Performance evaluation of the Zipper duplex method

Transcription

1 Performance evaluation of the Zipper lex metho Frank Sjöberg, Mikael Isaksson, Petra Deutgen, Rickar Nilsson, Per Öling, an Per Ola Börjesson Luleå University of Technology, Division of Signal Processing, SE , Luleå, Sween Telia Research AB, Aurorum 6, SE , Luleå, Sween Abstract In this paper we present performance results for a new lex scheme, calle Zipper, for DMT-base VDSL systems on copper wires. This scheme ivies the available banwith by assigning ifferent subcarriers for the stream an ownstream irection. It has high exibility to ivie the capacity between the - an ownstream as well as goo coexistence possibilities with other systems such as ADSL. Simulation results show high bitrate performance in ifferent environments such as mixe ADSL an VDSL traf- c, uner raio frequency interference, an with ifferent backgroun noise sources. I. I Very high bit rate Digital Subscriber Lines (VDSL) [1, 2] is the latest igital subscriber line technique for high spee communication on twiste copper wires. VDSL is intene to use larger banwith an achieve higher bit rates than its precursor, e.g. Asymmetrical Digital Subscriber Line (ADSL) [3] an High bit rate Digital Subscriber Line (HDSL) [4]. The stanar for VDSL is currently being evelope an is iscusse in forums such as American National Stanars Institute (ANSI) an European Telecommunications Stanars Institute (ETSI). A signicant problem VDSL systems confront is near-en crosstalk (NEXT). This NEXT occurs when ata is transmitte simultaneously in both irections, on the same frequencies, an on several wires in the same biner gro. Systems that preominantly transmit in only one irection (such as ADSL) experience mostly far-en cross-talk (FEXT), a markely less severe problem than NEXT [5]. Avoiance of NEXT by iviing the channel capacity between the two irections has shape the existing VDSL proposals. Time ivision lex (TDD) [6] an frequency ivision lex (FDD) [7] are two ifferent proposals for iviing the capacity in time an in frequency, respectively. Recently we introuce a novel iscrete multi-tone (DMT)-base [8] lex scheme for VDSL, calle Zipper [9]. Zipper uses ifferent orthogonal sub-carriers in the stream an ownstream irections to ivie the capacity, thereby avoiing NEXT. Using several orthogonal signals gives Zipper both variable capacity allocation an high ADSL compatibility. II. Upstream Downstream Frequency Figure 1 The Zipper principle of capacity ivision. In this paper we evaluate the performance of the Zipper lex metho. We calculate achievable bit rates for ifferent scenarios consisting of ifferent types of wires an noise sources. In particular we consier Zipper performance in a VDSL only environment, in mixe VDSL an ADSL traf- c, an with ETSI moels for backgroun noise an raio frequency interference (RFI). T Z D M Zipper extens traitional DMT in two ways: it uses ifferent DMT-carriers in ifferent transmission irections (as shown in Figure 1) an it as a cyclic suffix (CS) to ensure orthogonality between the transmitte an receive signal (as shown in Figure 2). Zipper allocates ifferent sub-carriers for the stream an the ownstream. A sample allocation scheme is sketche in Figure 1. The allocation of the stream an ownstream sub-carriers can be one ynamically enabling runtime aaption of the bit rates. The stream part of a transmitte DMT frame can be moele as: ) j2kfs 2 t Re X [ k] e N 2 N+ CP + CS x ()= t t [, ] fs k I otherwise, (1) where I is the inex set for the stream carriers, N is the total number of sub-carriers, CP is the length (in samples) of the cyclic prex, CS is the length of the cyclic suffix an fs is the sampling frequency. Similarly the ownstream part of the DMT frame has an inex set Iown that is complementary to I (or a subset thereof).

