WO2005060182A1

WO2005060182A1 - Cellular communications system

Info

Publication number: WO2005060182A1
Application number: PCT/GB2004/005324
Authority: WO
Inventors: Stephen Mclaughlin; Harald Haas
Original assignee: The University Court Of The University Of Edinburgh; International University Bremen
Priority date: 2003-12-18
Filing date: 2004-12-16
Publication date: 2005-06-30
Also published as: GB0329322D0

Abstract

A cellular communications system comprises at least one base station (1) establishing a macro cell (2), and a plurality of mobile stations (11, 12), each adapted to transmit a signal to said base station via a frequency division duplex (FDD) uplink frequency band (6) and to receive a signal from said base station via an FDD downlink frequency band (7). The mobile stations are also adapted to communicate directly with each other using time division duplex (TDD) via the FDD uplink and/or downlink frequency band (6, 7), to forward a signal from the base station (1) to another of the mobile stations and to forward a signal from another of said mobile stations to the base station (1) also via the FDD uplink and/or downlink frequency band (6, 7). A mobile radio/telephone unit for implementing the method is also provided.

Description

CELLULAR COMMUNICATIONS SYSTEM

Background to the Invention

The present invention relates to a cellular mobile communications system, and in particular to a CDMA-FDD (code division multiple access - frequency division duplex) system, and a CDMA-TDD (time division duplex) air interface.

It has been shown previously by H. Haas and G.J.R Povey, "Capacity Analysis of a TDD Underlay Applicable for UMTS", P1MRC 99, (Osaka, Japan) IEEE, pp A6-4, September 12-15, 1999, that the concept of spectrum sharing using a TDD underlay concept can provide additional flexibility and capacity in a CDMA system.

Due to increasing asymmetric traffic on the air interface, one communication direction of an FDD interface will be underused, provided that the uplink and downlink bandwidths are the same. The main idea behind the TDD underlay is to exploit the underused radio spectrum of a cellular CDMA- FDD system. A co-existing TDD interface with harmonized interface parameters utilizes the underused FDD frequency band for additional connections. Technically a TDD interface can be used within either the FDD uplink or downlink frequency band. A hierarchical system architecture consisting of pico cells which exclusively use FDD resources of a macro cell overlay was proposed by M.O. Sunay et al, "A Dynamic Channel Allocation Based TDD DS CDMA Residential Indoor System," in ICUPC '97, vol. 2 of 2, pp 238-234, IEEE, October 1997 and by L. Ma et al, "A Simulation Bed to Investigate the Feasibility of a TDD DS CDMA Residential Indoor System Underlay," in PIMRC '97, vol. 2 of 3, pp. 286-291, IEEE, September 1997. A dynamic channel allocation (DCA) decides whether to use the FDD uplink or dowTnlink frequency band. The TDD underlay concept is depicted in Figure 1. A macro cell base station 1 establishes a macro cell 2. At least one pico cell base station 3 establishes a pico cell 4 within the macro cell 2 at a distance from the macro cell base station 1. The macro cell base station cornmunicates with macro cell mobile stations 5, i.e. mobile stations not within any pico cell, via conventional FDD uplink and downlink bands 6, 7 respectively. The pico cell base station 3 communicates with pico cell mobile stations such as 8 within the pico cell using TDD. Also, as shown by the arrows 9, both the pico cell base station 3 and the pico cell mobile station 8 can communicate with the macro cell base station 1 using the FDD uplink frequency band 6.

It can be seen that the macro cell mobile station 5 interferes "with the pico cell base station 3 and the pico cell mobile station 8. In turn, the pico cell entities interfere with the macro cell base station 1. The proposed CDMA-TDD underlay in Sunay et al (supra) uses a PN code sequence with a cell unique phase offset relative to the CDMA-FDD macro cellular overlay. The authors reported substantial capacity gains without a significant deterioration of the QoS (quality of signal). The pico cells are assumed to consist of a single indoor base station and a single indoor mobile station and a re randomly distributed. However, severe interference can be anticipated if macro and pico cells are in close proximity (K. Takeo, "Improvement of Coverage Probability by Subband Scheme in CDMA Macro-micro Cellular System" in PIMRC '96, vol. 1 of 3, (Taipei, Taiwan), pp. 93-97, IEEE, October 15-18 1996). WO 00/07399 describes a TDD underlay system in which the TDD is only considered to be operated in the FDD uplink band due to an anticipated channel asymmetry in favor of the downlink.

