1(6) Austin, Texas March 2-6, 1998
T1E1.4/98-041
Standards project:
T1E1.4: VDSL
Title :
Pulse Shaping with Zipper Ð Spectral Compatibility and Asynchrony
Source :
Telia Research AB
Contact:
Mikael Isaksson1, Denis Mestdagh2 1
Telia Research AB, Aurorum 6, SE-977 75 LuleŒ, Sweden Fax: +46 920 75490 1 E-mail:
[email protected] 1
2
SGS-Thomson Microelectronics Avenue des Martyrs - BP 217 2 F-38019 GRENOBLE Cedex, France 2 Fax: +33 4 76 58 42 11 2 E-mail:
[email protected] 2
Authors:
Mikael Isaksson, Frank Sjšberg, Rickard Nilsson, Per …dling, Daniel Bengtsson, Denis Mestdagh.
Distribution: T1E1.4 Technical subcommittee working group Status:
Information
Abstract: We have investigated what the effects of adding pulse shaping and windowing to a
ZipperVDSL system would be. By adding short, pulse-shaped wings to the DMT symbol and accepting the, say, 3 % efficiency loss, the spectral sidelobes of the DMT-symbol are reduced. This provides several attractive features including the following: · The pulse shaping in the transmitter reduces the out of band power of the DMT-signal. · The spectral compatibility with other systems such as ADSL and CAP-VDSL is enhanced. · It becomes feasible to use Zipper-VDSL in a time-asynchronous mode, where a VTU-R only synchronizes itself to its VTU-O, while the VTU-R / VTU-O pair may be asynchronous to all other transceiver pairs. · The pulse shaping gives some increase in the depth of spectral notches, for HAM-bands etc. The windowing is a part of a powerful RFI-ingress suppressing method as proposed by Wiese and Bingham.
This contribution has been prepared to assist ANSI Standards Subcommittee T1E1.4. This document (T1E1.4/98-041) is offered as a basis for discussions and is not a binding proposal of Telia Research AB and SGS-Thomson. Telia Research AB and SGS-Thomson specifically reserves the right to add to, amend or withdraw the statements contained herein.
2(6) Austin, Texas March 2-6, 1998
1
T1E1.4/98-041
Introduction
Using pulse shaping and windowing in DMT systems is a topic often discussed and investigated. This contribution reports on the benefits and drawbacks of including pulse shaping in a Zipper-VDSL system [1,2]. Some of the results below are pertinent only to a Zipper-VDSL system, some more general. First we describe the pulse shaping and windowing that we have investigated. Thereafter, in Section 3, spectral compatibility issues with other systems present in the same binder group are discussed. The making of spectral notches and the ingress problem are also mentioned. With striking results, we ran simulations of Zipper-VDSL systems in a time-asynchronous mode. These are documented in Section 4. The expected, performance-killing NEXT, appears only as a mild degradation of the SNR curves. We see two immediate implications of this. If the synchronization between the transmitters on different pairs is lost, the performance hit is small. Also, if an operator so desires, Zipper-VDSL can run ÒasynchronouslyÓ, that is, with frame-synchronization on a line-by-line basis instead of on a binder-by-binder basis.
2
Pulse shaping and windowing the DMT frame
By pulse shaping the DMT frame at the transmitter, as shown in Figure 1, the frequency domain sidelobes of the DMT signal are reduced. To preserve orthogonality, the pulse shaping is applied only to extra samples added to the symbol for this purpose. The DMT-symbol is extended cyclically with b/2 samples at each end of the symbol before (prior to) the pulse shaping. Only these extra b samples are then shaped, as illustrated by Figure 1. To suppress radio frequency interference (RFI) a time window at the receiver can be used as proposed by Spruyt et al. in [3]. Windowing will also help to obtain high spectral containment in general, not only for RFI. The windowing is performed by first multiplying m samples at the beginning and m samples at end of the 2N+m block of samples. Then the outermost m/2 samples from each end are added to their corresponding samples at their respective opposite end of the 2N remaining block of samples, see Figure 1. To maintain orthogonality a symmetrical window is required, e.g. a raised cosine window, and the DMT-frame is cyclically extended m samples. The b+m extra samples result in a efficiency loss, a price to be paid for using this pulse shaping and windowing. In the simulations presented in the sequel, b has been 140 samples and m has been 70 samples. With our block length of about 4400 samples, the efficiency loss due to pulse shaping and windowing is just below 5%.
2N CP b/2
CS
DMT symbol m
m
b/2
Figure 1. Pulse shaping and windowing DMT frames.