2 CP CP +CP/f s Desire signal + NEXT signal = Receive signal CS CS +(CP+2N)/f s Figure 2 Timing iagram showing the NEXT symbol frame, the esire symbol frame, an the portion of ata extracte from the receive frame. t Power Spectral Density (B) a b a Receive signal b 9-NEXT on other carriers c 9-FEXT isturbing receive signal 9-NEXT isturbing receive signal Frequency (MHz) c CS = CS = 25 CS = 5 Amplitue NEXT impulse response h NEXT (t) Cable inpulse response h cable (t) Figure 4 Level of NEXT that appear if a too short cyclic suffix is use. presse as: r()= t x () t h ()+ t x () t h () t, cable own NEXT (2) +CP/f s Figure 3 Serimpose channel impulse response from NEXT an esire signal. Because Zipper transmits an receives simultaneously, the two network ens must be synchronize in both time an frequency in orer to maintain orthogonality. As both the stream an the ownstream contribute to a receive DMT frame, time synchronization is require to keep the signal contributions within one DMT frame. All transmitters in the access network (that may cause interference to each other) are synchronize to transmit simultaneously. Frequency synchronization between the two network ens is necessary to ensure the proper spacing between sub-carriers. However, in aition to synchronizing the transmitters an receivers, we a a cyclic suffix to ensure orthogonality between the stream an ownstream signals, thus preventing NEXT an near echoes. Traitional DMT uses a cyclic prex to preserve orthogonality between the carriers an prevent interblock interference [1], but Zipper as an extra cyclic suffix to preserve orthogonality between the stream an ownstream carriers. Consier a network terminal. With the Zipper scheme, it simultaneously transmits an receives. So the network terminal is not only receiving its intene signal but also NEXT from nearby transmitters plus its own transmitte signal which appears as near echoes. In Figure 3 we sketch a NEXT impulse response together with the cables impulse response. The esire signal is elaye secons ue to the propagation elay, but the isturbing signal arrives almost immeiately. The NEXT impulse response coul also represent a near echo impulse response. A receive signal at the central office can be ex- t where we have inclue only one NEXT isturber. The isturbing signal in (2) coul also represent near echoes. Since the esire stream signal is elaye, the fast Fourier transform (FFT) in the DMT receiver will use the part of the receive signal, r() t, that is in the range CP +2 t + + f, CP N, (3) s fs as epicte in Figure 2. For the non-elaye isturbing ownstream signal to be orthogonal it nees to be cyclically extene so that it spans the whole range in (3). This is accomplishe by using a cyclic suffix that is equal to or longer than the propagation elay. Since there can be wires of ifferent length in an access network, the cyclic suffix has to be as long as the elay in the longest wire-pair. Figure 4 shows how much NEXT is spresse by a cyclic suffix of ifferent lengths. For this case a cyclic suffix of 6 samples woul have completely spresse the NEXT. To summarize this section, Zipper is a DMT-base system transmitting orthogonal signals over ifferent sub-carriers in ifferent transmission irections. Maintaining signal orthogonality at the receiver en puts two key system requirements: synchronization among all transmitters at both ens a cyclic suffix to compensate for propagation elay III. F C Zipper is very exible lex scheme. This is because it uses a large number of sub-carriers to ivie the capacity an these sub-carriers can be assigne ynamically (even after the system is installe an running). This gives the avantage that any esire ratio between an ownstream

3 ADSL stream ADSL ownstream Arbitrary transmission irection.1 MHz 2 MHz 1-2 MHz Figure 5 A Zipper subcarrier assignment emonstrating possible coexistence with ADSL. bit rates can be chosen at any time. The exibility in subcarrier assignment also allows a Zipper base VDSL system to be spectrally compatible with other systems. A valuable feature for VDSL systems is the ability to coexist in the same biner gro as other systems, such as ADSL. A reasonable conition for coexistence between ADSL an VDSL is that neither system introuces NEXT to the other. This can be achieve if the both ADSL an VDSL transmit in the same irection in the common frequency ban. With Zipper the lowermost sub-carriers, those where ADSL exists (uner 2. MHz), may be partitione such that only FEXT is introuce between ADSL an VDSL [11], as epicte in Figure Table 1. Simulation parameters. Simulation parameters AWGN 14 Bm/Hz System margin margin =6B Coing gain coe =3B SNR-gap =98. B Sampling frequency 22 Mhz Use banwith 3 khz - 11 MHz Number of carriers 248 Length of CP 1 samples Length of CS 22 samples ( 2 m) IV. P 2 We have calculate the bit rate performance of Zipper for four ifferent backgroun noise environments. The rst case is a