GB2332817A - Downlink frequency overlay for cellular radio system - Google Patents

Abstract

A frequency plan for a downlink frequency overlay system in a fixed wireless access network wherein each of a plurality of cells operates a set of frequency division duplex pairs. An overlaid unpaired downlink frequency is provided within a cell area, the downlink frequency being selected from a frequency division duplex pair operated in a different cell. Preferably, a tri-sectored centre excited hexagonal cellular arrangement provides six frequency division duplex pairs per sector, and one additional unpaired downlink frequency overlay frequency per sector, increasing capacity on downlink transmission.

Description

DOWNLINK FREQUENCY OVERLAY FOR CELLULAR RADIO SYSTEM Field of the Invention The present invention relates to a frequency plan and a method of allocation of frequencies for a cellular wireless network, particularly, although not exclusively to a fixed wireless access network.

Background to the Invention In a fixed wireless access (FWA) telecommunications system, subscribers are connected to a backbone telecommunications network by means of radio links in place of traditional copper wires. Each of a plurality of subscribers is provided with a subscriber radio terminal at their subscriber premises. A base station provides cellular coverage, typically in urban environments over a 5 km radius, with the plurality of subscriber radio terminals. Each base station may be connected to a backbone network, eg a Public Switched Telecommunications Network (PSTN) switch via a conventional transmission link, known as a backhaul link, thereby providing the plurality of subscribers with access to the PSTN. A single base station can serve of the order of up to two thousand subscribers, making the installation and maintenance cost of a fixed wireless access system lower than that of an equivalent copper wire access network.

In geographical area, a plurality of base stations are deployed.

Communication between a subscriber radio terminal and its corresponding base station is via a local wireless radio link. Each local wireless link between a radio base station and a subscriber radio terminal comprises an uplink from a subscriber antenna to the radio base station antenna, and a downlink transmitting from the radio base station antenna to the subscriber antenna. Downlink beam coverage is provided in a nominally hexagonal cellular pattem. Each radio base station operates either an omni-directional beam or a plurality of broad sectorized beams encompassing all subscribers in a cell or sector for receive and transmit, whereas each subscriber radio terminal operates a directional pencil beam directed at the base station for receive and transmit. In a prior art fixed wireless access deployment, although each nominally hexagonal cell is served by a base station located nominally at the center of the cell, conventional technologies permit the base station antenna to be located non-centrally within a cell area.

In the prior art fixed wireless access system a frequency spectrum allocation for the uplink is typically of a same bandwidth as a frequency spectrum allocation for the downlink. For example, the uplink may be allocated 15 - 17 MHz bandwidth in an available radio spectrum, and the down link may be allocated a further 15 - 17 MHz bandwidth of frequency spectrum. The uplink and down link spectrums are spaced apart by typically around 50 MHz, referred to as duplex spacing.

Typically, the uplink frequency allocation of 15 MHz is subdivided into a plurality of 300 KHz slots each occupied by a separate canier frequency, giving 48 uplink carriers. For a 17 MHz uplink band, divided into a plurality of 300 KHz uplink frequency slots, 54 uplink caniers are available. Similarly, the allocated downlink frequency spectrum is subdivided into a plurality of 300 KHz downlink frequency slots, being symmetric with the uplink frequency allocation.

The 300 KHz frequency slots are allocated to a plurality of radio base stations of the network over a geographical area according to a repeating frequency reuse pattem. To minimize the likelihood of interference, adjacent cells, or sectors within each such cell are allocated distinct groups of radio frequencies selected so as to minimize the likelihood of a transmission with any other nearby cell (or sector of a cell) causing interference. On the uplink, in a three of nine reuse pattem, every ninth frequency is reused, so although only 18 of the 54 available carrier frequencies are used per cell, the frequency pattern can be reused indefinitely, and an allocation of subscriber radio terminals to base stations giving service to around 2000 subscribers per cell can be replicated indefinitely over a geographical area.

Referring to Fig. 1 herein, there is illustrated an example of a downlink frequency allocation plan for a prior art arrangement of hexagonal cells within a fixed wireless access network, having a deployment of omnidirectional base station antennas. Each of a plurality of base stations provides coverage for a nominally hexagonal cell area. A cluster pattem of seven frequency division duplex pairs F1 - F7 covering seven cells is duplicated across the network. In order to avoid interference between adjacent base stations, and subscriber transmissions, distinct frequency groups are allocated as between adjacent cells.

