One of the most effective and efficient methods of deploying high-speed digital services to business and residential customers is to use one of the many forms of DSL (Digital Subscriber Loop) technologies over copper telephone wires. This approach has become very popular in the last 20 years due to the fact that copper wires are already deployed almost everywhere and are quite easy to access, both at the Central Office (CO) and at the remote cabinet or the customer premises.
However, one of the main limitations of DSL technology is that the data capacity of copper wires decreases significantly as the length of the copper loop increases. Therefore, customers located more than a few kilometers from the Central Office can not be provided with high data speeds over copper wires.
One way to mitigate this issue is to use multiple copper pairs to the same customer location, thereby increasing the total data rate of the resulting multi-pair copper link. This method is typically referred to as “bonding” of copper pairs.
Another method for extending the reach of DSL services is to utilize repeaters. These are devices that are installed in intermediate locations on the copper loop, and contain one or more transceivers that receive and re-transmit the signal from neighboring devices. The resulting repeatered copper link is composed of multiple shorter segments that are connected to each other with repeaters. As a result, the capacity of the original long copper loop is increased to the capacity of the longest of these multiple shorter segments.
Repeaters can also be combined with bonding to further increase the rate and reach of DSL services by using multiple copper pairs, each of which is partitioned into multiple repeatered segments.
The deployment of repeaters faces many operational challenges including, but not limited to, installation procedures, proper electrical grounding and shielding, providing power over the copper wires, and remote troubleshooting and management to avoid the need to dispatch a technician to the field every time there is a problem with one of the multiple repeaters.
However, the main reason why repeaters are not widely used today is their potential for generating significant spectral interference to residential DSL services, which have become ubiquitous in the last decade. Spectral interference between different high-bitrate services in a copper binder is caused by the fact that each copper pair acts as an antenna. The signal transmitted on each copper pair, which is intended for the receiver located at the other end of that copper pair, is also inadvertently picked up by all of the neighboring copper pairs, because those pairs are not individually shielded from each other. This creates the well-known phenomenon of “cross-talk”, aptly named for the effect it caused in the early days of the telephone, when the telephone discussion taking place on one line could sometimes be overheard by the people conversing on a different line.
Due to the physical characteristics of copper pairs, and in particular due to the average length of the twist between the two copper wires making up each pair, the cross-talk coupling between different pairs increases exponentially with the frequency of the transmitted signal. But this cross-talk coupling is only one of the three factors that determine the strength of cross-talk; the other two are the strength of the disturbing transmitter and the sensitivity of the disturbed receiver at any given frequency. For example, if the transmit frequency band of a transmitter is different than the receive frequency band of a neighboring receiver, then there will be almost no cross-talk from that particular transmitter to that particular receiver.
This strong dependence of cross-talk interference on the overlap of transmit and receive frequency bands is one of the main reasons for the proliferation of DMT (Discrete Multi-Tone) technology in devices used for deployment of residential DSL services, such as ADSL, ADSL2, ADSL2+, VDSL, and VDSL2 services. Almost all DMT devices utilize Frequency Division Multiplexing (FDM), which means that they use different frequency bands for upstream and downstream transmission. Therefore, their upstream transmitters generate very low Near-End Cross-Talk (NEXT) interference to the downstream receivers of neighboring lines, and their downstream transmitters generate very low NEXT interference to the upstream receivers of neighboring lines.
In fact, in the case of FDM DMT services, the only potentially significant cross-talk is Far-End Cross-Talk (FEXT), generated from their upstream or downstream transmitters to the upstream or downstream receivers of neighboring lines. Since FEXT is generated by transmitters that are located at the other end (i.e., the “far end”) of the link, it attenuates as it propagates through the copper wires until it reaches the victim receivers. Therefore, FEXT becomes weaker as the distance between the disturbing transmitter and the victim receiver increases, and thus has a noticeable effect only when that distance is relatively short, typically less than 4-5 Kft (1.2-1.5 km). As a result, in typical deployment scenarios where most loops are longer than 4-5 Kft, FEXT is not a significant source of interference.
