GB2280087A

GB2280087A - Cellular radio system with hopping

Info

Publication number: GB2280087A
Application number: GB9313851A
Authority: GB
Inventors: Kevan Hobbis; Paul Golding
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1993-07-05
Filing date: 1993-07-05
Publication date: 1995-01-18
Anticipated expiration: 2013-07-05
Also published as: FI943217A0; CA2126947A1; GB9313851D0; GB2280087B; FI943217A; SE9402345D0; DE4423582A1; SE9402345L; JPH07154851A

Description

CELLULAR RADIO SYSTEM WITH HOPPING

Field of the Invention

2280087 This invention relates to a cellular radio system comprising a plurality of base stations, each base station having means for hopping from frequency to frequency. Separately and in addition, the invention relates to a base station.

BackL-round of the Invention Current GSM (Groupe Speciale Mobile) base station transceiver systems (BTS's) are typically multi-carrier units which can implement base - band hopping of channel data. Base-band hopping is the ability for data associated with a logical air interface channel to be hopped across all or some of the physical channels (the radio frequency carriers) to improve system performance.

There are two ways of frequency hopping channel-coded data. In the first method channel-coded data associated with one channel is always transmitted from the same radio, but is transmitted at a different frequency on a per GSM-timeslot basis. This is achieved by retuning the radio for each timeslot. The other method is to not retune the radio, but instead switch the channel-coded data between radios on a per GSMtimeslot basis so that it is still transmitted at a different frequency on a per GSM-timeslot basis. Both of these implementations, or a combination of the two, can take place within one BTS The first method is called Fast Synthesiser Hopping and the second called Baseband Hopping.

There are difficulties associated with hopping at the radio frequency band. Hybrid combiners (passive devices) are used to combine two carriers together to the same antenna, but each incurs a 3dB loss. When more than three carriers are combined then the loss seriously affects the output power at the antenna. When more than three carriers are combined then a remote tuned combiner (RTC) is used reducing the loss per carrier to less than 3dB. However the RTC cannot re-tune on a per timeslot basis. For this reason base band hopping is used, so that the RF combining is simpler and therefore not as much loss is incurred, 0 such that the power at the antenna is closer to the actual PA output power.

Accordingly, radio transceiver modules are generally provided within the same base station transceiver system, each radio transceiver module having a transmitter tuned to a different carrier.

Baseband hopping involves switching channel coded data from a channel coder in a base station transceiver system to any one of the radio transceiver modules within the same base station transceiver system.

A transceiver architecture which provides a predefined number of radio transceiver modules within a single base station transceiver system, to enable base-band hopping between those modules, is infleNible. Currently four or five radio transceiver modules are generally provided in a single base station transceiver system in a single cabinet. To provide more than five such modules would require a larger cabinet. To provide fewer than four modules would be uneconomic in a single cabinet designed for four or five modules.

Reference will be made herein to CEPT which stands for "Comit6 Europ6en des Postes et T616communications" and is a standardizing authority.

:K 0 Summary of the Invention

0 0 0 0 According to the invention, there is provided a base station of a cellular communication system comprising serial interface means for direct connection over a time divided serial bus (e.g. a CEPT hnk) to a base station controller, means for receiving traffic data from the base station controller through said serial interface means and a channel coder for channel coding the traffic data and selectively outputting it onto the serial bus for retrieval by another base station.

In this manner, the serial link that connects the base station to the base station controller is used for carrying channel coded traffic data between base stations. This arrangement enables a single carrier base station to be connected to a CEPT 2 Mbps link and for other base stations to be connected to the link, with base-band hopping data being transferred between the base stations, thereby allowing the base stations to hop as a group, even though not physically coupled by any connection other than the CEPT link.

It is through the time divided serial bus and the base station controller that the base stations communicate with the outside world, in the form of a connection fi-om the base station controller to a switch of a public switched telephone network.

The base station alternatively transmits the coded data as a radio signal, depending upon a hopping sequence.

The invention also provides a cellular radio system comprising a plurality of base stations, a base station controller for controlling the base stations and a data communications link communicating between the base station controller and the base stations, wherein each base station comprises means for hopping from frequency to frequency through a hopping sequence, characterised by means for outputting hopping information to the link for informing the base stations of the hopping sequence of each base station and means in each base station for receiving this information and controlling its hopping sequence accordingly.

A detailed description of a preferred embodiment of the invention will be given, by way of example only, with reference to the drawings.

1 Brief Description of the Drawings

Fig. 1 shows a prior art BTS implementing base-band hopping.

Fig. 2 shows a number of base stations in a system according to the preferred embodiment of the invention.

Fig. 3 shows part of a time division multiplex slot allocation on the air interface and the serial bus interface of Fig. 2.