No two adjacent cells utilize a same or like carrier frequency. Carrier frequencies are reused between base stations which are sufficiently far apart from each other, so as not to cause interference with each other thereby increasing the overall capacity of the fixed wireless access network by reuse of carrier frequencies.

Referring to Fig. 2 herein there is illustrated a downlink frequency plan for a prior art tri-sectored center excited cellular arrangement, in which each base station at the center of a corresponding respective nominally hexagonal cell radiates three beam pattems per hexagonal cell, each beam having different frequencies to other beams in the same cell. Greater frequency reuse is achieved in the tri-sectorized arrangement as compared with the omni-directional cells of Fig. 1 herein. In the tri-sectored center excited cellular arrangement of Fig. 2 each base station is adapted to transmit and receive on distinct frequencies over directional downlink and uplink beams within the cell. In Fig. 2, the symbols A1-A3, B1-B3, and C1-C3 are used to indicate distinct downlink frequency groups allocated to individual sectors. Carrier frequencies are chosen to be sufficiently far apart from each other so as not to interfere with adjacent sectors or cells. In each nominally hexagonal cell, there are 18 downlink carrier frequencies. Each downlink frequency group comprises a set of 6 downlink carrier frequencies. For example, frequency group Al of first sector 200 comprises down link frequencies f, - f6. The same frequency group Al is re-used in spaced apart first tier frequency re-use sector 201.

Thus, in the prior art case Fig. 2, in a frequency plan for a tri-sectorized arrangement, there are 18 downlink carrier frequencies per cell (6 downlink carrier frequencies per sector) and each downlink carrier frequency is paired with a corresponding respective uplink carrier frequency in a time division multiplexed frequency division duplex pair.

Typically in a 17 MHz uplink case, each base station may operate 18 uplink carriers, 6 per sector, in a tri-sectored arrangement. Each carrier frequency is separated into 10 bearer time slots, providing 60 uplink bearer time slots per sector (180 bearers per cell). Of these, 2 to 6 bearer time slots per sector are reserved for an access channel, through which subscriber radio terminals request access to the radio base station, leaving 54 bearer time slots per sector available for subscriber usage. Each subscriber radio terminal operates two subscriber lines, so taking account of the bearers reserved for access channels, up to a maximum of 27 radio subscriber terminals in a sector can communicate with a base station at the same time. However, as usage of subscriber terminals is statistical in nature, up to approximately 600 to 700 subscribers per sector can be accommodated since not all subscribers communicate at once.

Similarly, in the 17 MHz downlink band, the downlink frequency allocation at each base station is 18 carriers per cell, each downlink carrier corresponding to an uplink carrier in a frequency division duplex pair. In each sector, there are 6 downlink carrier frequencies, corresponding with the 6 uplink frequencies, to form 6 frequency division duplex pairs per sector. As with the uplink carrier frequencies, the downlink carrier frequencies are time division multiplexed into a plurality of bearer timeslots. Some of those bearer timeslots are used as a downlink broadcast channel which advertises available bearer timeslots to all subscribers within a sector.

For circuit switched services carried over the wireless link, where those services are characterized by having symmetric constant data rate traffic both on the uplink and downlink, eg voice traffic, the prior art symmetric allocation of frequency spectrum between the uplink and downlink beams is relatively efficient.

However, for services which entail an asymmetric data rate requirement as between the uplink and the downlink, for example where the volume of traffic data on the uplink differs greatly from a volume of traffic data on the downlink, a symmetric frequency spectrum allocation for the uplink and downlink beams is inefficient. For example, taking an instance of a subscriber making Intemet communications on a user terminal, connected to a subscriber radio terminal, a request for data sent to an Intemet service provider on the uplink may comprise a transmission of packets of tens or hundreds of Bytes. On the other hand, service data provided by the Internet service provider may comprise data units of the order kBytes or MBytes. Such data is downloaded over the downlink. In a circuit switched application, the bandwidth is reserved and available for use for uplink and down link communications throughout the duration of a communications session. During the download of data from the Intemet, the uplink path remains reserved for use by the subscriber, although no data traffic may be actually flowing on that uplink.

