GB2373405A - Determining an attenuator code for a receiver's geographical region and selecting an attenuator using the attenuator code - Google Patents

Abstract

Apparatus and methods for use with wireless communications systems are described, in particular for setting up such communications systems. In a first aspect the invention provides a method of selecting an attenuator for a receiver comprising determining a geographical region in which the receiver is located, determining an attenuator code for the receiver's region, and selecting an attenuator using an attenuator code. Preferably an attenuator is selected from a predetermined set, and preferably the attenuator code is determined using an address or address code for an approximate location of the receiver. The invention also provides a related computer system and a map for selecting an attenuator for a receiver. The method, system and map simplify the selection of an attenuator to reduce the risk of a receiver being overdriven, and facilitate the setup of a receiver in a wireless communications system by a relatively unskilled technician.

Description

COMMUNICATIONS SETUP SYSTEMS This invention relates to apparatus and methods for use with wireless communications systems, and in particular to apparatus and methods for setting up wireless data transmission systems.

With increasingly heavy use of the Internet, both domestic and commercial users are demonstrating ever increasing demands for greater bandwidth to allow faster access to data and to provide high quality sound and video services at acceptable quality levels.

At present the majority of home Internet connections are made by dialling up a local service provider by telephone, using either a land line connection or a mobile telephone.

In such communication systems the bulk of the cost is in the last few kilometres of "local loop"cable connection to a given subscribers home. By contrast, radio and television sounds and pictures are transmitted over the air to many subscribers simultaneously.

Increasingly, the potentially high bandwidth capability of fibre optic cable and copper cable connections is being exploited to provide fast Internet access for business and domestic subscribers. This is seen as a more appropriate use of data communication pathways since cable connections are thought to be intrinsically capable of providing higher bandwidth services than radio links, which suffer from spectrum congestion and data rate limiting factors such as multi-path fading.

The majority of so-called cable modems operate using a communications protocol known as DOCSIS (Data Over Cable Service Interface Specification), versions 1.0 and 1.1 of which are hereby incorporated by reference. This standard, which is well-known to those skilled in the art, is also known as CableLabs (Trade Mark) Certified Cable Modems project. The radio frequency interface specification is set out in documents SP-RFI-105-991105 and, for version 1.1, SP-RFIvl. 1-in5-000714, which are hereby incorporated by reference. Other relevant documents include cable modem to customer premises equipment interface specification SP-CMCI-104-000714, cable modem termination system network side interface specification SP-CMTS-NSI-101-960702,

cable modem Telco return interface specification SP-CMTRl-IOl-970804, operations support system interface specification SP-OSSIvl. 1-102-000714, and base line privacy plus interface specification SP-BPI±I05-000714.

DOCSIS is a North American specification and there is an equivalent European specification generally referred to as EuroDOCSIS, to which reference may also be made. Broadly speaking, EuroDOCSIS corresponds to North American DOCSIS, but with an altered frequency plan (EuroDOCSIS uses 8MHz channels whereas North American DOCSIS uses 6MHz channels) to take account of the use of PAL/SECAM analogue television signals rather than the slightly narrower bandwidth NTSC analogue tv system used in North America. Further information on the DOCSIS specification can be found at www. cablemodem. com and at cablelabs. com.

The characteristics of cable connections particularly optical fibre data connections, are very different to those of radio data transmission links. Optical fibres provide large bandwidths and low transmission losses and are often used, for example, for longdistance, high-speed telecommunications. Optical fibres are also immune to electromagnetic interference and the multi-path effects encountered in rf transmission links.

The DOCSIS specification defines interface requirements for cable modems for highspeed data distribution over cable television networks, and it is widely used in the cable television industry, particularly in the USA, for the provision of high-speed Internet services over a cable television infrastructure. When used in this way it provides an additional Internet Protocol (IP) data path from a cable operator's head end (s) to local subscribers. The specification also defines optional operations support system functions for, inter alia, security, network management, accounting, and configuration and fault management. The specification includes physical, link and network specifications and provides definitions for aspects of the system including features such as rf levels, multiplexing, contention control, and the like.

To provide such an additional data path, an interface device is located at the head end of the cable network. This device has an Internet interface for sending and receiving data to and from the Internet, and a cable interface, coupled to the cable TV distribution system. A downstream output is up-converted to a suitable part of the cable network frequency spectrum and is broadcast to all connected subscribers/customers. An upstream input receives signals that have been sent by a particular subscriber or customer. At the customer end cable modem customer premises equipment is located at the customers'homes. A cable side of this equipment is connected to the cable TV network by coaxial cable, and a customer interface is provided comprising a standard 10 Base T Ethernet connection for a Personal Computer.

