GB2361842A

GB2361842A - Skewed frequency hopping

Info

Publication number: GB2361842A
Application number: GB0010304A
Authority: GB
Inventors: Behzad Mohebbi; Robert Ivor Davies; Luis Lopes
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-04-28
Filing date: 2000-04-28
Publication date: 2001-10-31
Also published as: GB0010304D0

Abstract

Time slots in a cell carrier frequency utilise different frequency hopping plans. Frequencies allocated to the frequency hopping plans which experience a lesser degree of interference with surrounding frequency hopping plans are hopped onto more frequently than those with a greater level of interference. Additionally the time slot in which a call is being processed may regularly be changed to an alternative slot.

Description

2361842 CE30301P 1 SKEWED FREQUENCY HOPPING This invention relates to

frequency plans for use in cellular communications systems, and more specifically to frequency hopping in such frequency plans.

In a cellular communications system, the area over which service is provided is divided into a number of smaller areas called cells. Each cell is served from a base transceiver station (BTS) which has a corresponding antenna or antennas for transmission to and reception from a user station, normally a mobile station (MS) or handset.

A cellular communications system is operated according to a frequency plan. A frequency plan consists of a plurality of frequency plan settings applied to the various components of the system. Such settings typically include a specific radio frequency or set of frequencies assigned to each respective BTS, which frequency or frequencies thus apply to the corresponding cell. Other frequency plan settings forming part of the frequency plan are typically a cell colour code which a given cell will transmit. One example of a cellular communications system is a global system for mobile communications (GSM) system. In a GSM system, the cell colour code is contained within the base transceiver station identity code (BSIC).

The overall specification of the above-mentioned frequency plan settings constitutes a frequency plan of a cellular communications system. However, frequency hopping can increase the overall efficiency of a frequency plan in GSM systems.

In a fixed frequency plan, any one call uses the same frequency allocated to it by the cell in which the MS is located, unless it is changed to a different cell. When frequency hopping is utilised, a call moves through a set of frequencies allocated to that cell. This is achieved by both the mobile station and BTS moving through the allocated frequencies in synchronisation. The reason that frequency hopping is utilised is because some of the frequencies allocated to a cell may have interference associated with them, either with neighbouring cell frequencies or CE30301P 2 otherwise, and some will be clean frequencies. The use of frequency hopping allows both clean and interfering frequencies to be hopped onto and off again, thus spreading the use of the interfering frequencies (and thus the interference experienced by MS's) around the network. Typically, a hop occurs 250 times per second, i.e. the frequency being utilised to make a call changes 250 times every second.

Frequency hopping schemes can be used in wireless communication networks such as mobile, indoor or wireless local loops to overcome interference and noise limitations of a fixed air interface, as discussed above. Whilst the noise performance of a frequency hopping feature depends upon the channel characteristics and spectrum allocation of the system to which it is applied, the interference performance depends upon the correct frequency allocation of the system users, i.e. the MS's.

Additionally, there exist within areas of system coverage specific areas where certain frequencies "fade". Frequency hopping allows a better frequency to be used where a fade occurs, thus improving the overall quality of the system from the perspective of an MS.

It should be appreciated that any system can only tolerate a set level of interference, if it is to retain a required level of performance. As an example, a GSM system requires that the carrierlinterference ratio (CII) be no less than 9d13 for Additive White Gaussian Noise (AWGN). Additionally, the overall gain of a system utilising frequency hopping results from the deliberate introduction of controlled interference. In view of these two factors, it is important that the frequency plan for a frequency hopping network ensures that the requirements for the C/I ratio are met. Furthemore, the frequency plan must ensure that the desired bit error rate (BER) and frame erasure rate (FER) for the required grade of service are met.

The management of interference required to ensure that the above conditions are met requires the fine tuning of probabilities, i.e. the probability of interference with CE30301P 3 other elements of the system controlled by the frequency plan. Therefore, at the present time, the control resolution of a frequency hopping system is limited by the number of individual frequencies included in the system for hopping purposes.

Frequency hopping systems used currently utilise each frequency allocated to the hopping list equally, irrespective of the interference level associated with any one particular frequency in the list. Also, because each frequency used in a hopping list, which is associated with one cell for example, may be utilised in other hopping lists in surrounding cells, the interference profile of each frequency will be different.