Whilst in the case of fixed wireless access a circuit switched symmetric approach to allocation of transmission resources makes acceptable use of available radio bandwidth for largely continuous services having substantially symmetric data rate in either direction, (for example voice traffic, where a volume of traffic in one direction may be approximately equal to a volume of traffic in an opposite direction) it would be wasteful of the limited radio bandwidth across a fixed wireless access link to operate a circuit switched mode where data transfer is intermittent, or where data transfer is asymmetric in nature, having a large amount of data in one direction and a smaller amount of data in an opposite direction across the fixed wireless access link. An example of such an asymmetric intermittent usage pattern arises in subscriber access to the transmission control protocol/lntemet protocol (TCP/IP) I nternet, for example access to the World Wide Web (WWW), User Groups, file transfer protocol (FTP), or Bulletin Board. Such Intemet interactions typically take the form of a relatively short information request transmitted by a subscriber on an uplink (eg of the order of a few tens of bytes) each information request potentially resulting much larger amounts of data being downloaded to the subscriber via a downlink (eg many Bytes or even M/Bytes of data).

Referring again to Fig. 1 herein, the following example illustrates an imbalance between uplink and downlink interference. For a prior art arrangement of a plurality of, for example, 6 subscriber transceivers communicating with a single base station in an omnidirectional cellular base station layout, on a base station to subscriber downlink, assuming a frequency reuse of 1, there is only one possible base station interferer for a subscriber radio terminal having a directional beam 101 aimed at base station 102. However, for a base station receiving uplink frequencies omni-directionally, there are potentially 6 cells from which interference can occur around each base station, these being the first tier frequency reuse cells. On the uplink, since each base station has 6 subscriber transceivers in its surrounding cell, there are potentially six interferers surrounding each base station, one in each of the adjacent first tier frequency reuse cells to that base station. Thus, there is an in-built interference imbalance between the downlink and the uplink which manifests itself as a difference in carrier signal to noise and interference ratio (CNIR) between downlink and uplink paths.

The CNIR imbalance is mitigated due to the statistical nature of transmissions. Whilst the downlink beams are non-statistical, ie always on, the uplink beams are statistical, only transmitting when connections are made.

However, under circumstances where many subscribers communicate simultaneously, for a single subscriber wireless link, the worst case interference on the uplink exceeds the worst case interference on the downlink.

Taking as a comparison, an example of a prior art center excited hexagonal cell having a trisected uplink beam pattern as shown in Fig. 2 herein, instead of an omni-directional uplink beam pattern, the uplink interference position improves compared to the omni-directional case, since the trisected uplink beam receives interference from only two first tier frequency reuse cells, assuming a frequency reuse factor 1. However, there is still an imbalance in potential worst case interference between the downlink and the uplink for communication between a subscriber and a base station.

In a fixed wireless access network deployment having a plurality of subscribers each communicating with a base station, under conditions of services of asymmetric data rate, having a symmetric frequency spectrum allocation for a downlink and uplink path for each subscriber represents an inefficient use of frequency spectrum.

However, in many applications, a symmetric frequency allocation is all that is available, due to prior allocation of frequencies by license. Frequencies may become available due to decommissioning of legacy equipment using symmetric uplink and downlink frequencies and any replacement equipment must make use of the symmetric frequency allocations becoming available.

Summary of the Invention According to a first aspect of the present invention there is provided a method of allocating frequencies to a plurality of base stations of a cellular radio network in which an uplink frequency spectrum allocation and a downlink frequency spectrum allocation are divided into a plurality of frequency division duplex pairs, each said frequency division duplex pair comprising an uplink frequency and a corresponding respective downlink frequency, said method comprising the steps of: assigning a plurality of said frequency division duplex pairs to each said base station; and assigning an unpaired downlink frequency to at least one said base station.

The base stations may operate omni-directional beams in a center excited configuration on the downlink, or may operate a plurality of sectorized beams.

The sectorized beams may comprise a center excited tri-sectored arrangement having three nominally 1 20C beams per nominally hexagonal cell, or alternatively, a base station may operate a tri-cellular arrangement in which three radially extending beams cover three corresponding nominally hexagonal cells.