The feasibility of the . TDD underlay, provided that the base station separation distance is properly chosen, was confirmed by-W. Wong et al, "Feasibility Study of TDD- and FDD-CDMA Frequency Sharing Cellular Networks" in ICC '99 (Vancouver, Canada), pp. 531-535, IEEE, June 6-10 1999 and "Frequency Selection Strategies for Hybrid TDD/FDD-CDMA Cellular Networks", in ICUPC '98 (Florence, Italy), pp. 1152-1156, IEEE, October 5-9 1998. The frequency usage when operating the TDD underlay in a system with two air interfaces (FDD and TDD) and harmonized frame structures is shown in Figure 2. The components of the TDD pico cell or indoor cell are followed by the letter "i" (e.g. BSi designates pico cell base station) whilst the FDD macro cell or outdoor cell entities are marked with an additional "o" . Note that the TDD underlay may be used to accomplish cell-independent channel asymmetry in the TDD subsystem without asynchronous time slot overlaps. Furthermore, the flexibility on the air interface is increased as, for example, uplink radio resources can be converted into downlink capacity.

The UTRA-TDD (UMTS terrestrial radio access - time division duplex) standard originally included provision for ODMA (opportunity driven multiple access), although the implementation was not finalized. The main feature of the standard covered signaling slot allocation and methods for building neighbor lists. A routing protocol was noticeably absent and ODMA has now been removed from UTRA-TDD, amid concerns about increased power consumption, especially from users not involved in calls being used as relays.

Summary of the Invention

It is an aim of the invention to provide a particularly effective system making more efficient use of radio resources by means of a TDD underlay in an FDD system.

The present invention provides a cellular communications system comprising at least one base station establishing a macro cell, and a plurality of mobile stations, each adapted to transmit a signal to said base station via a frequency division duplex (FDD) uplink frequency band and to receive a signal from said base station via an FDD downlink frequency band; said mobile stations also being adapted to communicate directly with each other using time division duplex (TDD) via the FDD uplink and/ or downlink frequency band, to forward a signal from the base station to another of said mobile stations and to forward a signal from another of said mobile stations to the base station, also via the FDD uplink and/ or downlink frequency band.

Thus, ad-hoc mode operation is an integral part of the invention. It is commonly accepted that ad-hoc mode operation can only be deployed when using TDD. In order to be applicable to pure FDD systems, the invention makes use of the fact that future data services impose an imbalance on the uplink and downlink, in favor of the downlink. The FDD mode of operation has no inherent ability to cater for asymmetrical traffic. Therefore, whilst the downlink frequency band will be highly loaded, the uplink spectrum will be greatly underused. Provided that certain geometrical constraints are met (see H. Haas et al, PIMRC '98, IEEE, no 94-94; Proc. UMTS Workshop, pp. 151- 158, Nov. 26-28, 1998; Colloquium on UMTS Terminals and Software Radio, IEE, pp. 771-776, April 26, 1999; PIMRC '99, IEEE, pp A6-4), the FDD uplink spectrum can be used to carry TDD traffic - in particular, TDD ad-hoc traffic at the cell boundaries.

In some cases, the FDD uplink frequency band may be more heavily loaded than the downlink band. Thus the FDD downlink band may be used for the direct TDD communication between mobile stations according to the invention. It is also possible to use both the FDD uplink and downlink frequency bands as separate channels for direct TDD communication. A group of said mobile stations may form a virtual cell substantially within said macro cell, one mobile station of said group forwarding signals from and to the base station to and from other mobile stations of the group respectively. Part of said virtual cell may extend outside the macro cell.

In a particular embodiment of the invention, the mobile stations are prevented from direct TDD communication unless they are further than a predetermined distance from the base station.

Communications between the base station and the mobile stations may in particular use the UTRA (Universal Mobile Telecommunications System - Terrestrial Radio Access) standard.

The present invention also provides a mobile radio/ telephone unit adapted to transmit a signal to a base station via a frequency division duplex (FDD) uplink frequency band and to receive a signal from said base station via an FDD downlink frequency band; said mobile unit also being adapted to communicate directly with another mobile unit using time division duplex (TDD) via the FDD uplink and/ or downlink frequency band, to forward a signal from the base station to another mobile unit and to forward a signal from another mobile unit to the base station, also via the FDD uplink and/ or downlink frequency band.

The mobile unit may also be adapted to forward a signal received from a further mobile unit to yet another mobile unit, using TDD via the FDD uplink and/ or downlink frequency band. Brief Description of the Drawings

A particular embodiment of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows the known TDD underlay system described above;

Figure 2 shows frequency usage when operating the known TDD underlay system;

Figure 3 schematically shows a system according to the present invention;

Figure 4 shows the system of Figure 3 with a plurality of virtual cells; and

Figure 5 is a graph of average throughput against average offered load for embodiments of the invention and for a known system.

Detailed Description of the Invention

As shown in Figures 3 and 4, a virtual pico cell or ad-hoc cluster 10 is formed near the boundary of the macro cell 2. One mobile station 11 in the virtual pico cell acts as a gateway mobile station, forwarding signals from the base station 1 to other mobile stations 12 in the virtual pico cell and. forwarding signals from the other mobile stations 12 of the base station 1. Communications in the virtual pico cell 10 are in the TDD mode using the FDD uplink frequency band of the macro cell.