Figure 1 shows a simplified block diagram of a prior art data-over-cable system 100 using DOCSIS equipment. Cable network 102 typically has a tree and branch structure with a single trunk input 103, normally an optical fibre, fanning out to a plurality of branch outputs, coupled to fibre optic and/or coaxial cable links to local subscribers. In this specification"subscriber"has a broad meaning encompassing any user of a data communications system and/or recipient of a data communication service and is not limited to only registered or paying customers or users.

Normally, (although not always), the"input"and the"outputs"are bi-directional. In the DOCSIS specification one frequency band (for example 120 to 850 MHz) is allocated to downstream links from the head end to the subscriber ends whilst a second frequency band (for example below 80 MHz) is allocated to upstream links from subscribers to the head end or hub. Data broadcast from the head end is normally received by all the subscribers, and correspondingly signals may be sent from the subscribers to the head end. Signals sent to the head end simultaneously from two different subscribers may collide.

A subscriber link is coupled to a cable modem 116 located at the subscriber's premises.

Cable modem 116 provides an output to a communications port on a personal computer 118. The DOCSIS-compatible cable modem 116 forwards Internet Protocol traffic, for example using transparent bridging or network-layer forwarding, and may also support other network layer protocols. The link from the cable modem to PC 118 is typically an Ethernet connection (lOBase-T, IEEE 802.3) ; the PC runs standard TCP/IP software.

Fibre optical cable 103 is coupled to physical interface 104 which in turn is coupled to a Cable Modem Termination System (CMTS) 106. The CMTS 24 provides network layer forwarding or transparent bridging for Internet Protocol traffic to a network side interface (NSI) 108. The NSI 108 typically comprises an ATM (asynchronous transfer mode) network and links to one or more local server (s) 112, remote server (s) 110 and the Internet 114. The servers include a network management server, a billing server.

Other network components, not shown in Figure 1, include one or more central office switches, ISP routers, and frame relays, DHCP (Dynamic Host Configuration Protocol) and TFTP (Trivial File Transfer Protocol) servers, and associated databases. Both cable modem 116 and CMTS 106 function as IP hosts. The system can transparently transfer IP traffic of a variety of types including broadcast and multicast IP Group addressing.

In the DOCSIS specification, broadcast data is encrypted and subscriber-specific data is extracted by a subscriber's cable modem. Upstream bandwidth from a subscriber to the head end is divided into a stream of mini-slots and the CMTS controls access to the slots by the cable modems, determining which mini-slots are subject to collisions (subscribers initially transmit a short"request to send"message). Further information on the DOCSIS specification and its operation can be found at the web sites referenced above.

As already outlined, rf data links suffer from a number of problems including multi-path fading which is caused when a receiver picks up signals from a transmitter which have reached the receiver by travelling over paths of different effective lengths. This causes data bits originally emitted at different times to arrive at the receiver at the same time causing intersymbol interference and notches in the receiver frequency response. Typically these problems begin to appear at data rates of a few 100K bits per second and above and are particularly apparent at the microwave frequencies (1 GHz and above) which are necessary both to provide the required bandwidth and to avoid conflict with existing spectrum usage allocations. Typical error correction schemes for rf data transmission employ complicated adaptive equalisation, the use of significant redundancy and data interleaving and, in some wireless LAN systems, spread spectrum techniques.

For these reasons, the provision of wireless local loop data services has heretofore focused on providing specific tailor-made solutions involving sophisticated error correction and techniques for improving spectrum efficiency, that is, traffic carrying capacity for a given bandwidth. Whilst high speed wireless data links are available the equipment required for robust operation is complex and expensive, typically costing

E250K for head end equipment and around L2, 000 for subscriber end equipment. By contrast, known cable data transmission systems are relatively simple and cheaptypically subscriber end apparatus costs only a couple of hundred pounds.

Wireless Internet access at an affordable price is highly desirable as it would allow new service providers to offer services without having to rely on using the existing copper or fibre optic cable infrastructure. There is also a need for the improved subscriber mobility and facilitate fast Internet access whilst on the move, which a wireless system could offer. However the cost of this equipment and the difficulties in installing it at domestic premises provide a significant barrier to mass adoption of wireless local loop Internet access schemes.

The above problems with the provision of wireless local loop data services are addressed by an invention described in the applicant's earlier UK patent application, number 0001901. 8 of 27 January 2000, according to which there is provided a wireless data communication system comprising: a DOCSIS-compatible cable modem network system having a data input couplable to the Internet; a radio frequency (rf) transmitter coupled to the cable modem network system and to a transmit antenna; at least one radio frequency (rf) receiver coupled to a receive antenna ; and at least one DOCSIScompatible cable modem coupled to the microwave receiver, and having a data output; whereby data from the data input is transmissible to the data output.

In other aspects the earlier invention provides a method of communicating Internet

Protocol data over a wireless rf link comprising : encoding the data using a DOCSIScompatible cable modem network system to provide a radio frequency (rf) encoded data output; transmitting the encoded data over a wireless link to a receiver; receiving the encoded data using the receiver ; and decoding the encoded data using a DOCSIScompatible cable modem to provide a decoded data output and a use for DOCSIScompatible cable modem devices in such a method and system.