In a preferred implementation, each said base station operates a plurality of sector antennas; each said sector antenna is assigned a plurality of said frequency division duplex pairs; and at least one said sector antenna is assigned a said unpaired down link frequency.

Each said sector antenna may be assigned at least one said unpaired downlink frequency.

Preferably each said sector antenna operates a set of 6 said frequency division duplex pairs, and at least one said unpaired down link frequency.

Preferably a first base station operates a set of frequency division duplex pairs and at least one unpaired downlink frequency, said unpaired downlink frequency being selected from a set of frequency division duplex pairs operated by a different base station to said first base station.

Brief Description of the Drawings For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Fig. 3 illustrates schematically an example of a fixed wireless access system according to a first specific embodiment of the present invention, adapted for supporting services having asymmetric data transmission rates as between uplink and downlink; Fig. 4 illustrates schematically a frequency re-use plan for a plurality of base stations and a plurality of subscribers according to a specific method of the present invention.

Detailed Description of the Best Mode for Carrying Out the Invention There will now be described by way of example the best mode contemplated by the inventors for canying out the invention. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one skilled in the art, that the present invention may be practiced without using these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

Further, the techniques disclosed herein are applicable in a number of frequency division duplex systems, eg North American AMPS (Advanced Mobile Phone System), PCS (Personal Communications System), or CDMA (Code Division Multiple Access). Further, it will be realized that whilst the specific embodiments and methods disclosed herein describe specifically a fixed wireless access application, the general techniques employed are equally suitable for application in satellite links, and cellular wireless systems in general, including mobile systems.

Specific embodiments and methods according to the present invention may take advantage of a difference between the carrier to noise and interference ratio quality as between the uplinks and downlinks arising from the use of highly directional antennas at the subscriber radio terminals whereby the worst case downlink carrier to noise and interference ratio is better than the worst case uplink carrier to interference and noise ratio, to introduce an overlaid channel for communicating packet switched data.

Referring to Fig. 3 herein, there is illustrated schematically a fixed wireless access system according to a first embodiment of the present invention. A radio base station 300, comprises a main antenna 301 and main transceiver 302 and may also be provided with an auxiliary antenna 303, and a supporting auxiliary transmission apparatus 304. The radio base station 300 serves a plurality of subscriber radio terminals 305, each comprising a subscriber transceiver 305 and a subscriber antenna 306. Communication between the base station main antenna 301 and subscriber transceiver 305 is by means of the conventional fixed wireless access link, represented in Fig. 3 by the bidirectional arrow 307, comprising a frequency division duplex pair consisting of a first frequency f1 used for an uplink between the subscriber transceiver and the base station main antenna, and a second frequency f2 used for downlink transmission from the base station main antenna to the subscriber transceiver. Additionally, a third wireless link 308 operating on the downlink only is provided for transmission at a third frequency f3. The unpaired downlink frequency f, may be transmitted from base station auxiliary antenna 303 to the subscriber premises or where auxiliary antenna 303 is absent, may be transmitted on main antenna 301. Base station 300 in a tri-sectored arrangement in an embodiment where auxiliary antenna 303 is present, provides a separate base station main antenna 301 for each nominal 1200 azimuth sector, as well as a separate base station auxiliary antenna 303 for each 120" sector. The following description relates to an embodiment where paired downlink frequencies transmit from the main antenna 301 and the unpaired downlink frequencies transmit from the auxiliary antenna 303, it will be understood that in other embodiments transmission of all down link frequencies may be made from main antenna 301.

Base station 300 operates one or more unpaired frequencies on the downlink only. The base station main antenna 301 and base station main transceiver 302 operate a plurality (typically 6 per sector) of frequency division duplex pairs, each comprising an uplink carrier frequency and a corresponding downlink carrier frequency. The unpaired downlink canier frequencies operated by base station auxiliary antenna 303 and base station auxiliary transmitter 304 are additional to and overlaid on the frequency division duplex pairs. The additional overlaid unpaired downlink carrier frequencies can be accommodated on the downlink by taking advantage of the difference in CNIR between the uplink between a subscriber and the base station and the downlink, arising out of the spatial asymmetry of the broad sectorized beam of the base station antennas, and the narrow directional pencil beams of the subscriber antennas.

