Getting the best out of DAS

Enhanced mobile infrastructure is often a requirement for large venues and outdoor areas. With large amounts of people all in one place using their mobile devices to send photos, access the internet or transmit video simultaneously, it becomes difficult for mobile operators to provide enough network capacity to facilitate these high demands.

Putting up one large antenna will not deliver enough performance in these situations. Rather, the coverage and capacity must be focused to each seating section. This means that the wireless solution designs needs to be heavily sectorised to deliver defined capacity to each group of seats and other service areas. Another requirement is that the design should minimise soft hand-off from signals bleeding from one sector into another. As a result, the solution must be designed to accurately target the signal so as not to create overlap between sectors, or create isolation. Venue operators must also decide whether the coverage and capacity enhancement is intended to be permanent (as in a sports stadium) or temporary.

With this in mind, operators are increasingly using DAS to divide facilities into sectors using discreet, remote antennas. However, like any investment, they are constantly looking at ways to get more efficiency out of capacity for DAS while conserving costs, to ensure users in areas of high density are able to take full advantage of their mobile devices.

There are three key trends at work that can lead to lower costs and higher service quality in mobile networks; base station hotels, fibre conservation technologies, and Coordinated MultiPoint (CoMP) technology that improves service at the edges of the network. Whether used separately or together, these technologies will enable mobile operators to improve mobile services at large venues, to reduce churn, while maximising the use of their resources and keeping costs under control.

Three ways

With today’s fibre-fed DAS, mobile operators can locate base stations and DAS head-ends miles away from the stadium or arena where the DAS is deployed. By combining base stations into a central ‘hotel,’ operators can avoid having to find the space required at the venues themselves. In addition, they can share the base stations’ capacity among three different facilities, and reduce backhaul costs by backhauling traffic from one location instead of many. This model makes efficient use of base station resources while putting all of the base stations in one location – instead of three – for easier access and maintenance.

The trend toward base station hotels feeding multiple large venues points to the need for lots of fibre to connect the base station hotel to a DAS. Typically, each DAS head end requires one to three fibre pairs, but there can be a dozen or more head ends in a base station hotel. Finding the fibre to transport the DAS traffic between the base station hotel and the DAS-serviced venues can be problematic.

One solution to this is a new type of ’muxponder’, which is a multiplexer and transponder in one unit that will take in three 3.072Gb/s feeds and multiplex them into a 9.8304Gb/s transport over a single fibre pair. By combining three fibre pairs into one, the muxponder saves two-thirds of the fibre needed coming out of a base station hotel. These solutions are also perfect for neutral host architectures where it is necessary to transport full band, multi band RF to a designated service area, such as a stadium or an urban core, where there is high sectorisation and capacity strain on the network.

Base station hotels are becoming a popular strategy for mobile operators looking to get more efficiency out of base station capacity, while using DAS to serve large venues or urban cores. With muxponders, operators can extend these efficiencies out into the fibre network by slashing the amount of fibre needed to transport DAS traffic.

CoMP technology addresses service deficiencies for mobile devices at the edges of cells. When a mobile device is at the edge of the cell, the data rate drops off and the device starts looking to hand off its connection to the next cell. The signal levels are constantly changing and service is poor in these areas. The idea of CoMP is to get two or more cell sites to cooperate. One CoMP scenario is that the network can send the data to both base stations and, on a real-time basis, the base stations monitor the signal quality and can actually decide which of the base stations is in the best position to get that signal to the mobile device. In another form of CoMP, only one base station is transmitting but the other base stations in the area are aware of which time slots and frequency bands are being used to communicate, and they cooperate by saying ‘we won’t use that same block in our cell so we won’t interfere with you’.

Cooperation

So, under CoMP, all of the base stations in an area identify which mobile devices are in the lowest signal areas and figure out how to cooperate to get the best throughput to that mobile either by reducing interference or by sharing the responsibility of transmitting. CoMP is included in the LTE portion of the 3GPP specifications beginning with Release 11. It is an optional enhancement to the LTE-A air interface technology that was introduced in 3GPP Release 10.

One of the main challenges with CoMP is backhaul. In order to coordinate their operations, cell sites must be connected to each other as well as to the core network, so this means that mobile operators will have to install a lot of expensive new backhaul equipment between cell sites. However, by pooling base stations in a hotel, the backhaul requirements are easily and cost-effectively managed.

With base stations housed in a central hotel, the base stations can coordinate their efforts easily and then transmit signals through a DAS to the desired area. In fact, mobile operators have already determined that the only way to make CoMP work in the real world is to pool base stations in a hotel and use remote radio heads or DAS to distribute their signals. With CoMP technology, cell sites can become smart and dynamic, improving service to devices at the edge of a cell and creating happier customers.

Ultimately, as the demand for mobile network services anytime, anywhere increases, mobile operators are under huge amounts of pressure to find ways to deliver better wireless coverage, particularly in densely populated venues or events. They want to improve services to reduce churn, but they also want to maximise the use of their resources and keep costs under control. Base station hotels, fibre conservation technologies and CoMP work separately or together to help operators achieve these goals and essentially make the most out of DAS.