WO2004032548A1

WO2004032548A1 - Cellular radio telecommunication system

Info

Publication number: WO2004032548A1
Application number: PCT/GB2003/004294
Authority: WO
Inventors: Nicholas Dougall Johnson; Neil Philip Piercy; Christopher Moore
Original assignee: Ip.Access Limited
Priority date: 2002-10-04
Filing date: 2003-10-06
Publication date: 2004-04-15
Also published as: GB0323325D0; GB0222999D0; GB2394864A; AU2003274303A1

Abstract

There is described a cellular radio telecommunication system for reducing interference (both intra-cell and inter-cell) within indoor (40), and between indoor and outdoor cellular radio telecommunication coverage areas. The system comprises a plurality of head units (20) throughout the indoor coverage area (40) each being connected to a master unit (10) such that, in combination, they are seen by User Equipment (UE) (50) as a single base station using a single carrier. The master unit (10) continuously ranks the head unit’s (20) by comparing their uplink quality indicators (e.g. by received power or by signal quality) and selectively routes signaling and data traffic via the highest ranked head unit(s) (20). Accordingly, the transmit power of each head unit (20) and UE (50) can be independently controlled and (24) minimised. Interference is therefore reduced by (i) ensuring minimal transmit powers; and (ii) reducing the number of cell boundaries within the indoor coverage area.

Description

Cellular Radio Telecommunication System

This invention relates to a cellular radio telecommunication system, and in particular, but not exclusively, to a system for reducing interference within indoor, and between indoor and outdoor, cellular radio telecommunication coverage areas.

Cell planning in a cellular network is problematic in situations where indoor users are covered by both outdoor and indoor base stations. The existence of walls, ceilings and other obstructions which cause attenuation is compensated for by increasing the transmit power of the Base Stations (BS) and the User Equipment (UE) that is communicating with the BS . In the outdoor environment, the increased BS transmit power increases the interference between neighbouring cells (outdoor inter-cell interference) . It also increases the interference power as measured by every other UE within the same cell (outdoor intra-cell interference) . In the indoor environment, the increased UE transmit power 1 increases the interference as measured by an indoor

2 BS that may be operating on a similar frequency. 3

4 A more efficient solution is to provide cover for

5 indoor UE's from indoor BS's and provide cover for

6 outdoor UE's from outdoor BS's. In this case the

7 outdoor cells do not have to increase their transmit

8 power to reach indoor UE's with a corresponding

9 reduction of outdoor intra-cell and outdoor inter- 10 cell interference.

11

12. However, as a consequence, the interference between

13 outdoor BS and indoor BS (the outdoor to indoor

14 inter cell interference) will increase as the indoor

15 traffic amount increases . The worst case scenario

16 is that service levels and coverage as provided by

17 an outdoor BS is reduced in a zone located close to,

18 but just outside of, the indoor environment. 19

20 .The size of the zone, in which indoor to outdoor

21 inter cell interference is service affecting, can be

22 controlled by indoor cell location and indoor cell

23 transmit power. Indoor cell transmit power is a

24 factor of the indoor cell radius. Therefore, a

25 larger number of smaller radius indoor cells can be

26 used to provide indoor coverage with reduced

27 transmit power and thus reduced indoor to outdoor

28 inter cell interference. 29

30 The outdoor cellular network is disturbed least if a

31 large number of small radius cells are deployed in

32 the indoor environment. Unfortunately, in this scenario, the indoor environment suffers from an increase in the indoor inter cell interfence due to the large number of cell boundaries.

For a Code Division Multiple Access (CDMA) based system, intra-cell and inter-cell interference reduces cell capacity and range and increases the probability of call drop-out. Reduction of inter- cell interference and intra-cell interference is critically important in CDMA cellular systems due to the small number of carriers available for frequency planning.

According to a first aspect of the present invention there is provided a cellular radio telecommunication system for reducing cell phone signal interference in a cellular radio telecommunication network, said system comprising a plurality of head units (20) spatially separated throughout a first coverage area (40) , wherein each head unit (20) is connected to a master unit (10) and, in combination, the head units (20) and the master unit (10) operate as a single base station using a single carrier.

Preferably, cell phone signal interference is reduced both within the first coverage area (40) and between the first coverage area (40) and a second coverage area.

