Patent Application
20080039141
Embodiments of the present invention will now be described by way of example and with reference to the drawings, in which:
When considering a known system, the inventors realised that it should be possible to identify a need for a change in scrambling code, then select a new scrambling code and implement its use, in an automated procedure without requiring a human operator to effect the scrambling code change.
A first example will now be described in detail.
An example network is a Universal Mobile Telecommunications System (UMTS) terrestrial access network (UTRAN), which is a type of wideband code division multiple access (CDMA) network for mobile telecommunications. The UTRAN network is basically as shown in
The network 2 is a self-deploying network. A self-deploying network is one that learns about its current performance, for example in terms of its radio coverage and traffic capacity, and in consequence decides on changes in base station position, and also changes in transmission power level, such as of pilot signals, and then implements such changes without human intervention.
An example of the self-deployment process is shown in
Initially, the user terminals 12 are connected to the base station that provides a pilot signal received with the strongest power. At each time step (a), (b) and (c), the optimal positions of the base stations are calculated based on the distribution of current connections to mobile terminals. These optimal positions are shown in
The optimal position of a base station at any time depends on a variety of factors, such as best use of radio resources, costs, practical limits on suitable locations, legislation and public policy. As regards radio resources, transmission power and available frequency spectrum are important factors within a constraint as to maximum permissible transmission power.
In the UMTS network 2, each base station is associated with a scrambling code, sometimes referred to as a primary scrambling code. Accordingly, the scrambling code acts as a base station identifier. Each scrambling code is a complex sequence of 38400 chips, where a chip is a 1/r part of a spread symbol spread with a spreading code of length r. Each base station sends a standard pilot signal scrambled using its own scrambling code every 10 milliseconds, that being the duration of a UMTS frame. Such scrambled plot signals are sent on the Common Pilot Channel, CPICH.
A mobile terminal receives such a scrambled pilot signal and deduces, by symbol-by-symbol correlation with possible codes, which scrambling code was used so as to identify the base station from which the signal was sent. In UMTS networks, a total of 512 scrambling codes are defined.
To minimise interference of one base station on another, the scrambling code used by a base station should be unique within the coverage area of the base station including areas that overlap with the coverage areas of other base stations. However, this does not always occur. Therefore a need to assign another scrambling code can arise.
The scrambling code monitoring and assignment process used in the UMTS network consists of three main stages: identification of a need to change scrambling code, selection of a new scrambling code, and performing scrambling code change. These are considered in turn below. Correspondingly, as shown in
In this example system, base stations hold neighbour lists 11, that is, lists of the identities of neighbouring base stations and their currently associated scrambling codes. However, these neighbour lists cannot be assumed to be complete and reliable.
A pilot signal of an interfering base station that has been scrambled using the same scrambling code as a first base station, would merely appear as an echo of the first base station's own scrambled pilot signal. Also mobile terminals cannot distinguish between base stations that use the same scrambling code. However, a readily identifiable impact of more than one base station using the same scrambling code in an area of overlapping coverage is an increased amount of interference experienced by those base stations. In the network 2, it is an increase in interference that is used to identify a need to assign replacement scrambling codes.
As shown in
In an alternative embodiment (not shown), the radio network controller 8 can collate received signal data so as to calculate signal to interference ratio for each base station that it controls.
As a next step, a determination is made, (step h) as to whether or not the signal to interference ratio, either uplink or downlink, falls below an acceptable level. The level at which the ratio becomes considered as unacceptable being, for example, when the ratio goes below, say, 50% of its normal value. If the ratio is acceptable, a return (step i) to a fresh measurement of signal to interference ratio is made. Otherwise, assignment of another scrambling code for the base station is required (step j).
In this example network 2, additional information, known as neighbour lists, identifying neighbouring base stations and their currently used scrambling codes are stored in each base station 4. The base station requiring a new scrambling code excludes from the normal full set of 512 available codes those codes that are known to be used by neighbouring base stations, then selects randomly from among the codes that remain (step k).
The base station then applies the selected replacement scrambling code (step l) as explained more fully below.
As shown in
The base station is then reprogrammed to use the new scrambling code and rebooted with the new scrambling code being used thereafter. The power of the pilot signals from the base station is then increased until the cell size becomes as before the scrambling code change operation.
An identifier of the new scrambling code is then transmitted to neighbouring base stations, so that they can each update their own neighbour lists, so as to have available the information that that scrambling code is currently used by one of its own neighbours.
After the change of scrambling code, the base station performs further measurements (note return step m in
To identify a need to change scrambling code, another parameter that can be measured in place of signal to interference ratio is number of false handovers from a base station per unit time. A false handover is where a mobile terminal attempts to connect to a base station which it identifies, based on scrambling code, as being the correct base station but fails in that connection as the mobile terminal lacks the necessary authentication key-codes because it is not, in fact, the correct base station.
Some systems have base stations that hold complete and reliable neighbour lists. Then, a base station can select a scrambling code that does not conflict with others simply by searching its neighbour list to exclude scrambling codes that there is use by its neighbours. If any neighbour is found using the same scrambling code, then the base station selects a new scrambling code and performs a scrambling code change operation so as to use the new scrambling code.
In some systems, where a relatively large number of scrambling codes are usable, a new code can be selected randomly from the whole group of codes without consideration of which might be already in use. The selected code is then changed to. If an improvement in base station performance results, for example in terms of signal to interference ratio, then use of the selected code is continued. Alternatively, if insufficient improvement is noted, a further scrambling code is randomly selected, from among the complete set, and tried. This approach is possible in some UMTS systems because UMTS systems have up to 512 different scrambling codes available.
In some systems, base stations are able to select between multiple frequencies and/or access technology standards, for example IEEE 802.11 (which relates to wireless local area networks), IEEE 802.16 (which relates to wireless metropolitan area networks), Global System of Mobiles (GSM), and UMTS. Such a base station has multiple radio interfaces, each using different one frequency band and/or access technology. They can transfer connections with mobile users from one radio interface to another so as to free up a radio interface without dropping calls. A new scrambling code is assigned to the freed-up radio interface, which is then re-booted so as to use the new scrambling code. Call connections are then handed over to the re-booted radio interface. A simple example of this process is shown in
As shown in
As shown in
In some systems, a base station can change to using a new scrambling code without re-booting. As shown in
The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