Referring to Fig. 4 herein, there is illustrated schematically a downlink frequency plan according to a specific implementation of the present invention.

Each of a plurality of nominally hexagonal geographical cell areas are served by a corresponding respective centrally located base station 300 as described with reference to Fig. 3. Each base station operates a tri-sectored downlink azimuth beam pattern, in each sector operating a group of six frequency division duplex pairs, each comprising a downlink frequency and an uplink frequency, as described in the prior art arrangement of Fig. 2. In each sector, one or more additional unpaired downlink frequencies are transmitted eg by corresponding auxiliary base station antenna 303. In a preferred implementation, the unpaired downlink carrier frequency supports a plurality of downlink distribution channels for distributing packet switched data, eg Internet data. The separate unpaired downlink frequency is transmitted from a corresponding respective auxiliary base station antenna 303 provided one per sector, at central base station 300.

Whilst the subscriber antenna operates a narrow directional radiation beam exhibiting a relatively high gain, the base station antenna is typically less focused than the subscriber antenna beam, being much broader and exhibiting significantly lower gain. The relatively narrow, directional subscriber antenna pencil beams, typically may have a 3dB beamwidth of about 20 and a gain of around 18 dBi whereas the base station antenna beam may typiclly have a -3 dB beamwidth of the order 85 , and occupies a 1200 sector. Consequently, the downlink is less susceptible to interference than the uplink, because the base station antenna receives signals from a relatively broad azimuth, whereas the subscriber antenna receives signals with a relatively narrow azimuth.

In the specific implementation presented herein, as the subscriber radio terminals are mounted with highly directional antennas, the downlinks of the fixed wireless access network are generally less vulnerable to interference than the uplinks. Different traffic types can tolerate different levels of interference. For circuit switched traffic, for example carrying voice transmission, a high integrity of transmission is required since the nature of the traffic is such that intermittence of a connection cannot be tolerated. However, for packet switched data a higher degree of connection intermittence can be tolerated since packet switched data is less delay sensitive than voice data, and can often be retransmitted. This may enable an overlayed downlink frequency reuse scheme to be employed on the auxiliary downlink beam without causing excessive interference to symmetric traffic using the same network. For instance, in a tri-sectored deployment scenario, the auxiliary downlink beam of each sector may reuse a selected frequency from the same frequency reuse group, thereby providing a total of 3 extra downlink frequencies per cell.

The frequency plan of the best mode herein takes advantage of the differences in subscriber antenna beamwidth and base station antenna beamwidth, and of a difference in canier to interference and noise ratio (CNIR) between the uplink and downlink, to provide at least one extra downlink carrier frequency per sector compared with the prior art case of Fig. 2, the extra downlink frequency being unpaired, having no corresponding uplink frequency.

For each sector, the additional one or more unpaired downlink carrier frequencies are selected out of a frequency group assigned to a sector other than a first tier frequency re-use sector.

Taking the case of a 17 MHz uplink and downlink bandwidths, the available uplink bandwidth is divided into 54 300 kHz frequency slots, each uplink frequency slot having a corresponding respective downlink frequency slot, the downlink bandwidth being divided into 54 300 kHz down link frequency slots. A single uplinkldownlink frequency division duplex pair F comprises an uplink frequency slot fj and a corresponding downlink frequency slot fD such that: F=fU+fD Eighteen frequency division duplex pairs are allocated to each center excited hexagonal cell, six frequency division duplex pairs being allocated per 1200 sector. Additionally, one or more unpaired downlink frequencies fD may be allocated to each hexagonal cell. In the best mode herein, one unpaired downlink frequency is allocated per sector of a tri-sectored center excited cell.

However, in the general case the number of unpaired downlink frequencies allocated per cell may be dependent upon local propagation characteristics and local interference conditions at the base station. It may be difficult to incorporate an extra downlink carrier frequency at some base station sites, whilst at other base station sites, more than one unpaired downlink frequency may be incorporated.

In the best mode herein, as illustrated in Fig. 4, each sector is allocated a single unpaired downlink carrier frequency.

In each cell, the unpaired downlink carrier frequency is selected from downlink carrier frequencies which are not operated in the same hexagonal cell.

