0L[HG6LJQDO&LUFXLWVDQG6\VWHPV 0RGHP7HFKQLTXHV. The Modem as an example of a mixed signal system

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1 0L[HG6LJQDO&LUFXLWVDQG6\VWHPV 0RGHP7HFKQLTXHV The Modem as an example of a mixed signal system Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.1

2 ['6/[[['LJLWDO6XEVFULEHU/LQH2YHUYLHZ High speed modem techniques for wired telephone connection High speed modems, CAP and DMT DMT for ADSL (Asymmetrical Digital Subscriber Line) Advantages of DMT over QAM and CAP Main functions in digital processing Circuit parts in the Analog Front End G.lite, the low cost implementation of xdsl Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.2

3 +LJKVSHHGPRGHPV&$3DQG'07 Modems with data streams of more than 128 kbit/s do not make use of the standard 64 kbit/s channels that are used for voice and ISDN High speed modems are connected to dedicated modems in the Central Office that are directly linked to the digital network Two types of modulation are used now, CAP (Carrierless Amplitude Phase modulation) and DMT (Discrete Multi Tone modulation) In CAP amplitude and phase are modulated around a (suppressed) high frequency carrier with a high symbol rate. The digital data is mapped on a discrete number of amplitude and phase steps In DMT the digital data are distributed over a high number (typically 128 to 256) carriers. The symbol rate per carrier is low (4 kbaud) and the number of bits per symbol can be 15 in the most advanced systems. Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.3

4 '07IRU$'6/$V\PPHWULFDO'LJLWDO6XEVFULEHU/LQH 255 subcarriers, spacing khz, 0 to Mhz Symbol rate per carrier 4 khz, modulation between 0 and 15 bit per symbol Maximum bitrate per carrier is 15 * 4k = 60 kbit/s. QAM modulation, maximal 8 bit resolution (256 amplitude levels) on I vector and 7 bit resolution on Q vector ADSL combined with POTS, first subcarrier in use on khz ADSL mostly not used in duplex, lower frequencies for upstream data, higher frequencies for downstream data (Frequency Division) Carrier 6 to 31 are used for upstream, carrier 16 is used for reference, carrier 32 on nyquist frequency is not used for data 25 carriers available for upstream, maximum theoretical bitrate of 25 * 60 kbit = 1.5 Mbit, practical values between 64 and 640 kbit Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.4

5 '07IRU$'6/FRQWLQXHG '070RGXODWLRQ LPF HPF P OTS DMT 4k 26k subcarrier spacing 1.1M Hz Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.5

6 '07IRU$'6/FRQWLQXHG (FKR&DQFHOODWLRQ )UHTXHQF\'LYLVLRQPXOWLSOH[LQJ POTS upstream POTS upstream downstream downstream Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.6

7 '07IRU$'6/FRQWLQXHG 615IRU%(5 Bits/symbol QAM constellation size Required SNR QAM constellation: 4 bits: 2 bit I and 2 bit Q 2(1,1) 4 (QPSK) 14.5 db 3(1,2) 8 19,3 db 4(2,2) 16 21,5 db 5(2,3) db 6(3,3) db 7(3,4) db 8(4,4) db 10(5,5) db 12(6,6) db 15(7,8) ,8 db QAM constellation: 6 bits: 3 bit I and 3 bit Q Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.7

8 '07IRU$'6/FRQWLQXHG Carrier 33 to 255 are used for downstream, carrier 128 is used for reference, carrier 256 on nyquist frequency is not used for data 222 carriers available for downstream, maximum theoretical bitrate of 222 * 60 kbit = Mbit, practical values between 4 and 8 Mbit Line conditions tested during start-up, bitrate per carrier adjusted In operation BER per carrier monitored, bits are shifted to other carriers if needed Signal levels for POTS in dbm/600ω, for ADSL in dbm/100ω Maximum signal level in Central Office is 20.4 dbm (3.31 Volt RMS, noiselike signal, peak to peak voltage is about 33 Volt) Max. signal Remote Terminal 12.5 dbm. (1.33 Volt RMS, peak to peak voltage about 13 Volt) Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.8

9 '07IRU$'6/FRQWLQXHG Channel Response RFI Bridged Tap Crosstalk Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.9

10 '07IRU$'6/FRQWLQXHG Bits Attenuation Bits Frequency Frequency Frequency Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.10

11 $GYDQWDJHVRI'07RYHU4$0DQG&$3 DMT Discrete Multi Tone CAP Carrierless Amplitude Phase modulation QAM Quadrature Amplitude Modulation QAM and CAP mainly differ in the way they are generated: CAP is in nature an analog quadrature implementation with a sine/cosine mixer QAM is suited for time discrete digital implementation QAM and CAP have a high symbol rate on one carrier and therefore a wide side-band spectrum DMT has a low sybol rate (only 4k) and many carriers. Therefore is not very sensitive to spiky noise and RF interference. It can adapt easily to line conditions Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.11