In a further aspect the earlier invention provides a wireless local loop system designed to provide the same functionality as a cable modem system over a cable TV network in such a way that a cable modem on a customer premises designed to the DOCSIS technical specification for use over a cable TV network can be plugged into the wireless local loop system without modification and successfully provide the same functions.

In a still further aspect the earlier invention links equipment intended to be placed at the head end of a cable TV network (by virtue of its compliance with the DOCSIS technical specification) with equipment on a customers premises that is intended to be connected to the termination of a cable TV network (by virtue of its compliance with the DOCSIS technical specification) by means of a wireless transmission system.

The applicant recognised that the tree and branch structure of a cable network is an environment which has functional similarities to a point to multi-point microwave communication system, in that both systems share requirements for selective data reception and for collision handling for data sent by customers/subscribers to the head end or, in the case of an RF system, to a local base station.

The applicant also conducted a number of experiments and discovered the unexpected result that DOCSIS-compatible cable modem equipment provides acceptable performance levels when used in a wireless local loop system, particularly when operating at relatively high frequencies, such as microwave frequencies where radio wave propagation is relatively directional. This runs counter to earlier technical prejudices and allows a significant advance in the price/performance of wireless local loop systems to be achieved by exploiting existing mass-produced cable modem equipment.

The above-described wireless data communication system need only provide a one-way link from a base station to the subscribers, as a return path can be provided by other means, for example the public switched telephone network (PSTN). However, preferably the system employs transceivers at both the base station and subscriber ends to provide a bi-directional radio frequency link. This simplifies both lower layer communications such as re-requesting packets with errors, and higher level data communications such as web-telephony and video. The system can be used with RF signals of any frequency but preferably it is used at frequencies above 1 GHz, and more preferably at around 10GHz-20GHz although, for greater directionality and bandwidth,

frequencies above 20-40GHz, may be employed.

The above-described wireless system is primarily envisaged for the transmission of Internet data (which includes text, audio and video data), but it can also be used for transmitting other data, for example suitably packaged MPEG data and/or ATM data.

Advantageously DOCSIS forward error correction is enabled for improved robustness.

In a preferred arrangement there is a plurality of local subscribers each with a transceiver and DOCSIS compatible-cable modem, to provide a plurality of bidirectional rf links. To reduce the effects of multi-path it is preferable that the subscribers'antennas are relatively directional, for example with a gain of 3, 6,9, 12 dBi or greater. Advantageously a patch antenna or path antenna array can be used to provide sufficient directionality. Radio frequency digital data transmission systems tend not to degrade linearly but, to a first approximation, either function acceptably or fail to function at all, which facilitates the determination of how well a system for a plurality of subscribers is working.

The present invention concerns an improvement to this earlier system, although its use is not limited to such systems.

Cable modems using the DOCSIS standard require a received signal level falling within a specified range to operate correctly. This range is +15 dBm to-15 dBm, and the received level is nominally set at 0 dBm in a wired cable modems.

Wireless cable modems suffer from losses is received signal level due to, inter alia, free space losses and weather conditions such as rain attenuation, that do not affect a wired system. This means that distance from the point of transmission is critical to operation.

In a wireless system, subscriber modems are nominally set up for operation at the maximum envisaged transmission range, typically 5 to 10 kilometres. Any modem at greater than the maximum range from the point of transmission is considered out of range, and proper reception of the signal is not guaranteed. This limitation is acceptable and can be taken into account when designing the transmission system. A problem arises, however, when the customer is within the maximum operational range.

Within the maximum range the reduced free space losses have the result that stronger signals are received. When the received signal is significantly stronger than that received at the maximum range at the edge of a serviced area, the nearby receiver system may be overdriven, resulting in an increased bit error rate, unreliable operation and, under extreme conditions, damage to the receiver.

It is known to use an attenuator in the front end of an rf receiver to bring the signal level into an acceptable range, to stop the receiver being overdriven. Such an attenuator is normally connected between the antenna and the receiver. The value of the attenuator may be determined by, for example, measuring the actual received signal level and subtracting from this the desired input signal level for the receiver. This is relatively straightforward in an engineering laboratory but presents difficulties when installing attenuators in the field. These difficulties are compounded when relatively unskilled installation technicians are employed to install wireless transceiver equipment at subscribers'domestic premises. Signal strength measurements can be difficult to carry out, time consuming, and are not always reliable because of, for example, temporary environmental factors such as varying weather conditions.

According to a first aspect of the invention, there is therefore provided a method of selecting an attenuator for a receiver comprising, determining a geographical region in which the receiver is located, determining an attenuator code for the receiver's region, and selecting an attenuator using the attenuator code. The attenuator code may comprise an attenuator value specifying a required degree of attenuation or it may comprise other information for selecting an attenuator, such as a colour code.