Fig. 4 shows a table of time slot allocations as recorded by a base station controller of Fig. 2.

Detailed DescriTAion of the Preferred Embodiment In the prior art base station transceiver system of Fig. 1, there are provided a number of radio channel units (RCUs), 10, 11 connected through digital radio interfaces (DRI's) 12 and 13 to a pair of 64 Mbps MM buses 15. Also connected to these buses are a kiloport switch 16 and a Megastream (trademark) interface 17. The Megastream interface (MSI) 17 is connected to a CEPT link 18 for communication with a public switched telephone network (PSTN).

The entire apparatus of Fig. 1 is enclosed in a single cabinet. In addition to the RCUs 10 and 11, there are two or three further RCUs and DRI's.

Each DRI has a channel coder and decoder (not shown). As shown, one of the buses 15 is an output bus and the other is an input bus. To achieve baseband hopping, traffic data is received by the DRI 12 from the WM bus 15. It is channel coded and re-transmitted to the TDM bus 15. After routing through the KSW 16 it is then received by another DRI 13 for transmission by the RCU 11. In this manner a given channel coder always encodes the traffic for a given logical channel. The coded data is routed to one of a number of RCUs each of which is transmitting on a fixed frequency. Hence the channel coded data is hopped through a sequence of frequencies by routing to the appropriate RCUs for transmission.

The channel coder also deals with the other half of the same conversation by virtue of the fact that the RCU 10 connected to the DRI 12 re-tunes it's receiver on a per timeslot basis in the same sequence as the switching of the channel coded data between RCUs. In this manner the channel coder on the DRI 12 always deals with both halves of the same conversation for a given logical channel.

The above arrangement is inconvenient because the need for the dual buses 15, the megastream interface 17 and the kilo port switch 16 makes the architecture inflexible.

Referring to Fig. 2, a preferred embodiment of the invention shown in which a number of radio channel units (RCUs) 20, 21 and 22 are coupled to a single CEPT serial time divided link 23 via an interface 40, 41, 42 respectively. Each of these RCU's is effectively a self-standing base station. Each has an RF transceiver 24 (comprising a receiver 24' and a transmitter 24"), a front end processor (FEP) 25 and eight channel coders and decoders, of which two are shown 26 and 27.

It will be appreciated that a number of base stations 20 to 22 can be connected to the CEPT link 23, three being shown as a mere example. Also connected to the CEPT link 23, is a base station controller (BSC) 30 which includes means for transcoding between coded speech and digitized or analog speech and also includes a connection to a switch of a public switched telephone network. It will be appreciated that the BSC 30 is generally located remote from the base stations 20, 21 and 22. It should also be noted that the base stations 20 to 22 themselves may be dispersed geographically. The transcoding means may be remote from the BSC, located at or near the connection into the PSTN (the "switey').

In the diagram of Fig 2, there are shown dotted arrows indicating the flow of data. There are eight logical channels numbered 0 to 7. Codec 26 handles logical channel 0. Codec 27 handles logical channel 1 etc.

The example is given of logical channel 0.

Outbound traffic data arrives from the transcoder at the BSC 30 at a data rate of approximately 13 Ups. This data, speech coded but not channel coded, resides on the CEPT bus 23 in certain predetermined time slots described below. One of the base stations, in this case, base station 2 1, identifies itself as having responsibility for channel coding of that channel. This responsibility is indicated to base station 21 from BSC 30 which acts as a "master' for control of the base stations. Channel coder 31 codes the traffic data by adding error correction coding and by interleaving the data over time, and it outputs the channel coded data at approximately 22 Ups onto the CEPT link 23. This channel coded data is referred to as base-band hopping data (13BH data). The % 0 0 0 BBH data is now presented on the link 23 for retrieval by any one of the base stations connected to the link. Base station 20 identifies itself as currently having responsibility for transmission of logical channel 0 and accordingly base station 20 retrieves the BBH data and this is input to front end processor (FEP) 25. FEP 25 merely controls the passing of the data to RF unit 24 for direct transmission on the appropriate GSM slot. Meanwhile, RF transceiver 24 is tuned by FEP 25 to receive the corresponding in bound physical channel for that particular logical channel. This tuning of RF transceiver 24 is fast synthesizer hopping and is relatively simple on the receive carrier.

The received inbound physical channel is passed to channel decoder 26 responsible for logical channel 0 and channel decoder 26 performs reverse interleaving and performs error correction to produce traffic data at 13 1-bps, which it outputs onto the CEPT link 23, for retrieval by the base station controller 30.