For example, a base station 400 may operate three frequency groups, one per sector over three corresponding respective 120C sectorized areas. For example, in Fig. 4, each frequency group comprises six frequency division duplex pairs and one unpaired downlink frequency. The frequency groups may be constructed as follows: A'1 = F1, F2, F3, F4, F5, F6 + f19 A'2=F7, F8, F9, F10, F", F,2 +f2o A'3 = F,3, F,4, F,5, F,6, F,7, F,8 + f21 The unpaired downlink frequencies D are selected from frequency division duplex pairs of adjacent cells, ie from frequency groups B'1 - B'3; C'1 - C'3.

For example, for sector 401 in Fig. 4, frequency group A'1 comprises a group of paired downlink frequencies f, - f6 each having a corresponding respective uplink frequency, plus one or more additional unpaired downlink frequency fN selected from a sector other than a first tier frequency reuse sector also operating frequency group A'1. For example, the additional unpaired downlink frequency in sector 401 may be selected from paired frequency groups B'1-B'3 or C'1-C'3.

In one implementation, the unpaired downlink frequencies of each sector may be fixed to their corresponding sector. For example unpaired downlink frequency f19 may be permanently assigned to sector 401, having paired downlink frequencies f1 - f6, so that downlink frequency of group A'1 comprises downlink frequencies f, - f6 of paired frequency duplex pairs F, - F6, and unpaired downlink frequency f,g.

In another implementation, the unpaired downlink frequencies may be dynamically selected from frequency groups of non-first tier frequency group reuse cells in real time. In the second implementation, the downlink distribution channels may be statically fixed to a single unpaired downlink carrier frequency or may float from unpaired to paired downlink carrier. For example, an unpaired downlink frequency of a first sector 400 operating paired frequencies group F, F6 (including paired downlink frequencies f, - f6) may be selected from a paired downlink frequency of a non-reuse paired frequency group A'2 of a sector 402 for a first compared with that of the access channel, which tends to increase in accordance with the number of RF carriers, for example one ALOHA slot per RF carrier. This is because the sector broadcast information remains identical across all the RF carriers within a sector. This makes it possible to use the spare capacity for the low throughput Intemet traffic in the uplink direction.

Advantageously, since the specific embodiments and methods of the present invention do not necessarily rely upon a conventional air interface protocol (AIP) of a symmetric fixed wireless access system, conceptually, an overlaid system bypassing both the air interface protocol and the conventional switch interface protocol stack is possible. The overlay system may have its own transmission only, low-cost radio co-located with the existing fixed wireless access at the base station site.

Advantageously, specific embodiments and methods of the present invention may enable seamless integration and overlay of an asymmetric data network with existing fixed wireless access networks. Also enabled are use of simplified base station radio equipment for the additional unpaired down link frequency, which requires only a transmit capability. The use of a packet mode downlink protocol may also allow efficient utilization of available downlink bandwidth.

Claims (5)

Claims:

1. A method of allocating frequencies to a plurality of base stations of a cellular radio network in which an uplink frequency spectrum allocation and a downlink frequency spectrum allocation are divided into a plurality of frequency division duplex pairs, each said frequency division duplex pair comprising an uplink frequency and a corresponding respective downlink frequency, said method comprising the steps of: assigning a plurality of said frequency division duplex pairs to each said base station; and assigning an unpaired downlink frequency to at least one said base station.

2. The method as claimed in claim 1, wherein: each said base station operates a plurality of sector antennas; each said sector antenna is assigned a plurality of said frequency division duplex pairs; and at least one said sector antenna is assigned a said unpaired downlink frequency.

3. The method as claimed in claim 1 or 2, wherein each said base station operates a plurality of sector antennas; each said sector antenna is assigned a plurality of said frequency division duplex pairs; and each said sector antenna is assigned at least one said unpaired downlink frequency.

4. The method as claimed in any one of claims 1, 2 or 3, wherein each said sector antenna operates a set of 6 said frequency division duplex pairs, and at least one said unpaired down link frequency.

5. The method as claimed in any preceding claim, wherein a first base station operates a set of frequency division duplex pairs and at least one unpaired downlink frequency, said unpaired downlink frequency being selected from a set of frequency division duplex pairs operated by a different base station to said first base station.