Preferably, the first and second coverage areas are indoor and outdoor coverage areas respectively. Preferably, the system employs Code Division Multiple Access (CDMA) based air interface.

Preferably, the master unit (10) provides co- ordination and control functions to each head unit (20) .

Preferably, each head unit (20) transmits identical common control downlink channel information.

Preferably, both downlink and uplink signals of each head unit (20) are offset in time relative to those of the other head units (20) in the system.

Preferably, the timing offsets are controlled such that the earliest arriving signals and latest arriving signals at a User Equipment (UE) (50) within the coverage area have a time offset which is less than or equal to the maximum time delay expected by the UE (50) .

Preferably, the phase or timing reference for the downlink channels are dedicated pilots.

Preferably, the master unit (10) ranks the head units (20) by comparing uplink quality indicators.

Preferably, the uplink quality indicators used to rank the head units (20) are the power received and/or signal quality received. Preferably, the master unit (10) routes dedicated downlink channel information to a UE (50) via one or more of the highest ranked head units (20) .

Preferably, one or more of the head units (20) with the highest ranking independently receive uplink dedicated signals from a UE (50) .

Preferably, the master unit (10) continuously re- ranks the head units (20) as a UE (50) moves through the coverage area (40) .

Preferably, the head units (20) perform downlink channel power control independently of the other head units (20) .

Preferably, the downlink channel power control at a head unit (20) is optimised in response to the signal strength received from a UE (50) at said head unit (20) .

Preferably, the head units (20) perform uplink channel power control independently of the other head units (20) .

Preferably, the uplink channel power control at a head unit (20) is optimised in response to the signal strength received from a UE (50) at said head unit (20) .

Preferably, PRACH acknowledgement is performed independently at one or more head units (20) . Preferably, PRACH acknowledgement is performed independently at one or more head units (20) .

Preferably, two or more head units (20) receive signalling and data traffic from a UE (50) in order to increase signal diversity and thereby improve signal reception.

Preferably, uplink data is routed to the master unit (10) via two or more head units (20) , the data from each head unit (20) being constructively combined thus reducing its error rate.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing in which :-

Fig. 1 shows a schematic of a system for reducing interference relating to indoor cellular radio telecommunication networks.

As shown in Fig. 1, there is provided a Master Unit 10 connected to a plurality of Head Units 20. The Head Units 20 look similar, in terms of air interference, to cells in a normal cellular deployment. The Master Unit 10 provides co- ordination and control functions to the Head Units 20. The combination of multiple Head Units 20 and the Master Unit 10 operate as a single "distributed" base station and use a single carrier. In use, the Master Unit 10 controls the Head Units 20 an such a way that the intra-cell interference is reduced, the probability of call dropout is reduced and the inter-cell interference between indoor and outdoor cells is reduced without sacrificing capacity. Each head unit 20 provides a coverage area 30 that is as a subset of the total coverage area 40 of the cell. A UE 50 that is within the total coverage 40 of the cell will also be within the coverage area 30 of one or more of the head units 20.

The invention is particularly applicable to CDMA based cellular systems. The following description is specific to UTRAN (both FDD and TDD modes of operation) , however it is equally applicable to other air interface standards based upon CDMA. The UTRAN term for base station is "nodeB" and the distributed base station that is the subject of this invention will be referred to in this context as a "distributed nodeB" .

The time synchronisation between head units 20 must be controlled in order to ensure that the signals transmitted by several head units 20 and received at a single UE 50 can be constructively combined at the UE 50. The carrier phase of multiple head units 20 cannot be synchronised and so perfectly time-aligned signals may interfere. The minimum resolvable distance between multi-path components at the UE 50 is 1 chip period (each chip period is approximately 260ns in UTRAN FDD) , so signals must be transmitted from each head unit 20 at least 1 chip period away from any other head unit's signal. The maximum time offset between head units is determined by the maximum delay spread expected by the UE 30. This small and controlled time offset between head units 20 introduces time diversity into both the uplink and downlink paths. To the UE 50, this time diversity appears as delay spread in the downlink channel and the UE's 50 receiver constructively combines the signals.