On a frame by frame (or timeslot by timeslot) basis, FEP 25 causes transceiver 24 to retune to a difference receive frequency. The incoming channel coded traffic data on the new frequency is again passed to channel decoder 26 for decoding of logical channel 0 (on different timeslots, to channel decoder 27 etc for channel decoding of the different channels). At the same time, a different one of the base stations 20 to 22 retrieves the BBH data for the corresponding outbound channel from the link 23 and transmits the outbound traffic data. Thus channel coder 26 continuously handles the downlink half of the logical channel 0 conversation. The other half of the conversation is received through the RF base station, by re-tuning the receiver, and hence channel coder 26 always deals with both halves of the same conversation The other half of the conversation is received through the RF part of base station 21 and decoded via channel coder 31. Meanwhile, channel coder 26 is continuously decoding the inbound half of a conversation received through RF part 24 of base station 20 and continuously encoding the other half of that conversation, for transmittal fi-om the various base stations on a per - timeslot or fi-ame by frame basis.

A number of base sites 20 to 22 hop in synchronism under the control of the BSC 30. In the case will be described where four base stations (20 to 22 plus one additional base station not shown) hop together as a set.

1 0 For reasons explained below, in the case of a 2 Mbps CEPT link, four base stations are the maximum number that can hop in a set on a single CEPT link. The base station controller 30 defines the set of base stations in the hopping sequence, defines the starting point for hopping and defines the order of hopping. The frequencies on which the base stations will hop are determined by the predefined frequencies of the four base stations. Hopping is conducted in rotation by each base station retrieving base-band hopping data from different timeslots on the link 23 in rotation. Each base station transmits. in rotation, different logical channels. The receiving part of the RF section of each base station is retuned on each hopping step and a single logical channel is received through that receiver section over a discontinuous frequency, that is to say, a frequency that hops through four frequencies in rotation. In the meantime, the other half of the conversation is being transmitted by a different base station in rotation. That other half of the conversation is, however, always coded by the same channel coder that performs the decoding for the first half of the conversation.

Referring to Fig. 3, the time divided data on the air interface for one of the base stations and for the CEPT link is shown. The air interface 10 1 is divided into frames of 8 slots per frame, forming eight logical channels. One GSM timeslot is equivalent to five or four CEPT frames 103.

The CEPT link 102 supports two Mbps. The GSM interface 101 requires 22 kbps. In addition to the single logical channel shown in Fig. 3, there are an additional seven logical channels for one base station and there are four base stations on a CEPT link.

A CEPT link is divided into thia-ty two timeslots per frame giving thirty two 64 kbps channels, which are utilized for data and link synchronization according to the CEPT format. Accordingly to this format, thirty timeslots are available for traffic and two are reserved for link synchronization and control. In the present system, these thirty timeslots are used as follows. For each base station, five slots are used for base-band hopping data and two for traffic data. This provides 64kbps full duplex data for traffic channels (at approximately 13k-bps per channel). It also provides 3201,-bps full duplex for transfer of baseband hopping data between BTS's. This extra bandwidth is required for per timeslot RF control information such as power level, etc and also includes a eydical redundancy check so that the integrity of the information can be checked on receipt at the transmitting FEP/RCU. Fig. 3 shows five slots shaded in the CEPT link 102. These five slots are used for base-band hopping for one base station.

Fig. 3 also indicates timeslot 16 in the centre of one of the frames 103. Timeslot 16 is used for control information. In the preferred embodiment of the present invention, this slot is also used for hopping control information.

Fig. 4 shows a table stored at BSC 30. The table shows the timeslots allocated to four base stations, A, B, C and D- Base station A is allocated timeslots 1 to 7 of the CEPT link. Two of these are for traffic data and five are for base-band hopping data. Base station B is allocated slots 8 to 14, shown in shaded outline in Fig. 3. Base station C is allocated slots 15 and 17 to 22. Slot 16 cannot be allocated because it is a control slot. Base station D is allocated slots 23 to 29. Slots 30 and 31 are vacant. Slot 0 is required for data link synchronization.

At the start of a hopping sequence, the base station controller transmits the table of Fig. 4 to each of the base stations 20 to 22 etc. Thisinformation informs each base station that there are four base stations in the hopping sequence and informs each base station as to where on the CEPT link the necessary base-band hopping data can be found. At the same time, the BSC initiates hopping by indicating the start of the hopping sequence. All of this information is transmitted on timeslot 16 of the CEPT link, With this information available, the four base stations can commence hopping in rotation on four allocated frequency carriers. Each receiver is tuned in rotation to the four received frequencies. B aseband hopping data is retrieved by a given base station from one of the four sets of timeslots shown in Fig. 4 in rotation. All channel coders at a particular base station output their channel coded data (base band hopping data) on the same slots on the CEPT link. The channel coders utilise the CEPT slots in rotation in synchronism with the air interference timeslot timing. This data is, however, retrieved by a different base station at each point in the hopping sequence. Thus, a different base station transmits that data on its particular frequency at each point in the hopping sequence.

