Title : Zipper-a duplex scheme for VDSL based on DMT Source : Telia Research AB Contact :

A duplex scheme for DMT-based VDSL systems is presented here. It offers a solution to the problem with NEXT and near-echoes in broadband communications. The scheme has high duplex efficiency, low latency, and attractive transmit power features. However, hardware verification remains. 7KLV FRQWULEXWLRQ KDV EHHQ SUHSDUHG WR DVVLVW $16, 6WDQGDUGV 6XEFRPPLWWHH T1E1.4. 7KLV GRFXPHQW (T1E1.4/97016) LV RIIHUHG DV D EDVLV IRU GLVFXVVLRQV DQG LV QRW D ELQGLQJ SURSRVDO RI 7HOLD 5HVHDUFK $% 7HOLD 5HVHDUFK $% VSHFLILFDOO\ UHVHUYHV WKH ULJKW WR DGG WR DPHQG RU ZLWKGUDZ WKH VWDWHPHQWV FRQWDLQHG KHUHLQ $16, 7 ( 12 $XVWLQ 7H[DV )HEUXDU\ , ,QWURGXFWLRQ This contribution introduces a duplex method for VDSL. The method, which we will refer to as =LSSHU, is an alternative to methods as frequency divided duplex (FDD) and time divided duplex (TDD). The Zipper method is based on discrete multitone (DMT) modulation. The Zipper duplex method offers suppression of near-echoes and near-end crosstalk (NEXT) to the level of the far-end crosstalk (FEXT). It also provides exceptional versatility when it comes to dividing capacity between upand downlink and adaptibility to interference. However, the results presented here should be considered as early results, as hardware verification remains. Further studies and development will be performed. The method is being studied and validated at Telia Research AB in experimental test-bench trials using an in-house developed VDSL (DMT) prototype. In VDSL it is desired to have data transmitted both in the downlink direction, from the optical network unit (ONU) to the network terminal (NT), and in the opposite direction, the uplink. Methods for dividing the available resources, most significantly the channel, are referred to as duplex methods. For VDSL, two schemes are often discussed: FDD and TDD. With FDD the available spectrum is partitioned into disjoint frequency bands, typically two, to be used for the upand down-link, respectively. Some advantages with FDD are the strong suppression of near-echoes and NEXT. Often the frequency bands are separated by analog filters to ensure this suppression. However, these filters can become costly and quite complex. Also, the fixed analog filters gives very little flexibility, perhaps the main disadvantage with FDD. Once the frequency bands are set they are difficult to change, H J , to adapt the way the capacity is split between the upand the down-link. We believe this to be enough inhibiting to rule out FDD as a candidate to become a standard VDSL duplex method. In a TDD system the channel is accessed by only one of the ONU and the NT at a time. The capacity division between upand down-link is achieved by the relative amount of time the ONU and the NT have access to the channel. With TDD near-echoes and NEXT can be eliminated but TDD suffers from a capacity loss due to a need to wait for the signal in the channel to fade out when the direction of transmission is to be changed. A trade-off can be made between this capacity loss and latency, as we will discuss in the sequel. We will compare the capacity and latency of TDD and the Zipper method. Echo-cancellation can be used to reduce near-echoes in all the mentioned duplex methods and can also, in itself, be seen as a duplex method where all the frequencies are used all the time on both transmission directions. To realize this in practice is, however, difficult as the demands on the echo-cancellation hardware becomes very hard to fulfill. We will not further investigate echo-cancellation as a stand-alone duplex method. The presentation will proceed as follows. The Zipper method is defined in Section 2 and some of its properties are discussed in Section 3. Interchannel interference (ICI) due to time-misalignment and frequency offsets is described in Section 4, while Section 5 compares the efficiency and latency of the Zipper method with that of TDD. Section 6 concerns the power spectra and time variations of the transmitted signal. Coexistence with ADSL is briefly covered in Section 7. Section 8 contains a a discussion of some further properties of Zipper and conclusions are drawn in Section 9. 7KH =LSSHU GXSOH[ PHWKRG The =LSSHU method is a duplex method for systems employing DMT line-coding. The elements of the Zipper method are: • GLIIHUHQW VXEFDUULHUV are used in upand down-stream directions • WLPH V\QFKURQL]DWLRQ is performed between all transmitters in the ONU and the NTs • an extension of the cyclic prefix is added to compensate for propagation delay. $16, 7 ( 12 $XVWLQ 7H[DV )HEUXDU\ , The Zipper method is best illustrated with a graphical example. For the case of symmetric communication, L H , when the capacity is divided equally between the upand down-link, every second subcarrier can be used for the downlink and uplink, respectively, as is illustrated in Figure 1 below. This gives a ]LSSHU-like frequency pattern from which the method draws its name. As we will discuss in the sequel, this way of splitting the capacity has many advantages. The method allows the bandwidth to be split between the network terminal (NT) and the optical network unit (ONU) in almost any proportion, as long as different subcarriers are used in different directions. In addition to the standard requirements on a DMT system, preserving orthogonality between the signals in opposite directions introduces some extra requirements, such as the extension of the cyclic prefix, to suppress NEXT and near echoes. Frequency “ Zipper” Downlink Uplink )LJXUH 3RVVLEOH ZD\ RI V\PPHWULFDOO\ VSOLWWLQJ WKH FDSDFLW\ LQ D 3=LSSHU ́ SDWWHUQ 'HVFULSWLRQ RI WKH =LSSHU GXSOH[ PHWKRG Time dispersion is handled in DMT-systems by introducing a cyclic extension of the DMT-symbol, often referred to as a cyclic prefix (CP) [1]. In this section we show how the orthogonality between the up and downstream signals is maintained by using WLPLQJ DGYDQFH and an extra cyclic extension. 1HDU HFKRHV We start with studying the requirements on the time-synchronization and the extra cyclic extension for the reduction of near-echoes. In order to maintain orthogonality between the received signal and the disturbing transmitted signal, they have to be sufficiently aligned in time. This requirement is fulfilled by use of this extra extension, here denoted F\FOLF VXIIL[ (CS). An efficient way to perform synchronization is to use WLPLQJ DGYDQFH so that the transmitters at the ONU and the NTs start transmitting at the same time, as is depicted in Figure 2. The transmission of one upand one downstream signal is shown in Figure 2, at three different points in time: L transmission; LL reception; and LLL transmission of a new frame. During the reception of the shown frame, it is only disturbed by RQH frame in the other direction due to the cyclic suffix. This always holds if the length of the cyclic prefix and the cyclic suffix added together is longer than the time-dispersion and the propagation delay of the channel. Thus orthogonality between the signals can be preserved with timing advance and an additional cyclic extension. $16, 7 ( 12 $XVWLQ 7H[DV )HEUXDU\ , Optical Network Unit (ONU) Channel Network Terminal (NT)