bile networks were designed primarily to pro- vide outdoor coverage. While many users receive decent coverage
Delivering with DAS M
Tony Lefebvre describes the use of distributed antenna systems to provide a range of in-building services with the performance demanded by today’s users
obile phones are now standard business tools inside corporate of- fi ces. Unfortunately, current mo-
UMTS and HSPA. As a result, each of the sub- scribers in a given area will need 5–10 times as much available data capacity. With LTE reaching inside buildings, there
inside buildings when using 2G services such as GSM or CDMA, the picture changes as we move to 3G (UMTS, HSPA, EV-DO, etc.) and 4G (LTE, WiMAX) services. With 3G and 4G services, strong signals are required to achieve the performance necessary for mobile data and video applications, and capacity de- mands for these services require many more cells than previous network technologies. While 3G users may get coverage in the exterior-facing offi ces of a building, those far- ther inside the building often have poor cov- erage. Distributed antenna systems (DAS) are a means of addressing these issues, delivering indoor coverage and capacity for in-building mobile services.
Service requirements In addition to coverage (getting the signal to the user), another requirement is capacity. T ird-generation services provide a few hun- dred kilobytes per second of downlink data service at best, but with 4G services promising multiple megabytes of downlink data service, the network must support suffi cient capacity to provide every one of potentially hundreds of simultaneous users in a building with a large amount of bandwidth. Published estimates by major carriers sug-
gest that LTE service will be in the range 5–12 Mbit/s on the downlink and 2–5 Mbit/s on the uplink. T is is far greater than the 0·8- 1·2 Mbit/s downlink and 0·2–0·4 Mbit/s on the uplink promised for 3G services such as
Tony Lefebvre is director of product management for TE Connectivity. TE’s Wireless and Services division provides in- novative solutions for wireless, wireline, cable and enterprise networks around the world. the company is a leader in distributed antenna systems (DAS) for in- building and microcellular networks.
will be far denser user populations to be served. LTE represents a leap in the amount of data that must be delivered to each subscriber. It must serve denser populations, and as a result cells will have to be smaller.
Using DAS to create small cells Conventional thinking about delivering in- building services is to deploy a series of small cells (pico or femtocells) throughout the struc- ture. However, this approach presents chal- lenges. T e advantage of using these devices is that
they provide both coverage and capacity, and many of the smaller products are easy to install and require a minimum of space. With each cell you add, you gain more coverage and more capacity. But there are some disadvantages as well: • Each small base station must be managed, so using these as the only solution in a building that could require dozens or hundreds of units will create vast new demands of service pro- vider and user resources.
• Each small cell provides one or two frequency bands, so if a building needs to support service from multiple mobile operators, it may need more than one set of pico or femtocells in each cell
• Each small cell has a fi xed amount of capacity tied to the coverage area of the cell.
• Service providers have a limited amount of fre- quency spectrum at their disposal. Since each small cell in an adjacent area must use a diff er- ent frequency, small cells multiply the chances for interference and make it diffi cult to man- age available spectrum effi ciently. Distributed antenna systems (DAS) address
the problems of small cells. T ey extend the reach of cell base stations by extending base station signals through a series of small remote antennas. A DAS connects to a base station located in
the building’s data centre or telecommunica- tions room.
Inside the building, DAS main hubs connect to a carrier base station or repeater
A DAS approach off ers many signifi cant
benefi ts for mobile operators seeking to im- prove mobile services inside buildings:
• using DAS brings signals closer to users to deliver more uniform, high-quality coverage than base stations alone can provide
• DAS can carry multiple frequencies from a se- ries of connected base stations, serving multi- ple mobile operators with one set of antennas.
• using DAS improves spectral effi ciency be- cause directional antennas can control the area of signal radiation to help eliminate interfer- ence among adjacent sectors and channels.
• the capacity of a DAS can be scaled easily by adding more radio resources to the central location.
A choice of architectures Two basic types of DAS are on the market today: passive and active. Each has specifi c strengths and weaknesses when it comes to pro- viding in-building coverage for 4G networks. Passive systems use thick coaxial cable (2–3
centimetres in diameter) to distribute the wire- less signal. T e main distribution unit is con- nected to the base station and placed adjacent to it, and then the unit drives the signal over the coaxial cable. T e coaxial cable used to distrib-
LAND mobile November 2011
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