GB2308276A

GB2308276A - Cellular communication system and re-use pattern therefor

Info

Publication number: GB2308276A
Application number: GB9525271A
Authority: GB
Inventors: Jun Xiang
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-12-11
Filing date: 1995-12-11
Publication date: 1997-06-18
Anticipated expiration: 2015-12-11
Also published as: GB2308276B; GB9525271D0; ZA9610386B

Description

CELLULAR COMMUNICATION SYSTEM AND RE-USE PATTERN THEREFOR Background of the Invention This invention relates, in general, to a cellular communication system and is particularly applicable to a re-use pattern for such a cellular communication system.

Summarv of the Pnor Art As a consequence of the limited availability of frequency bandwidth for cellular communication systems generally, such as the pan-European Global System for Mobile (GSM) cellular communication, designers must employ frequency re-use techniques to optimise and increase cellular system capacity. More explicitly, a frequency bandwidth that is assigned to the communication system is divided into many channels (of equal bandwidth) that are themselves attributed to frequency groups. These frequency groups are then individually allocated to sectors that form a site (or cell), with the deployment of one set of frequency groups across many sites defining a cluster of sites within the communication system.As such, cell planning represents the distribution of frequency groups between a number of sectors in a cluster, while a repeat (or re-use) pattern for the system is indicated by the relationship between the number of sites that are covered by an integer number of sets of frequency groups. For example, a repeat pattern of 2 would be achieved from the deployment of two complete sets of frequency groups to cover a cluster containing 4 sites (with each site typically containing either three (3) or six (6) sectors). At present, however, many systems (including GSM and Digital Communication System (DCS) 1800) utilise a four-site repeat pattern.

As will be appreciated, allocation of the frequency groups to sectors may be on either a permanent or initial basis (with the latter instance providing a so-called frequency "hopped" system, such as envisaged in Code Division Multiple Access (CDMA)).

To date, repeat patterns offering a 2-site re-use have proved difficult to implement because of co-channel interference and, in particular, considerable adjacent channel interference (or splatter) prevalent in current repeat patterns. In these respects and as will be understood, co-channel interference occurs when different sectors use the same frequency groups, whereas adjacent channel interference occurs as a result of the adjacent location of contiguous frequency bands (channels).

Clearly, in a two-site repeat pattern, adjacent channel interference becomes an increasing problem.

In an attempt to mitigate against the effects of adjacent channel interference in a 2-site repeat pattern, frequency hopping schemes offer a potential solution to the problem, although such schemes are undesirable since they are relatively complex and commercially expensive (because of the necessity for sophisticated hand-over algorithms and the increased complexity of system infrastructure).

Summarv of the Invention According to a first aspect of the present invention there is provided a cellular communication system having a frequency bandwidth arranged into a plurality of frequency channels, the cellular communication system comprising neighbouring first and second sites each having sectors containing at least one frequency channel, wherein corresponding sectors in each of the neighbouring first and second sites have consecutive frequency channels from the frequency bandwidth, thereby producing a two-site re-use pattern.

In a preferred embodiment, neighbouring sectors within both the first and second sites observe a next but one frequency channel relationship with adjacent frequency channels in at least one neighbouring sector.

Indeed, by providing clusters having corner illuminated, six-sectored sites, the preferred embodiment of the present invention provides a two-site repeat pattern that avoids adjacent channels and hence adjacent channel splatter.

In a second aspect of the present invention there is provided a cellular communication system having a frequency bandwidth arranged into a plurality of frequency channels that are distributed on a consecutive frequency channel basis amongst a plurality of frequency groups, the cellular communication system comprising a cluster having neighbouring first and second six-sectored sites, each sector of each site containing a frequency group having at least one frequency channel and wherein: i) the first six-sectored site comprises a first frequency channel and at least five other frequency channels each having an integer multiple next but one frequency channel relationship to the first frequency channel, wherein neighbouring sectors in the first six-sectored site each contain a frequency group having respective frequency channels that observe a next but one frequency channel relationship with frequency channels in at least one neighbouring frequency group; ii) the second six-sectored site comprises a second frequency channel, consecutive in frequency to the first frequency channel, and at least five other frequency channels each having an integer multiple next but one frequency channel relationship to the second frequency channel, wherein neighbouring sectors in the second six-sectored site each contain a frequency group having respective frequency channels that observe a next but one frequency channel relationship with at least one neighbouring frequency group; and iii) consecutive frequency channels of the frequency bandwidth are assigned to corresponding sectors in each of the neighbouring first and second six-sectored sites to produce a two-site re-use pattern for the cluster in which consecutive frequency channels are alternated between the first and second six-sectored sites.

Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the DrawmEs FIG. 1 shows a prior art cellular communication system.

FIG. 2 shows a sectorised site typically utilised in the prior art cellular communication system of FIG. 1.

FIG. 3 shows a typical allocation of frequency bandwidth for the prior art cellular communication system of FIG. 1 FIG. 4 illustrates a 2-site repeat pattern according to a preferred embodiment of the present invention.

FIG. 5 illustrates a 2-site repeat pattern according to an alternate embodiment of the present invention.

Detailed Descnptlon of Preferred Embodiments FIG. 1 shows a typical prior art cellular communications system 10 in which a coverage area is defined by a number of sites (or cells) 12-22 represented in conventional hexagonal fashion. Each site 12-22 has a base station 24-34 responsible for controlling communication traffic in the respective site. Typically, the base stations 24-34 will be centrally located, although other positions may be desirable subject to surrounding terrain or propagation conditions. As will be understood, each base station 24-34 may receive 38 and/or transmit 40 signals from/to mobile communication devices 42-46 that roam throughout the communication system 10.

Furthermore, each base station (BS) 24-34 is responsive to an operations and maintenance centre (OMC) 49 arranged to have overall system control, which OMC 49 may be either on a regional or system basis (dependent upon the size of the communications system 10).

Each site 12-22 of the cellular communication system 10 of FIG. 1, is typically partitioned into six operational sectors 50-60 (as shown in FIG. 2), with each sector 50-60 serviced by a radio channel unit (RCU) 62-72 and associated transmit and/or receive antennas 74-84, respectively.

As previously explained, a cellular communication system has a limited frequency bandwidth 90 (shown in FIG. 3) that is divided into many contiguous frequency channels 91-97 (having equal portions of the available frequency bandwidth). In this respect, coincidence may dictate that the number of frequency channels for the system corresponds to the number of frequency groups, whereby each frequency group contains a solitary channel, however this is seldom the case.Therefore, channels are usually allocated (from a consecutive series of channels) to frequency groups on an incrementing and rotational basis, whereby a first channel 91 is assigned to a first frequency group, a second channel 92 is assigned to a second frequency group,... an nth channel 95 is assigned to a final frequency group, an (nth + 1) channel 96 is again assigned to the first frequency group, an (nth + 2) channel 97 is again assigned to the second frequency group, and so on (in a cyclic fashion) until all available channels for the frequency bandwidth have been assigned to frequency groups. As such, frequency groups need not contain equal numbers of channels.

From FIG. 3, the cause of adjacent channel interference (between sectors of cellular communication systems) is represented by boundaries (e.g. 98 and 99) between contiguous frequency channels.

A typical channel assignment protocol is illustrated in TABLE 1 immediately below: TABLE 1

Frequency Group a1 bi ci dl e1 11 Channel Number 1,13 2,14 3,15 4,16 5,17 6,18 Frequency Group | a2 | b2 | c2 I d2 | e2 a f2 Channel Number 7,19 8,20 9,21 10,22 11,23 12,24 It will be noted that TABLE 1 assumes that the frequency bandwidth 90 is sufficient to support 24 channels (carriers), such as Broadcast Control Channels (BCCH).

Now, turning to FIG. 4, a preferred embodiment of the present invention produces a two-site repeat (re-use) pattern that optimally balances the co-channel and adjacent channel interference levels. As can be seen, a coverage area 120 is defined by a number of conventionally-represented hexagonal sites (or cells) each having six sectors. Referring to adjacent sites 130 and 132 (which have been outlined in bold to facilitate recognition and which together form a cluster 134 that is repeated to produce a mosaic for the coverage area 120), the total combined number of sectors (namely, twelve sectors) in these two sites each receive a unique frequency group.

As such, each cluster requires twelve distinct frequency groups, with the communication system therefore requiring at least twelve distinct (contiguous) channels. Furthermore, in the preferred embodiment, channels are assigned to frequency groups in accordance with the assignment protocol tabulated above. However, it will be appreciated that there need only be a number of frequency channels corresponding to the number of sectors in the cluster 134, i.e. a minimum of twelve frequency channels are required for a cluster having two six-sectored sites. For the sake of illustration, another cluster 136 has been identified and specifically outlined in the coverage area 120 of FIG. 4.