This depends upon which frequencies are utilised in other such hopping lists, and where within the system this occurs, i.e. whether such an occurrence is geographically close to the cell in consideration or not. Furthermore, the interference environment experienced by a particular user/mobile station, is dependent upon the rhobile station location with respect to both the serving base transceiver station and surrounding base transceiver stations and also with respect to other mobile stations. Thus, the interference experienced by a specific mobile station varies with both location and time. Hence, the ideal set of hopping frequencies for a particular mobile station varies with time and location. As such, in a GSM system, the inclusion of a given frequency could degrade the BERIFER beyond acceptable levels, whilst the exclusion of such a frequency may place an excessive burden on the remaining frequencies.

In view of the foregoing, it is clear that a number of problems exist in the area of frequency hopping. In particular, the problem exists of how to achieve the required standard of service by utilising the available frequencies for hopping, whilst maximising the number of users able to be accommodated by the system.

The present invention aims to address some or all of the above problems.

According to the present invention there is provided, as claimed in the appendent claims, a method of skewing a frequency hopping scheme in a cellular communications system, and a cellular communications system operating a CE30301P 4 skewed frequency hopping plan, wherein different hopping plans are utilised by time slots in a cell carrier frequency.

This invention allows frequency hopping plans operating within an overall system 5 frequency plan to be skewed in order that better frequencies in the hopping plan, i.e. those subject to less interference or those less probable to experience interference, may be hopped to more often than those frequencies with more interference characteristics. As such, this invention addresses the disadvantages detailed above by enabling the skewing of a frequency plan towards preferred frequencies and increasing system capacity and/or call quality. Additionally, the method is provided for, and supported by, the ETSI standard.

According to a preferred embodiment of the present invention, the average system quality provided to a user is improved by changing the time slot in which a call is being processed regularly, to an alternative slot which utilises a different set of hopping frequencies. As such, the system performance is improved from the perspective of both the user and the operator.

In another aspect of the present invention there is provided a method of skewing a 20 frequency hopping scheme in a cellular communications system, wherein multiple hopping plans are utilised by individual time slots within a cell carrier frequency.

Additional specific advantages of the present invention are apparent from the following description and figures in which:

Figure 1 is an illustration of a prior art frequency hopping radio frequency plan;

Figure 2 is an illustration of one method of skewing a frequency hopping scheme; and Figure 3 is an illustration of the frequency allocation of each time slot in a carder frequency as dictated by the present invention.

The present invention is now described with reference to the accompanying drawings as detailed above.

CE30301P 5 As may be seen in Figure 1, which depicts a typical frequency hopping plan 102, there are two exemplary overlapping systems (or cells) A 104 and B 106. Each of these systems utilises a separate hopping list 108,110. However, it is apparent that the hopping list 108 of system A, and the hopping list 110 of system B, both utilise frequencies flo and f2o.

In general there are two types of frequency hopping that are used. Cyclic hopping and pseudo-random hopping. The above example and the present invention utilise the second of these, although both are supported. Accordingly, pseudorandom hopping will be briefly discussed.

Pseudo-random frequency hopping reduces the interference in a system because, when it is employed, the frequencies of neighbouring cells overlap much more rarely and it is not necessary to time synchronise the hopping systems in neighbouring base transceiver stations. The type of hopping used dictates the order in which the frequencies in a hopping list are hopped to. In the pseudorandom system, the order is pseudo-random as would be expected. The order is determined as a function of the mobile allocation (MA) i.e. the particular mobile station from the group currently under the scope of the hopping list, and the hopping sequence number (HSN). The HSN is an arbitrary number within a system dependent range, the value of which determines the order of the frequencies in the hopping list. As such, the order is given by:

f (AM, HSN) As an aside, every HSN results in a different frequency hopping order. An HSN of 0 results in the cyclic system. However, the above function always results in a uniform balance of the frequencies used. Finally, no two mobile stations under the same hopping list will be hopped on to the same frequency at the same time. This is prevented by the use of MA in determining the hopping order.