The Head Units 20 are, like normal UTRAN nodeBs, equipped with the ability to receive, process and report uplink signals .for UEs 50 transmitting to them. In addition, they also have the ability to receive, process and report signals when they are idle, in order to sense active transmissions which are being handled by nearby base stations. The Head Units 20 do not have standard lub interfaces and do not implement the NodeB Application Part (NBAP) , these functions being provided by the Master Unit 10. O&M interfaces are also terminated in the Master Unit 10.

All Head Units 20 will transmit the downlink common control channels with the same information content and on the same scrambling and spreading codes. All Head Units 20 will transmit the downlink common pilot channel and synchronisation channels with the same data content. This is possible due to the frame timing offsets between the Head Units 20. The resultant signal, as received by the UE 50, will contain a distinct and resolvable multipath ray for each Head Unit 20 that can be received given the location of the UE 50. This improves reception performance due to increased diversity.

In UTRAN FDD spreading code and scrambling code identify a dedicated uplink physical connection. Common channel uplink physical connections are similarly identified access signatures. The Master Unit 10 can rank the Head Units 20 in order of proximity to a particular UE 50, based on correlating the uplink measurements from all the Head Units 20 within the UE's 50 identity. There are sufficient codes defined within UTRAN FDD to permit unambiguous identification of all UEs 50 in a distributed nodeB. Ranking could be based upon received power, signal to interference ratio, or any other measure of received signal quality.

The Head Units 20 operate independently to receive and acknowledge PRACH bursts. In this way the PRACH power can be reduced since the closest Head Unit 20 will acknowledge, using AICH, before the UE 50 is forced to increase PRACH power to reach all head units. The master unit 10 will rank the head units 20 in order of proximity to the UE 50 based upon the PRACH received power level and other measurements of signal quality such as signal to interference ratio.

The Master Unit 10 will route the signalling and data traffic to one or more of the Head Units 20 which is closest to the TJE 50. More than one Head Unit 20 may be used to achieve reinforcement of the signal received by the UE 50. Again this is possible due to the timing offsets between the Head Units 20. The resultant signal, as received by the UE 50, will contain a distinct and resolvable multipath ray for each Head Unit 20 that can be received given the location of the UE 50. This improves reception performance due to increased diversity. The UE 50 will be instructed to use dedicated pilots as a phase reference for the channel, thereby avoiding any problems due to the multiple common pilot channels present within the distributed deployment coverage area.

Just as the downlink data may be multiply routed, so uplink data may be multiply received. If there are unused radio resources in nearby Head Units 20, they may be tuned to receive uplink data from a nearby UE 50. The uplink data so received may be routed to the Master Unit 10, and combined there, to further reduce the error rate in the data.

The Head Units 20 will implement downlink dedicated channel power control locally such that all Head Units 20 that receive power up commands from a UE 50 will increase their power output for that channel. Uplink power control is the responsibility of the Head Unit 20 deemed to be best placed to serve the UE 50 (that is the Head Unit 20 that is ranked the highest by the Master Unit 10 in terms of proximity to the UE 50) . For uplink synchronised systems (for example UTRAN TDD 1.28 Mcps option) the time synchronisation decisions are the responsibility of the Head Unit 20 deemed to be best placed to serve the UΞ 50.

In the case of power control and synchronisation commands the Head Units 20 will change the transmit power during a frame to either emphasise or de- emphasise a command. A Head Unit 20 that is not responsible for power control or synchronisation decisions will have a small or zero code power during the appropriate bits in the frame to de~ emphasis its command. Conversely, a Head Unit 20 that is responsible for power control or synchronisation decisions will increase the code power during the appropriate bits in the frame to emphasis its command.

As a UE 50 moves through the coverage area, the Master Unit 10 will change the routing of the signalling and data traffic, to maintain the connection with the UE 50, to maximise the traffic the traffic throughput of the network, and to minimise interference with the external network.

Thus, unlike a conventional cellular deployment, the UE 50 is not responsible for signal measurements to identify neighbouring Head Units 20 for use in controlling handover, but instead it cannot ^• distinguish between Head Units 20 and it is the responsibility of the distributed nodeB (the combination of Master Unit 10 and Head Units 20) to track each UE 50 through the system.