The applicant has recognised that a relatively simple, distance-based approach can be used to select an attenuator value. Thus the attenuator may be chosen based upon the geographical region in which the receiver is located, rather than upon more complex and potentially less reliable signal strength measurements. The precise value of attenuators selected need only be approximately correct because a receiver normally has a range of input signal levels which provide satisfactory performance.

Preferably, the receiver is an rf receiver, although the method may be adapted for other types of receivers such as, for example, an optical receiver. It is also preferable that the receiver receives line of sight transmissions from a transmitter as this improves the validity of an assumption underlying an embodiment of the method, that terrain variations have only a second order effect on received signal level.

In another aspect the invention provides a corresponding method of selecting an attenuator for a transmitter transmitting signals from a geographical region in which a transmitter is located to a remote receiver.

It has also been recognised that a limited range of attenuator values is sufficient to provide satisfactory performance, again because a receiver typically has a range of acceptable input levels. Thus in one embodiment the attenuator code indicates one of a predetermined set of attenuation values, preferably spaced at intervals of 1 dB, more preferably spaced at intervals of 5 dB. The attenuator code may comprise a simple colour code, each attenuation value having a different colour. This allows an installation technician to carry just a few different attenuator values and similarly reduces manufacturing costs as greater numbers of each attenuator value may be produced.

The invention also provides a computer system for selecting an attenuator for a receiver, the computer system comprising data memory storing data comprising a plurality of attenuator codes, each associated with a geographical area, input means for inputting data from a user; program memory storing program code for a processor; a processor, coupled to the data memory, to the input means, and to the program memory for implementing program code stored in the program memory, and output means, coupled to the processor, for outputting data to the user; the program code comprising code to, input geographical area data for a receiver, read an attenuator code associated with the geographical area of the receiver, and output the attenuator code to the user.

As described, for each geographical region there is a corresponding attenuator value, preferably selected from a limited range of values determined by the attenuator code.

According to a further aspect of the invention there is therefore provided a map for selecting an attenuator for a receiver, the map comprising, a plurality of geographical regions, each region having region identifier and an associated attenuator code for selecting an attenuator.

The map addresses the technical problem of selecting an attenuator for a receiver and allows this task to be performed by a relatively unskilled installation technician.

The applicant has further recognised that the geographical region in which the receiver is located may be determined using a mailing or street address or an address code such as a post code or zip code.

In another aspect the invention provides a computer system for selecting an attenuator for a receiver, the computer system comprising, data memory storing data comprising a geographical region identifier and corresponding region location data, input means for inputting data from a user, program memory storing program code for a processor a processor, coupled to the data memory, to the input means, and to the program memory for implementing program code stored in the program memory, and output means, coupled to the processor, for outputting data to the user, the program code comprising code, to input a geographical region identifier for the receiver, read the corresponding region location data from the data memory, determine an approximate distance of the receiver from a transmitter, determine an attenuator code using the receiver to transmitter distance; and output the attenuator code to the user.

In a preferred embodiment, the computer system calculates a distance from the receiver to a transmitter using an existing geographical database such as a post code or zip code database. The receiver post or zip code is used to determine an approximate location for the receiver allowing an approximate distance to the transmitter to be calculated. In this arrangement a position within the post or zip code area, such as a centroid of the area, is assumed for the receiver. Using an existing database avoids the problem of having to determine an exact location for the receiver. It is not essential that a post or zip code database is used as any database which can provide an approximate location for a receiver is suitable.

The attenuator code corresponding to an attenuator value may be calculated from the receiver-transmitter distance, as explained above, or it may be read from a database of geographical regions and corresponding attenuator codes which has been pre-prepared.

Such a database corresponds to the above described map and in this context"map"is to be understood as including electronically stored data for instructing a physical map.

Thus in another aspect the invention provides a database comprising geographical region identification data for a plurality of geographical regions, and for each region associated attenuation code data, for selecting an attenuator for a receiver. Thus an attenuator code for a geographical region in which the receiver is located may be determined by retrieving the code associated with the receiver's region from the database.

In a still further aspect the invention provides a method of generating such a map or database comprising selecting a geographical region, determining an assumed position for a receiver in the region, calculating a distance from the determined position to a transmitter, determining an attenuator code using a result of the calculation, storing the geographical region in a database in association with the attenuator code and/or inscribing the geographical region and the attenuator code onto a map, and then repeating the foregoing steps to generate a map and/or database for a plurality of regions.

In a preferred embodiment, the receiver forms part of a transceiver in a bi-directional communications system and the attenuator attenuates the level of input signal to the receiver input, but does not attenuate a transmitted output signal of the transceiver.