clean VDSL scenario with only AWGN as backgroun noise. The secon case consists of a mix of ADSL an VDSL services generating ADSL crosstalk in aition to the AWGN. The thir an fourth cases use the ETSI backgroun noise moel [2], while the fourth case also inclues raio frequency interference (RFI). All four cases inclue VDSL self-fext from 25 other users. Since Zipper uses DMT-moulation it is the bit-loaing [12] that etermines the bit rate of the system. The number of bits that can be loae onto carrier number k is calculate as [12] SNRk coe bk = log2 +1, margin where SNRk is the signal to noise ratio on carrier k, coe 1 is the coing gain, is the SNR-gap between the Shannon capacity an QAM-moulation [13], an margin is the system margin. System margin is the aitional amount of noise the system can tolerate without exceeing the allowe bit error rate (BER). By summing the number of bits given by the bitloaing we get the capacity of the system. Within the VDSL frequency ban there are certain frequency bans reserve for amateur raio users [2], so calle HAM-bans. To comply with the regulations for usage of these bans we are not allowe to transmit within these bans. So the carriers that correspon 1An SNR-gap of 9.8 B [13] is use to achieve a BER of approximately Figure 6 Achievable ownstream (8:1) asymmetrical bitrates, with AWGN an 25 self-fext istubers. to frequencies in the HAM-bans are not loae with any bits. Achievable bit rates have been calculate for ifferent 7 lengths of TP1 an TP2 wires [1], for a target BER of 1. The TP1 wire has a iameter of.4 mm an the TP2 wire has a iameter of.5 mm. Parameters use in the calculation are liste in Table 1. A. VDSL Environment Figure 6 shows achievable (8:1) asymmetrical bit rates versus wire length for the case with only self-fext an AWGN as backgroun noise. For wires shorter than 6 m there is no big ifference between the two type of cable, but for longer wires the thicker TP2 cable gives higher bit rates. This is because self-fext is the ominating noise source for shorter wires an AWGN for longer wires. The TP2 wire attenuates the signal less than TP1, thus it attenuates FEXT less also, which gives almost the same SNR for both types of wire for shorter lengths.

4 6 5 TP2, ADSL-compatible TP1, ADSL-compatible TP2, not ADSL-compatible TP1, not ADSL-compatible Figure 7 Achievable (8:1) asymmetrical ownstream bit rates for Zipper, with 25-ADSL systems an 25 self-fext. Figure 8 Downstream (8:1) bit rates, with ETSI backgroun noise moel A an 25 self-fext. B. Mix of VDSL an ADSL The results for the secon case, where Zipper coexists with 25 ADSL users in the same biner gro, are shown in Figure 7. The crosstalk moels an ADSL spectral power mask are specie by ANSI in [1]. To make Zipper compatible with ADSL the lower sub-carriers are assigne as shown previously in Figure 7. Figure 7 also shows the results for a case where the sub-carrier assignment is mae in such a way that the two systems are not spectrally compatible (every ninth sub-carrier is use in the stream irection). We can see that there is a clear avantage in making VDSL spectrally compatible with ADSL. But even when we avoi NEXT from the ADSL systems the performance is lower than in a clean VDSL-environment. This is because the power spectral ensity of the ADSL signal is 2 B higher than for VDSL, so FEXT from ADSL will be much stronger FEXT from VDSL in the common frequency ban. C. ETSI noise moel With the ETSI backgroun noise moel A ene in [2] we get the performance shown in Figure 8. The ETSI noise moel inclues AWGN an a mix of crosstalk from other existing services such as ADSL, HDSL, ISDN, etc. Comparing with Figure 7 we see that the performance with the ETSI moel is not much ifferent than the performance for the ADSL case. This is not completely unexpecte since the power spectral ensity of the ETSI noise moel resembles that of an ADSL system. D. ETSI noise moel plus RFI As a worst case scenario we have ae 8 broacast raio interferers to the ETSI noise moel A. These 8 RFI signals are those specie by ETSI [2] except for the two below 3 khz. At the customer sie, three of the RFI signals have signal power of -6 Bm an the other ve have -4 Bm signal power. The signal powers at the central office are 1 B lower. To spress this RFI we have use a time winow [14] at the receiver. To preserve the orthogonality after the winowing each DMT-frame is extene cyclically by 6 extra samples. Figure 9 show the performance for this case an Figure 1 shows the spectrum of the ETSI backgroun noise plus the RFI signals at the customer location (after winow an FFT). The bit rate performance showe to be just slightly lower than with only the ETSI noise moel. Table 2 shows the maximum length the wires can have for certain bit rates (both symmetrical an asymmetrical), for all ifferent noise scenarios Figure 9 Do wnstream (8:1) bitrates, with ETSI backgroun noise moel A plus RFI an 25 self-fext.