Nevertheless, there are two important deployment scenarios where FEXT is the dominant source of interference. The first one is the deployment of very high speed services using VDSL or VDSL2, where the loops are typically shorter than 5 Kft (1.5 km). The second one is the deployment of any FDM DMT service from a remotely-located cabinet, specifically when the cables that go from the cabinet to the customer locations also carry FDM DMT services deployed from the Central Office (CO). In this scenario, the cabinet is located much closer to those customer locations than the CO. Therefore, the downstream transmitters of the cabinet-deployed services are much closer to the downstream receivers of the CO-deployed services than their own transmitters, which are located at the CO much farther up the network. As a result, the downstream FEXT from the cabinet-deployed transmitters is quite strong, as it is attenuated only by a short distance, while the main downstream signal coming from the CO-deployed transmitters is relatively weak, as it attenuated by a much longer distance.
The problem of the strong FEXT generated by cabinet-deployed services to CO-deployed services is often referred to as the “near-far” interference problem, due to the fact that the disturbing cabinet-deployed transmitters are located “near” the victim receivers, while the victim receivers' own transmitters are located “far”, namely at the CO.
The first repeaters used Alternate Mark Inversion (AMI) or High Density Bipolar order 3 (HDB3) line codes to deliver T1 (1.544 Mbps) or E1 (2.048 Mbps) services over longer copper loops. These technologies made very inefficient use of frequency bands, utilizing almost 2 MHz of frequency spectrum to deliver a mere 1.544 or 2.048 Mbps over 2 copper pairs at distances no longer than 1-1.5 km. Later on, symmetric DSL standards such as HDSL (High-speed DSL), HDSL2, HDSL4 and SHDSL (Single-pair High-speed DSL) allowed deployment of the same T1/E1 services over repeatered copper links while making more efficient use of the frequency spectrum and reducing the number of repeater locations and repeatered segments compared with the original AMI/HDB3 methods.
Despite these advancements in repeater technology, the proliferation of residential DSL services resulted in a significant reduction in the deployment of repeaters. The main reason is that repeaters typically generate much stronger cross-talk into residential DSL receivers than non-repeatered services deployed from the Central Office. This is because repeaters are placed much closer to remotely located residential DSL receivers, and therefore they result in the aforementioned near-far interference problem.
Repeaters that utilize single-carrier technologies like HDSL, HDSL2, HDSL4, and SHDSL typically utilize the same frequency band for upstream and downstream transmission, and therefore they create strong FEXT and even stronger NEXT interference to adjacent DSL receivers. As a result, most countries have imposed significant restrictions on the use of repeaters in the outside loop plant. For example, the American National Standards Institute has issued recommendation T1.417, which specifies that repeaters using should only be deployed in North America with bitrate of approximately 768 Kbps per copper pair when used with HDSL4 technology, or approximately 634 Kbps per pair when used with SHDSL technology. This restriction is designed to reduce the frequency band of the disturbing signal to approximately 150 kHz, in order to minimize its overlap with the downstream frequency band of residential DSL services, which typically starts at 140 kHz. Other countries have imposed similar restrictions; for example, several European countries limit the bitrate of repeaters to 1 Mbps per copper pair.
As for FDM DMT repeaters, they have the potential for creating even stronger near-far interference than single-carrier repeaters. This is because most DMT repeaters operate their downstream transmitters in exactly the same higher frequency band used by the downstream receivers of residential DSL services. Since the strength of the cross-talk coupling between different copper pairs increases exponentially with the frequency of the transmitted signal, this means that the FEXT generated by FDM DMT repeaters can cause significant deterioration of the achievable bitrate rates of adjacent CO-deployed FDM DMT receivers. This potential for very strong near-far interference is the main reason why FDM DMT repeaters have not been deployed in any significant numbers thus far.
All these restrictions have severely limited the utility of repeaters for delivery of high-speed data services. For example, consider the problem of wireless backhaul, which involves providing a high-speed communications link between the Central Office and wireless base stations, so that these base stations can effectively provide high-speed data services to wireless subscribers. In the particular case where the required bitrate for the backhaul link is 20 Mbps, and the wireless base station is 10 km away from the Central Office, so that repeaters are required, it would take 26 copper pairs to deliver this service under a restriction of 768 Kbps per pair, and 32 pairs under a restriction of 634 Kbps per pair. Clearly, utilizing that many copper pairs for one 20 Mbps link is impractical and expensive.
Therefore, it would be highly desirable to install and operate repeaters in a way that provides significantly higher bitrates per copper pair and yet does not generate significant levels of spectral interference to residential DSL services.