The synchronisation channels, common pilot and common control channels can be transmitted at a lower power because they are transmitted by multiple Head Units 20 within the system and the mean path loss between the UE 50 and nodeB will be reduced. In this way the interference with the external macro network is reduced.

It will be appreciated that a UE 50 moving within the network of Head Units 20 will receive time- delayed copies of the data from each Head Unit 20, but that the UE 50 will treat these as multi-path copies and reconstruct hem in the usual manner. The UE 50 will therefore see the network of Head Units 20 as a single cell and the receive performance will improve due to diversity gain.

The use of local power control and small cell size will reduce the required power output on any specific code channel to achieve a given quality of service. In this way the intra-cell interference is reduced.

The problems associated with call dropout due to inter-cell interference at cell boundaries is not present within the distributed nodeB coverage area as described in this invention. All Head Units 20 transmit complementary signals that combine constructively. This is a consequence of the fact that all Head Units 20 are seen from a UE 50 perspective as a single nodeB.

Modifications and improvements may be made to the above without departing from the scope of the present invention.

Claims

1. A cellular radio telecommunication system for reducing cell phone signal interference in a cellular radio telecommunication network, said system comprising a plurality of head units (20) spatially separated throughout a first coverage area (40) , wherein each head unit (20) is connected to a master unit (10) and, in combination, the head units (20) and the master unit (10) operate as a single base station using a single carrier.

2. A system as claimed in claim 1 wherein cell phone signal interference is reduced both within the first coverage area (40) and between the first coverage area (40) and a second coverage area.

3. A system as claimed in claim 2 wherein the first and second coverage areas are indoor and ^• outdoor coverage areas respectively.

4. A system as claimed in any preceding claim wherein the cellular radio telecommunication system employs Code Division Multiple Access (CDMA) based air interface.

5. A system as claimed in any. preceding claim wherein the master unit (10) provides co-ordination and control functions to each head unit (20) .

6. A system as claimed in any preceding claim wherein each head unit (20) transmits identical common control downlink channel information.

7. A system as claimed in any preceding claim wherein both downlink and uplink signals of each head unit (20) are offset in time relative to those of the other head units (20) in the system.

8. A system as claimed in claim 7 wherein the timing offsets are controlled such that the earliest arriving signals and latest arriving signals at a User Equipment (UE) (50) within the coverage area have a time offset which is less than or equal to the maximum time delay expected by the UE (50) .

8. A system as claimed in claims 7 or 8 wherein the phase or timing reference for the downlink channels are dedicated pilots.

9. A system as claimed in any preceding claim wherein the master unit (10) ranks the head units (20) by comparing uplink quality indicators.

10. A system as claimed in claim 9 wherein the uplink quality indicators used to rank the head units (20) are the power received and/or signal quality received.

11. A system as claimed in any of claims 9 or 10 wherein the master unit (10) routes dedicated downlink channel information to a UE (50) via one or more of the highest ranked head units (20) .

12. A system as claimed in any of claims 9 to 11 wherein one or more of the head units (20) with the highest ranking independently receive uplink dedicated signals from a UE (50) .

13. A system as claimed in any of claims 9 to 12 wherein the master unit (10) continuously re-ranks the head units (20) as a UE (50) moves through the coverage area (40) .

14. A system as claimed in any preceding claim wherein the head units (20) perform downlink channel power control independently of the other head units (20) .

15. A system as claimed in claim 14 wherein the downlink channel power control at a head unit (20) is optimised in response to the signal strength received from a UE (50) at said head unit (20) .

16. A system as claimed in any preceding claim wherein the head units (20) perform uplink channel power control independently of the other head units (20) .

17. A system as claimed in claim 16 wherein the uplink channel power control at a head unit (20) is optimised in response to the signal strength received from a UE (50) at said head unit (20) .

18. A system as claimed in any preceding claim wherein PRACH acknowledgement is performed independently at one or more head units (20) .

19. A system as claimed in any preceding claim wherein two or more head units (20) receive signalling and data traffic from a UE (50) in order to increase signal diversity and thereby improve signal reception.

20. A system as claimed in any preceding claim wherein, uplink data is routed to the master unit (10) via two or more head units (20) , the data from each head unit (20) being constructively combined thus reducing its error rate.