Thus in a further aspect the invention contemplates an rf transceiver having an antenna connected to a coupler, such as a circulator or duplexer, an rf transmitter connected directly to the coupler, and an rf receiver connected to the coupler via an attenuator. In an embodiment where the transceiver includes a downconverter, the attenuator is preferably connected between the downconverter and the coupler. In a still further aspect the invention provides a system comprising a base station transceiver and a plurality of subscriber transceivers, each subscriber transceiver having an attenuator selected as described above.

These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows a simplified block diagram of a known data-over-cable system using DOCSIS equipment ; Figure 2 shows a wireless local loop data communication system; Figures 3a and 3b show, respectively, service areas for a radio communications network, and received signal strength zones for an rf transmitter; Figure 4 shows an outline schematic diagram of a DOCSIS-based data transmission system incorporating a wireless local loop; Figure 5 shows a schematic diagram of an rf transceiver for the system of Figure 4; Figure 6 shows a flow diagram for a computer program for selecting an attenuator for the transceiver of Figure 5; Figure 7 shows a schematic diagram of a computer system for implementing the program of Figure 6; and Figure 8 shows a simplified embodiment of a map for use in selecting an attenuator for the transceiver of Figure 5.

Referring to Figure 2, this shows a concept and schematic diagram of a wireless local loop data transmission and reception system. A base station 202 comprises a base station transceiver (not shown) coupled to an antenna 204 for transmission and reception of rf signals from and to the base station. The base station is also coupled to a data communications network 214, such as a cable tv network, for providing data services, such as Internet services, to service subscribers. Domestic homes 206a-c each house data communication service subscriber equipment comprising a subscriber transceiver (not shown) coupled to a respective subscriber equipment transceiver antenna 208a-c. Further base stations which are also coupled to communications network 214, such as base stations 210 and 212 in Figure 2, are provided for service subscribers in other areas.

Figure 3a shows an idealised wireless DOCSIS communications system 300 comprising three base station transceiver radio towers 308, 310 and 312, and three corresponding subscriber service areas 302,304 and 306. Referring again to Figure 2, antenna 204 of base station 202 is located on a tower such as radio tower 308 and subscribers 206a-c are located within a corresponding service area, such as area 302. In Figure 3a, service areas 302,304 and 306 are shown as hexagonal cells but, in practice, these will have irregular and overlapping coverage areas.

Referring now to Figure 3b, this shows received signal strength zones for transmissions from a radio tower 322 of the type illustrated in Figure 3a. Contours 324 to 330 represent idealised contours of equal received signal level, boundary contour 324 corresponding to a minimum received signal level, or, equivalently, a maximum design transmission range. Each successive contour 326,328 and 330 represents a step, for example, 10 dB, rise in received signal level. Thus if, for example, boundary contour 324 corresponds to a received signal strength of-86 dB at LOGEZ contour 330 corresponds, in the example to-46 dB. In Figure 3b, there are four received signal strength zones, zones A, B, C and D. In zone D 0 dB attenuation (i. e. no attenuation) is required; in zone C 10 dB attenuation is required; in zone B 20 dB attenuation is required; and in zone A 30 dB attenuation is required. The steps in attenuator value depend upon the transmitter output power, the receiver sensitivity and input signal level range, and the zone size-in other embodiments attenuator values of 0 dB, 5 dB, 10 dB and 15 dB are employed.

Referring now to Figure 4, this shows an outline schematic diagram of equipment 400 located at a base station 202 and at subscriber premises 206 in a wireless local loop system employing DOCSIS standard cable modem apparatus.

The subscriber premises equipment 400 comprises a microwave antenna 408 coupled to a microwave transceiver 406; the equipment may also include a preamplifier and/or low-noise block downconverter (LNB) (not shown) in some applications. The equipment also comprises a conventional DOCSIS cable modem 116 and a home PC 118, as shown in Figure 1. In one embodiment microwave transceiver 406 interfaces to the DOCSIS cable modem 116 by means of a simple coaxial cable connection.

The subscriber end equipment from transceiver 406 onwards towards the customer corresponds to that used in a conventional cable network system. Cable modem 116 provides a conventional Cat 5 connection for use with a hub, switch or router, or other Internet protocol hardware or, as illustrated, personal computer 118. Other types of customer premises equipment, for example an Internet enabled television or set top box, can also be used. In a typical installation the interface to PC 118 comprises a Cat 5 cable connection to an Ethernet NIC (Network Interface Card).

In some embodiments antenna 408 is a simple printed patch antenna, but a patch array or yagi may be used where greater directionality is desired. Antenna 408 is preferably selected taking into account physical size, bandwidth and filter requirements. If necessary to reduce the bit error rate, additional filtering or equalisation can be included in microwave transceiver 406. Microwave transceiver 406 may also include an up converter and/or power amplifier (not shown) for return transmissions from the subscriber to the base station at microwave frequencies.

