Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-04-07
2003-09-30
Maung, Nay (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S522000, C455S453000
Reexamination Certificate
active
06628952
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of increasing capacity in a fixed wireless access communications network. The invention also relates to a computer system for implementing the method, a computer program for controlling the computer system and a communications network deployed using the method.
2. Description of the Prior Art
Fixed wireless access communications networks use a fixed antenna at each subscriber premises where the antenna is typically directional. Each subscriber antenna communicates with a base station (to which it is directed, in the case that directional antennas are used). Each base station is in turn connected to a physical communications network such as a public switched telephone network via a transmission link. A typical base station supports many subscribers, for example 500 to 2000 subscribers in the IONICA (trade mark) system. The resulting fixed wireless access system is capable of delivering a wide range of access services such as POTS (public operator telephone spice), ISDN and broadband data.
When a fixed wireless access telecommunications system is initially deployed, a base station of a particular capacity is installed to cover a particular populated area. The capabilities of the base station are designed to be commensurate with the anticipated coverage and capacity requirement Subscribers' antennas are mounted on a building, for instance, on a chimney, and upon installation are normally directed towards the nearest (or best signal strength) base station or repeater antenna (any future reference to a base station shall be taken to include a repeater).
In order to meet the capacity demand, within an available frequency band allocation, fixed wireless access systems divide a geographic area to be covered into cells. Within each cell is a base station with which the subscribers' stations communicate; the distance between the cells re-using the same frequency being determined such that co-channel interference is maintained at a tolerable level. When the antenna on the subscriber premises is installed, an optimal direction for the antenna is identified using monitoring equipment The antenna is then mounted so that it is positioned towards the optimal direction.
As already mentioned, fixed wireless access systems divide a geographic area to be covered into cells. For initial planning and design purposes these cells may be represented as hexagons, each cell being served by a base station (in the centre of the hexagon) with which a plurality of subscriber stations within the cell (hexagon) communicate. When detailed cell planning is performed the ideal hexagonal arrangement can start to break down due to site constraints or for radio propagation reasons. The number of subscriber stations which can be supported within each cell is limited by the available number of carrier frequencies and the number of channels per frequency.
Base stations are expensive, and require extensive effort in obtaining planning permission for their erection. In some areas, suitable base station sites may not be available. One aim in the design of a fixed wireless access system design is to have as few base stations as possible, whilst supporting as many subscriber stations as possible. This helps to reduce the cost per subscriber in a fixed wireless access system. Another aim is to increase the traffic carrying capacity of base stations whilst at the same time keeping interference levels within acceptable bounds. This is referred to as trying to optimise or increase the carrier to interference level ratio. By increasing the traffic capacity the number of lost or blocked calls is reduced and call quality can be improved. (A lost call is a call attempt that fails.)
Cells are typically grouped in clusters as shown in FIG.
1
. In this example, a cluster of seven cells is shown. Within each cluster 7×6=42 frequencies are each used once. The term “frequency re-use factor” is used to refer to the number of sets of frequencies that the total number of available frequencies is divided into. In this case, the frequencies are divided into 7 sets, one for each cell in a cluster.
FIG. 2
shows how a larger geographical area can be covered by re-using frequencies. In
FIG. 2
each frequency is used twice, once in each cluster. The frequency re-use factor N is 7. Co-channel interference could occur between cells using the same frequencies and needs to be guarded against through cell planning.
When the capacity of a cell or cluster is exhausted one possibility is to sectorize each cell. This involves using directional antennas on the base station rather than omnidirectional antennas. The 380° range around the base station is divided up into a number of sectors and bearers are allocated to each sector. In this way more bearers can be added whilst keeping interference down by only using certain frequencies in certain directions or sectors. The frequency re-use factor is a product of the base re-use factor and the sector re-use factor.
Known approaches for seeking to increase system capacity include fixed frequency planning (FFP) which involves carefully planning reuse patterns and creating sector designs in order to reduce the likelihood of interference. For example,
FIG. 3
shows an example of a fixed frequency plan with a frequency reuse factor N of 4 and which is known as the “mirror method”. Each sector with the same reference numeral is constrained to use a specific set of frequencies that are different from the frequencies used by sectors with different reference numerals. There are four different reference numerals
31
,
32
,
33
,
34
, for each of four different frequency sets. The letters H and V are used to denote horizontal and vertically polarised frequency channels. However, fixed frequency planning is problematic because it is often difficult to map a frequency plan onto an actual communications network. This mapping process is complex, time consuming and adds to costs.
Further, when a communications network is deployed according to a fixed frequency plan, the base station locations and pattern of frequencies used for the communications links needs to adhere to the fixed frequency plan closely, in order for the benefits of the fixed frequency plan to be achieved.
The deployment of base stations in a fixed wireless access network is conducted such that anticipated use within a cell can be accommodated. Nevertheless, in some markets subscriber demand has far exceeded predictions. Further, the increasing use of the intemet has changed subscriber call characteristics considerably since subscribers will be connected to intemet service providers for extended periods of time—even if there is little flow of data. Difficulties arise in the maximisation of capacity whilst at the same time maintaining specified link performance levels for all subscribers. Whilst the frequency plans aim to reduce interference, specified link performance levels are not necessarily maintained for all individual subscribers. As subscribers are added to the fixed frequency planned network, there are knock-on effects on the link performance provided to other subscribers. Accordingly there is a requirement to increase the capacity of fixed wireless access networks.
The installation of excess base station capacity at the start of a network rollout has a serious problem in that it requires an expensive up-front investment in extra network infrastructure by the operator—and this infrastructure will take a considerable time to produce any return on the investment There is also a risk that this extra capacity may never be used, since the prediction of service take up by potential subscribers is not an exact science, and market conditions can change unpredictably. This approach amounts to a serious financial risk, that few operators wish to accept.
The capacity of the network can be increased by fitting extra equipment at the base stations. This technique has its limits, however, determined by the fundamental design of the equipment c
Chrystie Peter
Hankins Timothy R
Swindell Rachel
Thomas Mark K
Barnes & Thornburg
Gesesse Tilahun
Nortel Networks Limited
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