In the preferred embodiment, each site (e.g. site 130) contains a base station and associated infrastructure (not illustrated for the sake of clarity) that is similar to that described in relation to FIGs 1 and 2, and as will be understood. However, unlike the side-illumination sector coverage provided by each individual RCU of FIG. 2, each RCU for each sector in FIG. 4 is arranged to provide a corner-illumination of its respective sector, whereby each side of the hexagonally represented site is partitioned between frequency groups.

With regard to the arrangement of frequency groups (al-fl and a2-f2) containing consecutive (and perhaps contiguous) frequency channels of incrementing frequency (as illustrated in FIG. 3 and tabulated in TABLE 1) in each cluster, a first frequency group al (having the lowest frequency channel) and a last frequency group f2 (having the highest frequency channel) are nominally disregarded with respect to placement within particular sectors of the cluster (e.g. cluster 134). The remaining frequency groups bl-fl and a2-e2 are then nominally paired together by associating adjacent frequency groups, i.e. bl is associated with c1; dl is associated with el; fl is associated with a2; b2 is associated with c2; and d2 is associated with e2.

A first member (the lowest frequency member) of each of these pairs of frequency groups is uniquely assigned to a particular sector of a first site (e.g. site 130) of the cluster 134, whereas a second member (the higher frequency member) of each of these pairs of frequency groups is assigned to a corresponding (identically located/ positioned) sector of a second site (e.g. site 132) of the cluster 134.Assignment of the frequency group pairings continues on an adjacent and rotational basis such that all first members are assigned to the first site 130 and all second members are assigned to the second site 132, and each first or second member of the pair is side adjacent to at least one other next but one adjacent frequency group, i.e. on a sectorial basis bl is side adjacent to dl which is side adjacent to fl which is side adjacent to b2 which is side adjacent to d2 (in a first site 130), and cl is side adjacent to el which is side adjacent to a2 which is side adjacent to c2 which is side adjacent to e2 (in a second site 132).

By following this frequency group placement pattern, an empty sector that is side adjacent to both bl and d2 in site 130 appears and, similarly, an empty sector that is side adjacent to both cl and e2 in site 132 also appears.

Therefore, ten of the twelve possible sectors in cluster 134 are filled by the pairings, with the remaining two sectors receiving a pairing of the first frequency group al (having the lowest frequency channel) and the last frequency group 12 (having the highest frequency channel). More particularly, the first frequency group al is inserted into the empty sector of site 132 such that it is only side adjacent to c1, i.e. a next but one adjacent frequency group. Then, by default, the last frequency group 12 is inserted into the empty sector of site 130 such that it is only side adjacent to d2, i.e. a next but one adjacent frequency group.In this way, an interface 136 between sites of the cluster 134 (or 136) of the preferred embodiment will contain: (a) side adjacencies between (i) frequency group f2 and frequency group d2 (in site 130) and (ii) frequency group el and frequency group a2 (in site 132); and (b) half-side adjacencies between (i) frequency group f2 and frequency group el (in different sites) and (ii) frequency group d2 and frequency group a2 (also in different sites). As such, each frequency group pairing (containing adjacent channels that could potentially cause splatter if placed side adjacent to each other) is separated by a distance (diameter) of at least one site.

In the new two-site repeat pattern of FIG. 4, no adjacent frequency groups are found within the clusters (134, 136), and adjacent channel interference (splatter) is substantially reduced. However, since the physical separation of co-channel sectors is reduced in comparison with, for example, a foursite use pattern, a carrier-to-interference ratio (C/I) measurement for the co-channel of the preferred embodiment is reduced but nonetheless still provides adequate isolation.

Consequently, the preferred embodiment of the present invention advantageously provides a low-cost, two-site repeat pattern offering low adjacent channel interference and adequate co-channel isolation, while providing the inherent efficiency advantages associated with a reduced repeat pattern.

In an alternate (but less efficient) two-site repeat pattern (shown in FIG. 5), a coverage area 150 again contains a plurality of sites (represented in conventional hexagonal format) each having a base station and associated infrastructure (as will be understood). However, unlike the preferred embodiment in which sectors are corner-illuminated by individual RCUs, each RCU for each sector in FIG. 5 is arranged to provide a side-illumination of its respective sector.

With respect to the arrangement of frequency groups (al-fl and a2-f2) in each cluster of FIG. 5, a first frequency group al (having the lowest frequency channel) and a last frequency group 12 (having the highest frequency channel) are initially disregarded with respect to placement within particular sectors of a cluster. The remaining frequency groups bl-fl and a2-e2 are then again nominally paired together by associating adjacent frequency groups, i.e. bl is associated with cl; dl is associated with el; fl is associated with a2; b2 is associated with c2; and d2 is associated with e2.