At base station 202 a base station microwave transceiver 402 is coupled to a base station antenna 404. As with transceiver 406 and antenna 408, up and down converters, preand power amplifiers, and filters and equalisation (not shown) may also be included. A less directional or omnidirectional antenna or antennas is (are) preferred for base station 202 because there is usually a need to transmit to subscribers located over a range of directions. Antenna 404 may therefore be, for example, a dipole antenna. The transmission power of the base station is generally higher than that of a subscriber's transceiver, because of the greater coverage required, and also because of the different frequency and data rates of the down and up links.

Microwave transceiver 402 interfaces to the CMTS 106 of a conventional cable network system of the type illustrated in Figure 1. For simplicity details of the cable network system are not shown in Figure 4 but the skilled person will understand that the system will include a DOCSIS-compatible Cable Modem Termination System (CMTS) 106, coupled to a network side interface 108 comprising an ATM core and to the Internet 114, as well as other elements shown in Figure 1. The cable network system will also include a server complex including a DHCP server to verify each subscriber's MAC address, a caching server, and other local and remote servers coupled to the ATM core.

CMTS 106 is provided with an appropriate physical interface to microwave transceiver 402, which will in general separate downstream and upstream data. The network side of CMTS 106 will generally be coupled to a switch. The link between transceiver 402 and CMTS 106 may comprise a coaxial cable or a fibre optic cable; the link from CMTS 106 to the ATM core normally comprises a fibre optic cable and may also include an rf link such as a microwave link.

The single-cable interface to microwave transceiver 402, carries rf frequency multiplexed data. At an appropriate electrical interface, that is at this or another physical interface where such a single cable interface to the cable tv system exists, microwave transceiver 402 can be coupled to the cable modem network system with substantially no modification to the network system. Thus, in effect, the base station and subscriber rf transceiver equipment replaces the wired cable network 10 shown in Figure 1.

Figure 5 shows subscriber end equipment comprising an rf transceiver 500 suitable for use with the transmission system of Figure 4. The transceiver comprises a transmit/receive antenna 502 (although separate antennas could be employed) coupled to a duplexer 504 which provides signals received on antenna 502 to a received signal output and which provides signals received at a transmit signal input to antenna 502.

The transmit signal input of duplexer 504 is coupled to a transmitter 512 which in turn is coupled to an rf interface of a cable modem 514. The received signal output of duplexer 504 is coupled to an attenuator 506 to attenuate the received signal level, and an output of attenuator 506 is coupled to downconverter 508, which in turn is coupled to a receiver 510. Receiver 510 is also coupled to the rf interface of cable modem 514.

Attenuator 506 is preferably located prior to any sensitive input circuitry in the received signal path. A data interface of cable modem 514 is coupled to a home PC, as shown in Figure 4, or to a switch, router or some other network device.

As used herein,"receiver"and"transmitter"refer to apparatus providing an rf interface for cable modem 414 and they need not provide an interface for baseband data.

In one preferred embodiment the rf signals received by and transmitted from antenna 502 have a frequency of approximately 10 GHz, although in other embodiments higher (or lower) frequencies may be employed. Where increased receive data rates are desired higher frequencies are used, such as, approximately 28GHz or 40 GHz. Downconverter 508 downconverts the received signal to a lower frequency, normally around 1 GHz, for input to receiver 510.

A single co-axial cable couples cable modem 514 to receiver 510 and transmitter 512.

This cable carries rf frequency domain multiplexed signals in a frequency range of 0750 MHz, that is, different frequencies or frequency bands are used for upstream and downstream channels to and from transceiver 500. The signals to and from the cable modem and hence the signal input to the transmitter and signal output from the receiver comprise rf signals rather than baseband signals. Thus the receiver effectively operates as a downconverter and the transmitter as an upconverter and power amplifier. In some embodiments the"transmitter"and"receiver"may be combined as a transceiver.

The cable modem 514 converts rf signals on the co-axial cable linking it to receiver 510 and transmitter 512 to Ethernet (physical layer) signals on unshielded twisted pair (UTP) data output carrying, at the transport layer, Internet protocol (IP) data.

Cable modem 514 operates as defined in the DOCSIS standard. According to this standard, the cable modem adjusts the transmit power according to the received signal level, increasing the output power when the received input signal is at a lower level, with the aim of maintaining an approximate 0 dB signal level at physical interface 104 of Figure 1. As shown in Figure 5, attenuator 506 is located in the received signal path but not in the transmitted signal path. The effect of this is to ensure that cable modem 514 provides a high level transmit output signal, which is desirable for a wireless data transmission system as, generally speaking, such systems are more prone to unpredictable signal losses, particularly in the return path to the base station transceiver, as compared with wired cable systems.

Referring now to Figure 6, this shows a flow chart for a computer programme for selecting an attenuator for the transceiver of Figure 5.

