1 Multi-Gigabit Wireline Systems over Copper: An Interference Cancellation Perspective S. M. Zafaruddin, Member, IEEE and Amir Leshem, Senior Member, IEEE Abstract—Interference cancellation is the main driving mitigation for next generation digital subscriber line technology in enhancing the transmission rates over tele- (DSL) systems. phone lines above 100 Mbps. Still, crosstalk interference The coordinated processing of the signals over in multi-pair digital subscriber line (DSL) systems at all lines referred to as vectoring [8], is a viable higher frequencies has not been dealt with sufficiently. The upcoming G.(mg)fast DSL system envisions the use technique to deal with crosstalk in the DSL systems. of extremely high bandwidth and full-duplex transmis- This joint processing can be applied on the signals sions generating significantly higher crosstalk and self- transmitted from the central point to the end users interference signals. More powerful interference cancel- in the downstream and on the signal received at lation techniques are required to enable multi-gigabit the central point from the users in the upstream per second data rate transmission over copper lines. In transmissions. Since DSL channels are well con- this article, we analyze the performance of interference cancellation techniques, with a focus on novel research ditioned at lower frequencies, linear vectored pro- approaches and design considerations for efficient in- cessing is near optimal for crosstalk cancellation in terference mitigation for multi-gigabit transmission over the very high-speed digital subscriber line (VDSL) standard copper lines. We also detail novel approaches system [9], and it is recommended for the 106- for interference cancellation in the upcoming technologies. MHz G.fast system [1]. With further increase in frequency as in the 212-MHz G.fast system, and in the upcoming 424-MHz and 848-MHz G.(mg)fast I. INTRODUCTION systems, crosstalk is significantly higher than the Evolution of data transmission technologies over direct path thereby the channel matrices are no copper lines has been vary rapid. Recent extensions longer diagonal dominant. In the 212-MHz G.fast achieve bit rates of several gigabits per second [1]– system, linear cancellation schemes are shown to be [4]. This growth is possible by exploiting higher near-optimal [10]. Recently, lattice reduction based transmission bandwidth over the same telephone techniques are employed for the the 212-MHz G.fast lines [5]. However, an increase in the bandwidth system that incurs an extra run-time complexity increases the crosstalk among closely packed tele- [11]. It is interesting to evaluate the performance phone lines in a binder caused by the electromag- of linear and non-linear processing for G.(mg)fast netic coupling. Furthermore, the upcoming copper systems. It is well known that the nonlinear schemes are computationally more complex than the linear arXiv:2005.08264v1 [eess.SP] 17 May 2020 line technology might include full-duplex trans- mission creating problems of self-interference [6], techniques. This opens an exciting opportunity to [7]. In addition, subscribers can experience inter- develop computationally efficient crosstalk cancella- ference external to the binder. Novel interference tion schemes for the next generation multi-pair DSL cancellation techniques are required to realize a systems. multi-gigabit rate transmission over copper lines. The upcoming G.(mg)fast systems promise to In this article, we analyze the performance of in- achieve multi-gigabit data rates using full-duplex terference cancellation techniques, with a focus on (FD) transmissions, and by exploiting higher band- novel research approaches for efficient interference width than the previous standards [5]–[7]. This is in contrast to the VDSL system based on the frequency S. M. Zafaruddin is with Department of Electrical and Elec- division duplexing (FDD) and to the G.fast system tronics Engineering, BITS Pilani, Pilani-333031, India (email: based on the time division duplexing (TDD). Al- [email protected]). Amir Leshem is Faculty of Engineering, Bar-Ilan University, though FD transmissions can double the throughput, Ramat Gan 52900, Israel (email: [email protected]). self-interference or the echo signal can be detrimen- 2 Technology Duplexing Bandwidth Tone Spacing Symbol Rate No. of Tones Agg. Data Rate ITU-T VDSL2 FDD 17 MHz 4.3125 KHz 4 KHz 4096 100 Mbps G.993.2 (2006) G.fast 106 TDD 106 MHz 51.375 KHz 48 KHz 2048 1 Gbps G.9700 (2014) G.fast 212 TDD 212 MHz 51.375 KHz 48 KHz 4096 2 Gbps G.9700 (2017) G.9711 (Sept-2020) G. (mg)fast 424 FD 424 MHz 51.375 KHz 48 KHz 8192 5 Gbps G.(mg)fast 848 FD 848 MHz 102. 