The assignment of the frequency group pairings to corresponding sectors in each two-site cluster again occurs in an fashion identical to that previously described for FIG. 4, namely that members of each frequency group pairing are split between sites in the cluster and each first or second member of the pair is then positioned in a sector that is side adjacent to at least one other next but one adjacent frequency group. However, in the side-illuminated configuration of the alternate embodiment, interference points 152-160 (at each corner of every site) appear between adjacent frequency groups, e.g. bl and al or dl and cl or d2 and c2, resulting in a lower isolation for adjacent channel splatter (which may be overcome by adhering to strict hand-over regimes between sites, as will be appreciated by the skilled addressee).In this respect, it is noted that the interference experienced at interference points 152-160 in FIG. 5 arises between only two frequency groups (or channels) that potentially interfere at each point.

Therefore, hand-off to a base station responsible for any one of the four other non-interfering frequency groups (or channels) that also converge at that interference point could be acceptable.

In summary, a cellular communication system has a frequency bandwidth arranged into a plurality of frequency channels that are allocated on a consecutive frequency basis to corresponding sectors in each of neighbouring first and second sites. The present invention may be employed with any cellular communication system, such as time-division multiplexed systems (including those capable of supporting frequency hopping, if desired).

It will, of course, be understood that the above description has been given by way of example only and that modifications in detail, such as the orientation of each cluster and the rotational assignment of each frequency group pairing to particular but corresponding sectors in each cluster, may be made within the scope of the present invention. Furthermore, with respect to an available frequency bandwidth for the communication system, this may be constructed from two or more separate blocks of spectrum, in which blocks have some channels that have frequencies contiguous to one another.

Claims

1. A cellular communication system having a frequency bandwidth arranged into a plurality of frequency channels, the cellular communication system comprising neighbouring first and second sites each having sectors containing at least one frequency channel, wherein corresponding sectors in each of the neighbouring first and second sites have consecutive frequency channels from the frequency bandwidth, thereby producing a two-site re-use pattern.

2. The cellular communication system according to claim 1, wherein neighbouring sectors within both the first and second sites observe a next but one frequency channel relationship with adjacent frequency channels in at least one neighbouring sector.

3. The cellular communication system according to claim 1 or 2, wherein the first and second sites are corner-illuminated sites.

4. The cellular communication system according to claim 1 or 2, wherein the first and second sites are side-illuminated sites.

5. The cellular communication system according to any preceding claim, wherein the first and second sites each contain six sectors.

6. A cellular communication system having a frequency bandwidth arranged into a plurality of frequency channels that are distributed on a consecutive frequency channel basis amongst a plurality of frequency groups, the cellular communication system comprising a cluster having neighbouring first and second six-sectored sites, each sector of each site containing a frequency group having at least one frequency channel and wherein:: i) the first six-sectored site comprises a first frequency channel and at least five other frequency channels each having an integer multiple next but one frequency channel relationship to the first frequency channel, wherein neighbouring sectors in the first six-sectored site each contain a frequency group having respective frequency channels that observe a next but one frequency channel relationship with frequency channels in at least one neighbouring frequency group;; ii) the second six-sectored site comprises a second frequency channel, consecutive in frequency to the first frequency channel, and at least five other frequency channels each having an integer multiple next but one frequency channel relationship to the second frequency channel, wherein neighbouring sectors in the second six-sectored site each contain a frequency group having respective frequency channels that observe a next but one frequency channel relationship with at least one neighbouring frequency group; and iii) consecutive frequency channels of the frequency bandwidth are assigned to corresponding sectors in each of the neighbouring first and second six-sectored sites to produce a two-site re-use pattern for the cluster in which consecutive frequency channels are alternated between the first and second six-sectored sites.

7. The cellular communication system according to claim 6, wherein at least one frequency group contains more than one carrier frequency obeying the integer multiple next but one frequency channel relationship.

8. The cellular communication system according to claim 7, wherein the plurality of frequency carriers are allocated to the frequency groups on a cyclic basis.

9. The cellular communication system according claim 6, 7 or 8, wherein the first and second six-sectored sites are corner-illuminated sites.

10. A cellular communication system having a two-site re-use pattern substantially as hereinbefore described with reference to FIGs. 4 and 5 of the accompanying drawings.

11. A method of deploying frequency channels of a cellular communication system to sectors of two neighbouring sites to produce a two-site repeat pattern, substantially as hereinbefore described with reference to FIGs. 4 and 5 of the accompanying drawings.