At step S10 a postcode for a district or region in which a receiver (or transceiver) is located is entered into the programme. The programme then, at step S 1, uses the postcode to access a postcode database to retrieve a grid reference for the region or district at which the receiver (or transceiver) can be assumed to be located. The grid reference identifies a point within the region or district, preferably relatively centrally located, for example a centroid of the district. The information comprising such database is available commercially and on the Internet, for example, at www. streetmap. co. uk. A grid reference for the receiver's base station transmitter is then input into the programme. Alternatively, the base station transmitter grid reference can be determined using a second database in which a transmitter grid reference is associated with each postcode.

The programme then calculates, at step Sol3, the distance between the postcode grid reference and the base station grid reference, which approximates to the transmitter

receiver distance, and at step S 14, then calculates the base station transmitter to receiver path loss. The free space path loss may be calculated by PL (dB) 92. 5 + 20 log f+ 20 log d where F is the frequency in gigahertz (GHz) and d is the distance in kilometres between the transmitter and the receiver (where d is in miles"92. 5" should be replaced by "96. 6").

At step S 15 the programme then calculates the difference between the path loss to a receiver at the system's maximum design range (5 kilometres in the example shown in Figure 6) and the signal at the actual (assumed) receiver location. This difference represents the excess signal received by a receiver in the region or district as compared with the signal received by a receiver at the maximum range, and hence corresponds to the degree of attenuation required in the received signal path to provide a received signal which is the same as that which would be received by receiver at the maximum range. For example, at 10 GHz the path loss to a receiver at 4.68 kilometres is 130 dB and the transmitter of the base station and the receiver of the transceiver of Figure 5 are setup to provide 0 dBm into cable modem 514 at this range. At 1. 34 kilometres the path loss is 115 dB and thus an additional 15 db of attenuation is required to provide the same 0 db nominal signal level into cable modem 514.

At step S 16 the difference in path loss corresponding to the required attenuator value, is rounded down to the nearest 5db so that the received signal is slightly above, rather than below, the optimal level. Finally, at step S 17, the programme outputs either the required attenuator value or other attenuator code such as green for a 5db, amber for a 1 Odb attenuator and red for a dim db attenuator. Outputting an attenuator code is a preferable option when a programme is run on a basic hand held terminal for use by an installation technician.

In an alternative embodiment at step S 15 the programme calculates the difference between a signal level at the receiver and a signal level at a receiver at the system's maximum design range from a given radio tower. To calculate a signal level at a receiver the base station transmit power level is needed and this can be retrieved from a database of base station EIRP (Effective Isotropic Radiated Power) power levels accessed, for example, using the base station grid reference input at step Sol2. If desired the base station EIRP can then be adjusted to take account of the rain zone in which the base station is located, either automatically based upon the base station grid reference, or manually based upon manual input of the rain zone. Rain zones are graded A to H according to the expectation of rain in the zone and each rain zone corresponds to a figure, in db, by which the base station transmitter EIRP must be reduced to provide a target system availability of 99.9%. In the UK rain zone makes only a small difference, approximately 2-3 db. The base station transmitter power, optionally adjusted according to the rain zone, may then be used to calculate the expected signal level at the receiver and the expected signal level at 5 kilometres and the difference between these two signal levels may then be used to calculate an attenuator value suggested for the receiver, as set out above.

Referring now to Figure 7, this shows a schematic diagram of a computer system 700 suitable for use with a programme implementing the steps of Figure 6. In one embodiment the computer system is a small, hand held terminal suitable for outdoor use by an installation technician. The computer system 700 includes a keyboard 702 and pointing device 706 such as a touch pad, for data input by a user and a display 704 for output to the user. The system also comprises working memory 708, data memory 712, programme memory 714 and a central processor 716. All these components are linked by a data and communications bus 710. Data memory 712 comprises a postcode database for looking up a grid reference using a postcode and, optionally, also comprises a base station EIRP database as described above. Programme memory 714 comprises receiver setup code to perform the programme steps illustrated in the flow chart of Figure 6. This code is implemented by processor 716 as receiver setup application 716a. Working memory 708 is volatile memory such as RAM whilst data memory 712 and programme memory 714 is preferable non-volatile memory such as ROM.

Referring now to Figure 8, this shows a map 800 for use in selecting an attenuator for the transceiver of Figure 5. A base station radio tower 802 is located in a first postcode district 804 marked with attenuation code"A"corresponding to a first attenuation value, for example, 15db. Postcode districts 806, which are further from radio tower 802, are marked with attenuation"B"corresponding to a second, lower attenuator value, such as lOdb. Postcode districts 808 are still further from radio tower 802 and are marked with

attenuator code"C"corresponding to a still lower attenuator value, such as 5db, and postcode districts 810, further still from radio tower 802, are marked with attenuator code"D"which, in this example, corresponds to no attenuation. Broadly speaking postcode districts 804,806, 808, and 810 correspond to zones A, B, C, D of Figure 3b.