12 6LJQDOFRGLQJLQWKHWUDQVPLWWHU Each 0.25 ms (4 khz) a data frame is build taking data out of the elastic buffer After CRC encoding, scrambling, Reed Solomon encoding and interleaving, data frames are prepared for transmission Tone ordering gives 256 complex vectors in the frequency domain (nr. of bits per tone and gain scaling depends on line conditions, the digital data are mapped on the QAM constellation) With a 512 point IFFT a symbol of 512 time domain samples is created The symbol of 512 samples is filled up with a 32 samples long cyclic prefix After 68 data symbols a synchronization symbol with a nominal length of 544 samples is added making a superframe of 17 ms Samplerate is 544 * 69/17ms = Msamples/s = 256 * ksamples/s The Msamples/s output signal can be bandlimited in MHz analog frquency band. An oversampled DA is preferred for smooth interpolation Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.12

13 0DLQIXQFWLRQVLQGLJLWDOSURFHVVLQJUHFHLYHU In the receiver most digital operations are reverse to the operations in the transmitter. A function that is unique for the receiver is the time equalization which is necessary to compensate for the dispersion and reflection in the line: The telephone line (local loop) is at most 5.5 km long Propagation delay is about 30 µs with a dispersion of a few µs. The sampling must be synchronized to the MHz sample frequency of the transmitter MHz is commonly used as frequency in the DSP Each symbol consists of 512 samples plus the 32 samples cyclic prefix. This prefix is needed to avoid intersymbol interference or leakage Leakage between symbols can originate from the different delays for different frequencies in the band from 0 to MHz A 160 tap adaptive FIR filter on MHz (36 µs) is used for equalization Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.13

14 :KDWGRHV$'6/ORRNOLNHLQWKHWLPHDQGIUHTXHQF\GRPDLQ I I I Carrier 2 Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.14

15 $'6/LQWKHWLPHGRPDLQ Each symbol consist of 512 samples in the time domain that represents the summation of the 256 carrier signals and 32 cyclic prefix samples Carrier n (on n * khz) is exactly n periods long in the time period of 512 samples Leakage from other symbols is avoided by the cyclic prefix (guardband) of 32 samples When a FFT over 512 samples is executed some energy of the cyclic prefix may leak into the frequency domain, giving only a small noise like addition to the frequency spectrum Complex frequency vectors can be recovered with almost no leakage. Amplitude and phase of the vectors must be corrected for equalization and dispersion Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.15

16 &KDUDFWHULVWLFRIWKH$'6/VLJQDO DMT is a multi carrier technique: Distortion of one carrier is unwanted signal in another carrier For the maximum resolution of 15 bits/symbol and a BER = 10-7, a SNR of 55 db is needed on one carrier (maximum V pp = 1 or V RMS = 0.35) Mean carrier amplitude is V RMS,mean = 0.58 * 0.35 = 0.20 About 250 carriers contribute to the total power resulting in a RMS amplitude value of = for the ADSL signal Clipping must be less then 10-7 so the peak amplitude is about 5 times higher than the RMS value resulting in 9 = = ,max The (white) noise over the full 1.1 MHz bandwidth must be lower then 55G% + 10log 256 = 31G% relative to V pp = 1 Maximum amplitude 32.3 = 30 db, so the minimum S/N ratio is 61 db All components in the analog chain must have noise and distortion much lower than -61 db Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.16

17 &LUFXLWSDUWVLQWKH$QDORJ)URQW(QG There are many circuits parts in the total signal chain: DA converter Transmitter Splitter filter Transformer and hybrid 7HOHSKRQHOLQH Transformer and hybrid Splitter filter Receiver and gain control AD converter To maintain optimum signal quality all components in the chain must have a THD plus noise much lower than -70 db. The SINAD of the convertors must be 12 to 13 bit Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.17

18 *OLWHWKHORZFRVWLPSOHPHQWDWLRQRI['6/ G.lite is named to the ITU specification for a xdsl protocol that compared to ADSL has: Low cost but still offering up to 128 kbit upstream and 1.5 Mbit downstream Better compatability to POTS, no splitter filter needed Suitable as a very fast PC telephone line modem that can compete with cable modems G.lite is also a DMT technique with: 127 carriers covering a frequency band from 0 to 552 khz A constellation diagram 8 (4,4) with a maximum size of 256 Level adaption to prevent distortion on the line and interference with POTS Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.18

19 &KDUDFWHULVWLFRIWKH*OLWH$)(DWVXEVFULEHUVLWH For the maximum resolution of 8 bits/symbol and a BER = 10-7, a SNR of 34 db is needed on one carrier (maximum V pp = 1 or V RMS = 0.35) Mean carrier amplitude is V RMS,mean = 0.58 * 0.35 = 0.20 About 125 carriers contribute to the total power resulting in a RMS amplitude value of = for the ADSL signal Clipping must be less then 10-7 so the peak amplitude is about 5 times higher than the RMS value resulting in 9 = = ,max The (white) noise over the full 1.1 MHz bandwidth must be lower then 34G% + 10 log(128) = 13G% relative to V pp = 1 Maximum amplitude 22.8 = 27 db, so the minimum S/N ratio is 40 db All components in the analog chain must have noise and distortion much lower than 40 db Mixed Signal Circuits and Systems, A.J.M. van Tuijl, IC Ontwerpkunde, sheet 8.19