CE30301P 6 Referring back to Figure 1, it is clear that each frequency in each hopping list will be used one third of the time on average, giving a probability of occupancy Of 113. Similarly the probability of hitting the same frequency in both systems is given by:

(p(f)[f10,f20))2 x no. of ftequencies appearing (Y3)2 x 2 = Y9 (in both cells (i.e. f,, and f,,) = i.e. the probability of hitting f or f2 is Y9 This conventional frequency hopping plan utilises each frequency in a frequency hopping list for the same amount of time, irrespective of the relative interference levels associated with each frequency, as was mentioned above. This does not provide the least amount of interference across the system, a desirable characteristic. In order to overcome the problems already highlighted, it would be preferable if the better frequencies in the hopping list, i.e. those with less associated interference, were hopped to more frequently than the others. This would result in the controlled inclusion of less ideal frequencies in the hopping sequence. Such a result would be systematic in overcoming the problems detailed above, wherein the exclusion of a frequency from a hopping list, or its equal use with the other frequencies therein would degrade the system parameters beyond acceptable levels.

As such, the use of frequencies within a hopping list must be biased/skewed towards the better frequencies such that the required BERIFER may be achieved. In doing this, a fine resolution for interference control is also achieved.

As is described with reference to Figure 2, skewed hopping can be performed by the repeated inclusion of a preferred frequency in a hopping list. This obviously increases the probability of occupancy of that frequency. As may be seen in the frequency plan 202 of Figure 2, there are again two systems, A 204 and B 206, and each has a frequency hopping list 208,210. In each hopping list, the frequency not shared with the other system, i.e. fl and f2, is inserted twice so as to be hopped on to 50% (one half) of the time. Similarly, the amount of occupancy of the shared frequencies drops to 25% (one quarter) per frequency. Thus, the CE30301P 7 probability of hit for the shared frequencies is reduced to one eighth. This is a reduction of 44%. The reduction is depicted as follows:

2 no. of frequencies appearing) 4 (p(f)[f]O,f20]) X in both cells (i.e. f10 and f20) 4)2 x 2ys The reduction of 44% occurs without removing the shared frequencies from the hopping list and this results in a reduction in interference caused by the shared frequencies in each of the two lists 208,210. Therefore, the problems detailed above may be addressed in this way.

The GSM standard has not made any provision for skewed frequency hopping. Instead it utilises one of several different methods of specifying the frequency list. Details of these methods are, however, unnecessary for the appreciation of the present invention.

As already stated, the GSM (or ETSI GSM) standard does not allow repetition of frequency allocation in a frequency hopping list. However, the system does not preclude the use of different frequency systems for different time slots in a given carrier or on different carriers. Each carrier frequency allocated to a cell has eight time slots contained therein. Every time slot may be used to support a different MS utilising the system resources in that particular cell area. As such, the standard allows for each time slot in a carrier frequency to have different or similar frequency allocations. The list of frequency allocations within the entire system can be adjusted by the operation and maintenance center (OMC) and, as such, can be predetermined by the network user. In view of these provisions, it is allowable to provide a different hopping list for each time slot within a carrier, or to provide multiple hopping lists for a time slot. In addition, these hopping lists may be adjusted automatically or manually at different times, or they may be varied, i.e. switched on and off, depending on the traffic conditions (e.g. loading, measured interference environment etc.) experienced by the system at any one time.

CE30301P 8 As is now detailed with reference to Figure 3, the present invention utilises the capabilities of the GSM standard to perform skewed frequency hopping. In effect, the cell is skewed rather than the individual users relying upon that cell, which is the result of the description relating to Figure 2 (the latter is not described in the standard). Figure 3 shows the frequencies assigned to each time slot in the two cell system of Figures 1 and 2. As may be seen, adjacent time slots are allocated different frequency lists. In addition, neither cell uses the same shared frequency in the same time slot as does the other. This allocation of different frequency systems to the eight time slots of any given carrier frequency enables more emphasis to be placed upon a single frequency at the system level, in this example. In reality, a greater emphasis could be placed upon a number of frequencies at the system level.

In the example of Figure 3, for cell 1 the unshared frequency f, is allocated to 15 every time slot whilst shared frequency flo is only assigned to half of the time slots and shared frequency f2o is assigned to the other half. As such, the unshared frequency f, will be hopped onto 50% of the time and each shared frequency will be hopped onto 25% of the time. Therefore, the improvement of Figure 2 has been achieved within the provisions of the current GSM standard. This is a way of using a given frequency with a high probability of occupancy at a system level. It is also possible to ensure, by random/variable/adaptive (as will be described later) allocation of frequency systems to time slots during the call set up procedure, that a regular pattern between adjacent cells'time slot structures is avoided.