In a preferred embodiment the attenuator codes"A"to"D"are indicated by colours, as described above, with code"D"being left uncoloured or white.

The map is used by reading off an attenuator code corresponding to a postcode or zip code for an area in which a receiver or transceiver is located. An attenuator determined by the code is then selected and installed in the receiver or transceiver.

A map such as shown in Figure 8 may be produced by repeated application of the process of Figure 6 for each separate postcode district, either manually or automatically, for example by means of computer program.

No doubt many other effective alternatives within the spirit and scope of the present invention will occur to those skilled in the art, and it should be understood that the invention is not limited to the described embodiments.

Claims (25)

1. CLAIMS: 1. A method of selecting an attenuator for a receiver comprising: determining a geographical region in which the receiver is located; determining an attenuator code for the receiver's region; and selecting an attenuator using the attenuator code.
2. 2. A method as claimed in claim 1 wherein said attenuator code indicates one of a predetermined set of attenuator values and said selecting selects an attenuator from a set of attenuators comprising attenuators with attenuation values in said predetermined set.
3. 3. A method as claimed in claim 2 wherein said set attenuation values comprises attenuation values spaced at intervals of 1 dB or greater.
4. 4. A method as claimed in claim 3 wherein said set of attenuation values comprises attenuation values spaced at interfaces of 5 dB.
5. 5. A method as claimed in claim 1, 2,3 or 4 wherein said geographical region is determined using an address or address code for an approximate location of the receiver.
6. 6. A method as claimed in claim 5 wherein said geographical region is determined using a post or zip code for the receiver's location.
7. 7. A method as claimed in any preceding claim wherein said attenuator code is determined using a map of geographical regions and corresponding attenuator codes.
8. 8. A method as claimed in any one of claims 1 to 6 wherein said attenuator code is determined by calculating a distance from the receiver to a transmitter from which it receives signals.
9. 9. A method as claimed in claim 8 wherein said calculating comprises determining an assumed position for the receiver using location data for the geographical region of the receiver.
10. 10. A method as claimed in any preceding claim wherein the attenuator code comprises a colour code.
11. 11. A computer program to, when running, carry out the method of any preceding claim.
12. 12. A computer readable medium carrying the computer program of claim 11.
13. 13. A computer system for selecting an attenuator for a receiver, the computer system comprising: data memory storing data comprising a plurality of attenuator codes, each associated with a geographical area; input means for inputting data from a user; program memory storing program code for a processor; a processor, coupled to the data memory, to the input means, and to the program memory for implementing program code stored in the program memory; and output means, coupled to the processor, for outputting data to the user; the program code comprising code to; input geographical area data for a receiver; read an attenuator code associated with the geographical area of the receiver; and output the attenuator code to the user.
14. 14. A computer system as claimed in claim 13 wherein each said attenuator code indicates one of a discrete set of attenuator values.
15. 15. A computer system for selecting an attenuator for a receiver, the computer system comprising: data memory storing data comprising a geographical region identifier and corresponding region location data; input means for inputting data from a user; program memory storing program code for a processor; a processor, coupled to the data memory, to the input means, and to the program memory for implementing program code stored in the program memory; and output means, coupled to the processor, for outputting data to the user; the program code comprising code to: input a geographical region identifier for the receiver; read the corresponding region location data from the data memory; determine an approximate distance of the receiver from a transmitter; determine an attenuator code using the receiver to transmitter distance; and output the attenuator code to the user.
16. 16. A computer system as claimed in claim 15, wherein the program code further comprises code to determine an approximate transmitter signal strength at the receiver or a path loss between the receiver and a transmitter, to determine the attenuator code.
17. 17. A computer system as claimed in claim 15 or 16, wherein said attenuator code indicates one of a predetermined set of attenuation values and wherein said program code to determine an attenuator code comprises code to select an attenuation value from said set and to determine a corresponding attenuator code.
18. 18. A computer program comprising the program code of any one of claims 15 to 17.
19. 19. A computer readable medium carrying the computer program of claim 18.
20. 20. A map for selecting an attenuator for a receiver, the map comprising: a plurality of geographical regions, each region having region identifier and an associated attenuator code for selecting an attenuator.
21. 21. A map as claimed in claim 20 wherein each said attenuator code indicates one of a discrete set of attenuator values.
22. 22. A map as claimed in claim 20 or 21 wherein said attenuator code comprises a colour code.
23. 23. A computer readable medium storing data comprising a map as claimed in claim 20, 21 or 22.
24. 24. Use of the map of any one of claims 20 to 22 to select an attenuator for a receiver.
25. 25. A method, computer system, computer program or map substantially as hereinbefore described with reference to one or more of Figures 3 to 8.