Each of the sets of slots shown in Fig. 4 supports eight logical channels. All eight logical channels on a given physical channel hop together between frequencies. All eight channel coders or any 1 0 0 -g- combination thereof can likewise hop together through independent sequences. In the case of a logical channel that is not hopping the baseband hopping data can be sent to the CEPT and always read by the local FEP such that no hopping occurs. It is not necessary that a hopping set comprises four base stations. The architecture has the great advantage that any number of base stations can be added to the link up to a maximum of four. If a link of greater capacity is available, more than four base stations can be supported. Consider the case where there are three base stations in a hopping sequence, coupled to a single link and a fourth base station is added to the link, for example due to expansion of the network or filling of a gap in the network. In this case, the new base station identifies that slots 23 to 29 are vacant and transmits a signal to the BSC 30 over the control channel indicating that the new base stations wishes to use those slots and wishes to join the hopping set. The BSC sends its signal on the control channel to all the base stations informing the base stations that the hopping set now comprises four base stations and indicating the channel numbers of those four base stations. The BSC also indicates which timeslots are allocated to which base station. The BSC also instructs the base stations as to their respective starting channels. From this starting point for hopping, the base stations commence hopping in rotation.

There has now been described a novel use of a CEPT interface to carry channel encoded data from one base station to a remote base station in order to perform base-band hopping. The dynamic allocation of CEPT timeslots has been described for carrying base-band channel coded data in order to minimise the total delay of voice data through the local and remote base stations. Base-band hopping data is transmitted over the GSM A - bis interface. This feature allows the CEPT link to be used to switch the base-band hopping data between remote sites.

In order to keep the delay through the system at a minimum, the timeslots used for base-band hopping are allocated to each base station in a manner such that when base-band data becomes available, it is placed onto the CEPT line in the next available timeslot.

0

Claims

1. A base station (20, 21, 22) of a cellular communications system comprising serial interface means (40) for direct connection over a time divided serial bus (23) to a base station controller (30), means (3 1) for receiving traffic data from the base station controller through said serial intelface means, a channel coder (31) for channel coding the traffic data and selectively outputting it onto the serial bus for retrieval by another base station.

2. A base station according to claim 1, wherein the serial interface means are arranged to receive from said serial bus channel coded traffic data and means (24") are provided for transmitting that data as a radio signal.

3. A base station according to claim 2, wherein the traffic data is outbound traffic data of a given two-way communication and wherein a radio receiver (24') is provided for receiving channel coded inbound traffic data for that communication, and a channel decoder (26) is provided for decoding the inbound traffic data and outputting channel decoded inbound traffic data to the base station controller over the serial bus (23).

4. A base station according to claim 3, wherein the channel coded inbound traffic data comprises frames of data and the base station comprises hopping control means (25) coupled to the radio receiver (24') for tuning the radio receiver to receive different frequencies from frame to frame and also coupled to the serial interface means (40) for changing the timing of receipt of channel coded traffic data from the time divided serial bus, so as to receive channel coded traffic data from a channel coder of a different base station connected to the serial bus, thereby enabling hopping of inbound and outbound fi-equencies to be synchronised between base stations.

A cellular radio system comprising a plurality of base stations (20, 21, 22), a base station controller (30) for controlling the base stations and a data communications link (23) communicating between the base station controller and the base stations, wherein each base station comprises means (25) for hopping from frequency to frequency through a hopping sequence, characterised by means (30) for outputting hopping information to the link 1 0 for informing the base stations of the hopping sequence of each base station and means (25) in each base station for receiving this information and controlling its hopping sequence accordingly.

6. A system according to claim 5, comprising a finite set of base stations, wherein the base station controller outputs onto the Iink information including a start frequency for a predetermined hopping sequence and a sequence length, where the sequence length is related to the number of base stations in the set.

7. A system according to claim 6, wherein the finite set consists of three or four base stations.

8. A system according to claim 5, wherein the radio system is a time division multiple access system and the means for hopping are arranged to hop from frequency to frequency between frames of time slots, and wherein each base station comprises means (24) for receiving encoded interleaved traffic data over an r.f. interface and decoding and de-interleaving that traffic data and outputting the decoded de-interleaved traffic data onto the link (23) to the base station controller, characterised that each base station has means (26, 3 1) for (a) receiving uncoded. non- interleaved traffic data from the link (b) outputting encoded interleaved traffic data to the link and (c) receiving encoded interleaved traffic data from the link for transmission over the r.f. interface, whereby the system is adapted to control a base station to continually accept outbound traffic data and encode that traffic data even when the traffic data is to be transmitted by a different base station in the course of hopping.