750 KHz 96 KHz 8192 10 Gbps G.9711 TABLE I: Evolution of transmission over telephone lines technology with system parameters. Aggregate data rate is for each user upstream and downstream combined for FDD and TDD systems, and either upstream or downstream in the FD system. FD: full-duplex; FDD: frequency division duplexing; TDD: time division duplexing. tal for the data rate performance. It is interesting significant in G.(mg)fast. to note that the symmetric DSL technologies were based on the FD where the self-interference was dealt with an echo canceler [12]. However, in the II. OVERVIEW OF DSL TECHNOLOGIES central office (CO) topology, near-end crosstalk (NEXT) beyond 600 kHz was prohibitive, and later standards used either FDD or TDD. In this context, Discrete multi-tone (DMT) is the de facto multi- the performance analysis of existing echo cancel- carrier modulation for frequency selective DSL ers in the G.(mg)fast systems and development of channels. Of-course, there is a change in the param- novel techniques tailored for multi-gigabit data rate eters for different technologies. The main system pa- transmissions are desirable. NEXT cancelling was rameter is the sub-carrier spacing which is chosen to used and is still relevant in point to point multi- limit the number of tones to 4096 to accomplish per- input-multi-output (MIMO) topologies such as cer- tone processing. For the VDSL system with 17 MHz tain multi-pair symmetric high-speed DSL (SHDSL) bandwidth, the sub-carrier spacing is 4:3125 KHz. or 1=10 Gbps Ethernet. Unfortunately, in point to The sub-carrier spacing in the G.fast is 51:75 KHz multi-point, NEXT cancellation at the CPE is hard which accounts for 2048 tones and 4096 tones in the because of the separation of the receivers. It has 106 MHz and 212 MHz G.fast systems, respectively. been suggested that for certain topologies, proper The 424 MHz G.(mg)fast system employs 8192 power management of the signals can overcome this tones with a subcariier spacing of 51:75 KHz. In problem [6]. order to limit the number of tones, we anticipate that the sub-carrier spacing of the G.(mg)fast- 848 The lack of signal-coordination of the interfer- MHz system should be double compared the G.fast ence external to binder i.e., from other existing system. broadband services and through the usage of var- Duplexing techniques are another distinguishing ious electrical appliances in the vicinity of a CPE feature among different DSL technologies. These limits the possibility of its cancellation. The usual techniques separate the upstream and downstream practice to deal with the impulse noise is the use of to avoid the NEXT and echo signals in the DSL error-correcting codes together and physical layer systems. The VDSL standard separates the upstream retransmission. The use of noise reference modules and downstream in the frequency domain, known as has shown promise for impulse noise mitigation in the FDD. The latest G.fast standard employs a TDD VDSL system [13]. where the upstream and downstream are transmitted This article analyzes the performance of multi- at different times. The next generation G.(mg)fast gigabit rate DSL systems. In this paper our fo- targets a more ambitious full duplex transmission cus is on comparative study of FEXT cancellation to double the throughput with a provision of effi- (vectoring) techniques, which are the main building cient echo and NEXT cancellation. Table 1 presents block in VDSL and G.fast and will continue to be important parameters of the various technologies. 3 Distributed Users Frequency Impulse Noise Upstream Transmission Impulse Noise Downstream/Upstream CPE (G.fast) P o r t FD: G.(mg)fast Direct Path P o r t CPE (G.fast) FEXT Fiber Line DP Downstream Upstream TDD: G.fast NEXT FEXT CPE (G.fast) P o r t Self-Interference Alien FEXT (Echo) P o r t Upstream CPE FDD: VDSL G.(mg)fast Downstream Time Copper Lines Binder Downstream Transmission Vectored Processing Fig. 1: Duplexing techniques [left] and interference environment [right] in a multi-pair DSL system. FD: full-duplex; FDD: frequency division duplexing; TDD: time division duplexing; NEXT: near-end crosstalk; FEXT: far-end crosstalk; DP: distribution point; CPE: customer premise equipment. III. NOISE AND INTERFERENCE ENVIRONMENT inate from the end opposite to that of the affected receiver. Although the FEXT interference decreases Thermal noise caused by the random movement with line lengths, its impact remains significant and of electrons is inherently present in all communica- has adverse effect on the data rate performance. tion systems. This presents a noise floor which is generally considered with a power spectral density (PSD) of −174 dBm/Hz at the room temperature. The PSD of background noise is around −140 Another major cause of rate degradation is dBm/Hz for the VDSL frequencies which decreases crosstalk that originates from subscribers enjoying to −150 dBm/Hz for the G.fast system. The noise other services, and is referred to as alien crosstalk. floor for the G.(mg)fast systems needs to be mea- Such crosstalk exists in practical situations mainly sured.