-end Xtalk (NEXT) = US into DS, DS into US
Far-end Xtalk (FEXT) = DS into DS, US into US
useful signal
FEXTNEXT
DSP p. 11
ADSL (VDSL) Modems
Communication Impairments :
DSP p. 12
ADSL (VDSL) Modems
Communication Impairments (continued) :• Xtalk from other systems e.g. HPNA (Home Phone Network Alliance) vs. VDSL• Radio Frequency Interference (RFI) e.g. AM broadcast, amateur radio• Noise e.g. impulsive noise (=high bursts of short duration)• Echo due to hybrid impedance mismatch
Conclusion: Need advanced modulation, DSP,etc. !
DSP p. 13
ADSL (VDSL) Modems
Line Codes (modulation technique) :• ADSL (ANSI standard)
DMT (`Discrete Multi-tone Modulation’, a.k.a. multi-carrier modulation, orthogonal frequency division multiplexing (OFDM))
256 different `carriers’, each of them QAM-modulated, see below.
• VDSL
DMT or single carrier (CAP/QAM) systems ? =undecided..
DSP p. 14
ADSL (VDSL) Modems
Line Codes (modulation technique) :• DMT: Multiple carriers are QAM-modulated with a low-rate
bit stream. The high-rate bit stream is thus carried by dividing it into hundreds of low-rate streams. Modulation/demodulation by FFT/IFFT (see below)
DSP p. 15
ADSL (VDSL) Modems
DAC
S/P
FFT
FEQ
IFFT P/S
Tx clock
Discreteequivalent
channel
Rx clock
p(t)
Tx filter Channel Rx filter
ch(t) r(t) ADC
DSP p. 16
ADSL/VDSL Specs
ADSL specs (=ANSI standard)• 256 carriers• 512-point (I)FFT’s (or `similar’)• prefix length = 32 • carrier spacing fo= 4.3215 kHz• basic sampling rate is 2.21 MHz (=512*4.3215k)
VDSL specs (=proposal) • up to 4096 carriers• same carrier spacing• etc.
DSP p. 17
ADSL/VDSL Spectrum
• ADSL spectrum :
DSP p. 18
Equalization
Problem Statement :• ISI/ICI-free transmission if prefix length >= L-1
L is channel impulse response length
• In ADSL : prefix-length is 32 samples
channel length is….500 samples (approx.) !!!
Solution : • Insert time-domain equalizer for channel shortening
DSP p. 19
Equalization
ps: standard (zero-forcing) equalization means `shortening
to 1 non-zero tap’ (cfr. C(f).H(f)=1)
ps: complexity !
Example: 32 taps @ 2.2 MHZ -> 70 Mflops...
DSP p. 20
Equalization
DSP p. 21
Echo Cancellation
• Problem Statement : ADSL with overlapping up- and downstream bands requires
echo cancellation (adaptive filter)
Main problem = complexity
example : echo path with 500 taps @ 2.2 MHz -> ...Gflops
DSP p. 22
Conclusion
• Advanced features in ADSL/VDSL systems - DMT modulation, prefix trick & co - Equalization - Echo Cancellation - (Windowing) - etc…
• Confirms importance of digital signal processing techniques in present-day high-performance communication systems