5 Power spectral ensity (Bm/Hz) R2, ANSI, Sacramento, CA, Sept [2] Very high spee igital subscriber line (VDSL); part1: Functional requirements, Tech. Rep. DTS/TM631(raft), ETSI-TM6, Nov [3] K. Sistanizaeh, P. Chow, an J. Cioffi, Multitone transmission for asymmetric igital subscriber lines (ADSL), in Proc. Intern. Conf. Commun., pp , May [4] J. Lechleier, A review of HDSL progress, IEEE J. Select. Areas Commun., vol. 9, pp , Aug [5] J.-J. Werner, The HDSL environment, IEEE J. Select. Areas Commun., vol. SAC-9, pp. 7858, Aug Subcarrier inex Figure 1 Power spectral ensity of ETSI noise moel A plus RFI. Table 2. Reach for ifferent backgroun noise scenarios. bit rate Reach in meters for TP1/TP2 -wire (Mbps) AWGN ADSL-mix ETSI ETSI+RFI 52:6.4 16/16 11/11 13/13 12/12 26:3.2 86/15 76/9 75/9 69/83 13: /163 16/131 15/131 1/124 26:26 27/27 18/18 2/2 19/19 13:13 93/113 8/97 79/99 73/9 It shoul be note that HAM-raio interferers can be much stronger than the broacast interferers use in this case, but they can be almost completely cancelle with RFIcancellation methos such as [15], [16]. VI. V. C Zipper is a lex metho for VDSL that is very exible ue to the many sub-carriers that ivies the capacity between the - an ownstream irection. This allows Zipper to be spectrally compatible with other existing systems, such as ADSL. In this paper we have evaluate the performance of the Zipper lex scheme. We showe that it gives goo bit rate performance in coexistence with ADSL systems. The scheme is also shown to be robust against RFI. The best performance is obtaine in an environment with only VDSLsystems ue to the ecrease level of crosstalk. A The authors wish to thank Paul Petersen an Sarah Kate Wilson for their help an comments on this paper. R [1] Very-high-spee igital subscriber lines system requirements, raft technical report, Tech. Rep. T1E1.4/97- [6] J. M. Cioffi, J. S. Chow, J. A. Bingham, an P. Tong, An SDMT line coe proposal with rationale for sport, Tech. Rep. T1E1.4/96-88, ANSI, Colorao Springs, Co, Apr [7] C. Fael et al., Preliminary proposal for a single-carrier VDSL specication, Tech. Rep. T1E1.4/97-16, ANSI, Austin, Tx, Feb [8] J. A. Bingham, Multicarrier moulation for ata transmission: An iea whose time has come, IEEE Commun. Mag., vol. 28, pp. 514, May 199. [9] M. Isaksson, P. Deutgen, F. Sjöberg, P. Öling, H. Öhman, S. K. Wilson, an P. O. Börjesson, Zipper a exible lex scheme for VDSL, in Proc. Intern. Workshop on Copper Wire Access Systems, (Buapest, Hungary), pp. 9599, Oct [1] A. Pele an A. Ruiz, Frequency omain ata transmission using reuce computational complexity algorithms, in Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing, (Denver, CO), pp , 198. [11] D. Bengtsson, P. Deutgen, N. Grip, M. Isaksson, L. Olsson, F. Sjöberg, an H. Öhman, Zipper performance when mixing ADSL an VDSL in terms of reach an capacity, Tech. Rep. T1E1.4/97-138, ANSI, Clearwater Beach, Floria, May [12] P. S. Chow, J. M. Cioffi, an J. A. C. Bingham, A practical iscrete multitone transceiver loaing algorithm for ata transmission over spectrally shape channels, IEEE Trans. Commun., vol. 43, pp , Feb [13] G. Forney an M. Eyuboglu, Combine equalization an coing using precoing, IEEE Commun. Mag., vol. 29, pp. 2534, Dec [14] P. Spruyt, P. Reusens, an S. Braet, Performance of improve DMT transceiver for VDSL, Tech. Rep. T1E1.4/96-14, ANSI, Colorao Springs, Co, Apr [15] B. Wiese an J. Bingham, Digital raio frequency cancellation for DMT VDSL, Tech. Rep. T1E1.4/97-46, ANSI, Sacramento, CA, Dec [16] F. Sjöberg, R. Nilsson, N. Grip, P. O. Börjesson, S. K. Wilson, an P. Öling, RFI sression in DMT-base VDSL systems, to appear at Intern. Conf. on Telecomm. 98, (Chalkiiki, Greece), June 1998.