Transcript Module-3 : Transmission Lecture-7 (11/5/00)

Case Study ADSL Modems
Geert Leus
Fac. EEMCS, TUDelft
[email protected]
DSP
p. 1
ADSL Modems
Overview
• ADSL (VDSL) modems
Modem technology : voice-band vs. xDSL
Communication impairments (...call for DSP solutions)
• Equalization
• Echo Cancellation
DSP
p. 2
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. 3
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. 4
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
1972
1976
1984
1989
1996
DSP
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/TCM
128-QAM/TCM
simplex
simplex
EC-duplex
EC-duplex
EC-duplex
p. 5
ADSL (VDSL) Modems
PS: duplexing techniques
• Frequency division duplexing (FDD)
up
down
`downstream’ = central office -> remote user
`upstream’ = remote user -> central office
• Time division duplexing (TDD) :
`ping-pong’
• Full duplex with echo-cancellation (EC)
frequency
up
down
time
up & down
time & frequency
DSP
p. 6
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 highspeed internet access with cable modems.
DSP
p. 7
ADSL (VDSL) Modems
• ADSL : `Asymmetric Digital Subscriber Line’
Backbone
Network
Access
Node
max 3.5…5 km
copper wire
• VDSL : `Very High Speed Digital Subscriber Line’
Backbone
Network
Access
Node
cabinet (ONU)
optical fiber
copper wire
max 0.3…1.5 km
DSP
p. 8
ADSL (VDSL) Modems
• Distance/bitrate trade-off (upstream)
bitrate
(Mbits/sec)
40
VDSL
30
20
10
ADSL
1
DSP
2
3
4
distance (km)
p. 9
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
NEXT
DSP
FEXT
p. 10
ADSL (VDSL) Modems
Communication Impairments :
DSP
p. 11
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. 12
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. 13
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. 14
ADSL (VDSL) Modems
IFFT
P
/
S
Discrete
equivalent
channel
S
/
P
FFT
FEQ
DAC
Tx clock
DSP
p(t)
ch(t)
r(t)
Tx filter
Channel
Rx filter
ADC
Rx clock
p. 15
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. 16
ADSL/VDSL Spectrum
• ADSL spectrum :
DSP
p. 17
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. 18
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. 19
Equalization
DSP
p. 20
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. 21
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 highperformance communication systems
DSP
p. 22