3
Reducing the out of band power
The reduction of the out of band power obtained by pulse-shaping has some advantages. It allows Zipper to be more spectrally compatible with spectrally ÒlocatedÓ systems as ADSL or FDD-VDSL (e.g. CAP [4]), and the construction of required analog filters, such as POTS-filters, will also be an easier task. Figure 2 shows the out of band power for Zipper, with and without pulse shaping. In this case the subcarriers are assigned in such a way that we have one upstream- and one downstream band. The frequency bands are selected as one of the proposed FDD/CAP configurations for asymmetrical ratios [4]. The lower band is between 1.14 MHz and 2.6 MHz and the upper band starts at 3.1 MHz and occupies the rest of the bandwidth up to 11 MHz. From Figure 2 we find that when pulse shaping is used, the out of band power is below -120 dBm/Hz at the edge of the other band. When no pulse shaping is used this level is -95 dBm/Hz. Thus, the pulse shaping reduces the out of band signal power with 60 dB only 500 kHz from the edge of the band. This reduction was obtained with 2048 subcarriers and a pulse shaping overhead b equal to 140 samples.
3(6) Austin, Texas March 2-6, 1998
T1E1.4/98-041
The main benefit of including the windowing is that it gives efficient and effective RFI cancellation. This is well described in [3] by Spruyt et al. and by Wiese and Bingham in [5].
-60
-70
Signal power (dBm/Hz)
-80
-90
-100
-110
-120
-130
-140
-150
without pulse shaping with pulse shaping 0
0.5
1
1.5
2
2.5 3 Frequency (MHz)
3.5
4
4.5
5
Figure 2. Out of band power for Zipper with and without pulse shaping.
4
Asynchronous Zipper
As originally proposed, the Zipper duplex scheme [1,2] should work in a synchronous mode where all users in the same binder group are synchronized to make the NEXT interference between adjacent pairs orthogonal. We have now investigated if asynchronous transmission with the Zipper scheme is a possible alternative. However, synchronization and timing advance are still needed within each separate pair to maintain orthogonality to the near echoes, which otherwise would be a severe disturbance. That is, in the ÒasynchronousÓ mode synchronization, timing advance and the dimensioning of the cyclic suffix is done on a line-by-line basis, and not on a binder-by-binder basis. When Zipper operates asynchronously the NEXT will no longer be orthogonal. This is because the phase shift between two consecutive NEXT originating DMT frames can occur within the time period of the received DMT frame, as shown in Figure 3. DMT frames without pulse shaping Pulse shaped DMT frames
Received NEXT signals without windowing Received NEXT signals after windowing
Figure 3. Non-orthogonal NEXT occurrence. Since non-orthogonal NEXT may seriously degrade the capacity of Zipper, it is desirable to suppress this interference. Now, the pulse shaping and windowing comes to our help. By applying pulse shaping to the transmitted DMT-frames the out-of-band power of the NEXT is reduced. In addition, the windowing in the receiver will further minimize the reception of the non-orthogonal NEXT, cf. Figure 3. As we will see, the combination reduces the NEXT sufficiently for asynchronous Zipper-VDSL to be an alternative to the originally proposed synchronous Zipper scheme. However, to exploit the NEXT reduction, the up- and downstream carriers should be grouped in larger blocks (see Figure 2) than what is associated with time-synchronous Zipper, where a change of direction can be done with every subcarrier. The effects of pulse shaping and windowing on the NEXT-signal energies are shown in Figure 4. It shows the signal power, the NEXT reference signal without pulse-shaping and windowing, the NEXT signal after pulse-shaping and
4(6) Austin, Texas March 2-6, 1998
T1E1.4/98-041
windowing, and the FEXT signal at each subcarrier. More than 25 dB suppression of the non-orthogonal NEXT is obtained, when the carriers for the up- and down stream directions are grouped in blocks of 200 subcarriers. -60 VDSL-signal NEXT reference level FEXT NEXT without window & PS NEXT with window & PS
-70
Signal power (dBm/Hz)
-80 -90 -100 -110 -120 -130 -140 -150
0
200
400
600
800 1000 1200 Sub-carrier index
1400
1600
1800
2000
Figure 4. Non-orthogonal NEXT with and without pulse shaping and windowing.
4.1
Simulation of asynchronous Zipper
We have simulated the performance for asynchronous Zipper systems and compared it with synchronized Zipper systems. Table 1 shows the simulation parameters. For synchronized Zipper the cyclic suffix is dimensioned for 2000 meters (220 samples) but for asynchronous Zipper the length of the cyclic suffices are dimensioned individually for each wire. For the (8:1) asymmetrical bit rate we used the bands 2.0-2.6 MHz and 7.1-7.65 MHz for the upstream direction and the complement for the downstream, although the HAM bands were not used for transmission. In the symmetrical case the frequency bands 1.61-4.4 MHz and 7.1-10.1 MHz were allocated for the upstream. Table 1 Number of subcarriers Cyclic prefix length Cyclic suffix length Window length Pulse shaping length Background noise model Number of VDSL systems Cable type Used Bandwidth1 Transmit PSD-level SNR-gap2 System margin Coding gain
2048 100 samples 220 samples (max.) m = 70 samples b = 140 samples ETSI ÒAÓ 25 TP1 (0.4 mm) 300 kHz - 11 MHz -60 dBm/Hz 9.8 dB 6 dB 3 dB
Figure 5 and Figure 6 show the SNR for the down- and upstream directions, respectively, for the asymmetrical (8:1) rate, and Figure 7 and Figure 8 show the SNR for the symmetrical rate. There is a small loss in SNR at the edges of the transmission bands. Figure 9 and Figure 10 show the performance for synchronized and unsynchronized Zipper for the symmetrical rate and the asymmetrical (8:1) rate, respectively. We conclude that the performance loss is minor! Actually, there is even a performance gain for shorter wires. This is because a shorter cyclic suffix is needed on the shorter wires, resulting in higher duplex efficiency. Note though that we have taken the opportunity to change the direction of transmission in conjunction with a HAM band when possible. By doing so, we avoid some NEXT and get a ÒfreeÓ change of direction. 1 2
The HAM-bands listed in [6] are not used for transmission. SNR-gap G = 9.8 dB gives a bit error rate of approximately 10-7 [7].