In another aspect of this invention, the occupancy/interference pattern of each cell may be brought into conformity with that which would be prevalent if the system of skewing detailed with reference to Figure 2 were provided for by the standard, by using a technique named "slot hopping". The aspect described above, of skewing the frequency hopping scheme at a system level, has the advantages of addressing problems in the relevant area of technology and enabling an increase in system capacity over the currently used scheme. Slot hopping passes benefits on to the system user, i.e. individual MS's, by providing an average quality for MS's over the whole system.

CE30301P 9 The method of slot hopping operates as follows. Calls from MS's being serviced by a particular carrier frequency within a particular cell are regularly shuffled using a random algorithm. Specifically, calls are regularly transferred from one time slot in the carrier frequency to another time slot therein. The second time slot to which transfer is made is determined by a random procedure, such as the standard frequency hopping algorithm used in GSM systems. Slot hopping is carried out, for example, every two seconds. However, the interval during hops will be determined based upon system specific criteria, such as interference experienced and additional signalling load. The frequency allocation for each time slot is kept constant during slot hopping. As a result of this procedure, a more complete average of interfering conditions, channel occupancy and other such system characteristics, is achieved for each call. Thus, slot hopping benefits the individual MS making a call on the overall system.

The final characteristic of the present invention is referred to as "adaptive allocation of frequency". As mentioned previously, frequency hopping lists/schemes may be allocated to time slots within a carrier frequency either randomly, variably, or adaptively. Adaptive frequency allocation consists of monitoring the quality of reception of each frequency deployed for use by a BTS. The frequency skewness of that BTS may slowly be changed in order to optimise the performance of the system. The alteration of the frequency scheme in response to the measurements taken regarding reception may be made automatically or may be initiated manually. However, the first option is preferable.

With regard to the quality of reception assessment, this may be achieved by monitoring and accumulating the number of errors received during a training sequence, and adjusting the frequency systems accordingly. As such, local frequency use will depend upon traffic and propagation conditions.

This invention has been described paying specific attention to a two cell scenario in which only two of the three frequencies in either hopping plan experience any interference. However, it will be appreciated that this method is applicable to CE30301P 10 systems containing a great deal more cells, and to hopping plans with far more frequencies contained therein. This invention also applies to hopping plans where all the frequencies used experience a level of interference.

It will of course be understood that the present invention has been described above by way of example only, and that modifications of detail can be made within the scope of the invention.

CE30301P

Claims

1. A method of skewing a frequency hopping scheme in a cellular communications system, wherein different hopping plans are utilised by time slots 5 in a cell carrier.

2. A method of skewing a frequency hopping scheme in a cellular communications system, wherein multiple hopping plans are utilised by individual time slots within a cell carrier.

3. A method according to either of claims 1 or 2, comprising the steps of:

allocating hopping plans to each time slot, wherein the frequencies, from a set of frequencies available for hopping, with more associated interference are allocated to time slot hopping plans less frequently than those frequencies with 15 less associated interference.

4. A method according to claim 3, wherein the frequencies allocated to a time slot hopping plan comprise a subset of the set of frequencies available for hopping within a cell.

5. A method according to either of claims 3 or 4, further. comprising the step of varying time slot hopping plans in accordance with the traffic loading and interference of the cell.

6. A method according to any preceding claim, wherein the hopping plans are altered in response to the quality of reception of each frequency used in a cell.

7. A method according to any preceding claim, further comprising the step of changing the time slot in which a call is being processed, regularly, to an 30 alternative time slot.

8. A method according to claim 7, wherein the change is made about every few seconds.

CE30301P 12

9. A method according to either of claims 7 or 8, wherein the next slot to process the subject call is determined randomly.

10. A cellular communications system operable under a frequency plan that utilises frequency hopping, wherein each time slot of a carrier allocated to a cell within the system has a different hopping plan assigned thereto.

11. A method of skewing a frequency hopping scheme in a cellular communications system, substantially as hereinbefore described wkh reference to the attached figures.

12. A cellular communications system operable under a frequency plan that utilises frequency hopping, substantially as hereinbefore described with reference to the attached figures.