5(6) Austin, Texas March 2-6, 1998
T1E1.4/98-041
SNR for downstream, TP1, 745 m, 25 FEXT
SNR for upstream, TP1, 745 m, 25 FEXT
40
40 synchronous asynchronous
35
35
30
30
25
25 SNR (dB)
SNR (dB)
synchronous asynchronous
20
20
15
15
10
10
5
5
0 0
200
400
600
800 1000 1200 Sub-carrier index
1400
1600
1800
0 0
2000
Figure 5. SNR for the downstream direction, (8:1) asymmetrical rate.
30
30
25
25 SNR (dB)
SNR (dB)
35
20
15
10
10
5
5
1400
1600
1800
0 0
2000
Figure 7. SNR for the downstream direction, symmetrical rate.
200
400
Asymmetrical bit rates for TP1, 25 FEXT
600
800 1000 1200 Sub-carrier index
2000
1400
1600
1800
2000
Symmetrical bit rates for TP1, 25 FEXT 30
Synchronous mode Asynchronous mode
Synchronous mode Asynchronous mode
50
25
40
20 bit rate (Mbps)
bit rate (Mbps)
1800
Figure 8. SNR for the upstream direction, symmetrical rate.
60
30
15
20
10
10
5
0 0
1600
20
15
800 1000 1200 Sub-carrier index
1400
synchronous asynchronous
synchronous asynchronous
600
800 1000 1200 Sub-carrier index
40
35
400
600
SNR for upstream, TP1, 810 m, 25 FEXT
SNR for downstream, TP1, 810 m, 25 FEXT
200
400
Figure 6. SNR for the upstream direction, (8:1) asymmetrical rate.
40
0 0
200
200
400
600 800 length (m)
1000
1200
1400
Figure 9. Performance for synchronized and unsynchronized Zipper, downstream (8:1) asymmetrical rate.
0 0
100
200
300
400
500 600 length (m)
700
800
900
1000
Figure 10. Performance for synchronized and unsynchronized Zipper, symmetrical rate.
6(6) Austin, Texas March 2-6, 1998
5
T1E1.4/98-041
Conclusions
We have investigated the effects of pulse shaping and windowing on a Zipper-VDSL system. Advantages of the combined pulse shaping and windowing are: reduced out of band power; RFI-ingress reduction; and enhanced spectral compatibility with FDD/CAP-VDSL and ADSL. Another very attractive feature, especially regarding VDSLdeployment issues, is that pulse shaping and windowing makes asynchronous Zipper a possible alternative to the originally proposed synchronous Zipper. The non-orthogonal NEXT due to the asynchrony is reduced by the pulse shaping and windowing which results in a performance close to synchronized Zipper. Note also, if the binder-group frame-synchronization is omitted, other Zipper-parameters, such as the FFT-size and sampling rate, can then also be chosen independently from line to line.
6
References
[1] M. Isaksson, D. Bengtsson, P. Deutgen, M. Sandell, F. Sjšberg, P. …dling, and H. …hman, Zipper - a duplex scheme for VDSL based on DMT, ANSI T1E1.4/97-016, Austin, Texas, February 3-7, 1997. [2] M. Isaksson, P. Deutgen, F. Sjšberg, P. …dling, S. K. Wilson, and P. O. Bšrjesson, Zipper a flexible duplex method for VDSL, CWAS 97, pp. 95-99, Budapest, Hungary, October 1997. [3] P. Spruyt, P. Reusens, S. Braet, Performance of improved DMT transceiver for VDSL, T1E1.4/96-104, Colorado, April 22-25, 1996. [4] B. Waring et al. VDSL Line Coding, Modulation and Duplexing Method based on SingleCarrier Technology, ETSI TM6, TD-4, Madrid, Spain, January 1998. [5] B. Wiese and J. Bingham, Digital Radio Frequency Interference Cancellation for DMT VDSL, ANSI T1E1.4/97-460, Sacramento, CA, December 1997. [6] J. Cioffi, ANSI VDSL System Requirements, ETSI STC/TM6 971T10R0, March 10-14, 1997. [7] P. Chow, J. Cioffi and J. Bingham, A Practical Discrete Multitone Transceiver Loading Algorithm for Data Transmission over Spectrally Shaped Channels, IEEE Transactions on Communications, vol. 43, no. 2/3/4, February/March/April 1995.