Figure 5 shows the simulated average throughput, considering three scenarios. The first scenario has no peer-to-peer communication and is denoted "cellular". The second scenario, denoted "Ad-Hoe 0.5" assumes a relaying probability of 0.5 and the third scenario, denoted "Ad-Hoc 0.01" assumes a relaying probability of 0.01. Figure 5 shows that "Ad-Hoc 0.01" and "Ad-Hoc 0.5" achieve maximum throughputs that are respectively 24% and 13% higher than that achieved by "cellular". The use of ad-hoc clusters results in superior performance because they create more parallel channels for communication, thus improving the overall system trunking efficiency, leading to lower blocking rates and hence higher system throughput. The channel parallelism results from the fact that some fraction of the traffic generated in an ad-hoc cell will not leave the ad-hoc cluster. In addition, the cellular system benefits from reduced interference from the short-range peer- to-peer communication, where each peer-to-peer hop represents a communication channel. Therefore, the smaller the hops in the ad-hoc clusters, the less the offered load per channel and consequently the lower the blocking probability and the higher the throughput. This is evident from the results, where "Ad-Hoc 0.01" achieves a higher throughput than "Ad-Hoc 0.5". These results suggest that "Ad-Hoc O.01" and "Ad-Hoc 0.5" can respectively achieve maximum capacity and system spectral efficiency that are 20% and 1 % higher than that achieved by "cellular".

The invention thus exploits efficient spectrum sharing. Since no extra TDD spectrum is required, the invention is applicable to pure FDD systems.

It has been shown that the issue of increased battery drain for non-calling users is not a problem if only relays already using one or more of the TDD time slots are considered. For those involved in calls an increase in battery life will result on average for all users if only the transmitted power is considered. Users close to the base station may lose out, but for non-relaying they benefited from the lowest required power. Mobile stations at the edge of cells will see the greatest benefit, with the result that battery life is more consistent and longer on average. In order to reduce interference, mobile stations may be prevented from acting as gateways if they are closer to the base station 1 than an interference protection distance x.

Information sent to a recipient located within the same macro cell does not have to be routed via the base station, thereby rninimizing the bottleneck effect induced by the multi-point to single point transmission in the uplink. Dead zones at the cell boundaries are avoided.

Uplink information from all mobile stations in a virtual pico cell is accumulated at the gateway mobile station. Therefore a smaller number of high-data-rate mobile stations are communicating with the base station, reducing the multi-point nature of the uplink transmission. This is particularly effective in a CDMA system, as the capacity-reducing effect incurred by the near-far effect is minimized and thus the pole-capacity is increased.

All forms of the verb "to comprise" used in this specification have the meaning "to consist of or include".

Claims

1. A cellular communications system comprising at least one base station establishing a macro cell, and a plurality of mobile stations, each adapted to transmit a signal to said base station via a frequency division duplex (FDD) uplink frequency band and to receive a signal from said base station via an FDD downlink frequency band; said mobile stations also being adapted to communicate directly with each other using time division duplex (TDD) via the FDD uplink and/ or downlink frequency band, to forward a signal from the base station to another of said mobile stations and to forward a signal from another of said mobile stations to the base station, also via the FDD uplink and/ or downlink frequency band.

2. A system according to claim 1, wherein a group of said mobile stations form a virtual cell substantially within said macro cell, one mobile station of said group forwarding signals from and to the base station to and from other mobile stations of the group respectively.

3. A system according to claim 2, wherein part of said virtual cell extends outside the macro .cell.

4. A system according to claim 1, 2 or 3, wherein the mobile stations are prevented from direct TDD comrnunication unless they are further than a predetermined distance from the base station.

5. A system according to any preceding claim, wherein communications between the base station and the mobile stations use the UTRA (Universal Mobile Telecommunications System - Terrestrial Radio Access) standard.

6. A mobile radio/telephone unit adapted to transmit a signal to a base station via a frequency division duplex (FDD) uplink frequency band and to receive a signal from said base station via an FDD downlink frequency band; said mobile unit also being adapted to communicate directly with another mobile unit using time division duplex (TDD) via the FDD uplink and/ or downlink frequency band, to forward a signal from the base station to another mobile unit and to forward a signal from another mobile unit to the base station, also via the FDD uplink and/ or downlink frequency band.

7. A mobile unit according to claim 6, which is prevented from such direct TDD communication unless it is further than a predetermined distance from the base station.

8. A mobile unit according to claim 6 or 7, adapted to use the UTRA (Universal Mobile Telecommunications System - Terrestrial Radio Access) standard for coirirnunicating with the base station.

9. A mobile unit according to claim 6, 7 or 8, adapted to forward a signal received from a further mobile unit to yet another mobile unit, using TDD via the FDD uplink and/ o downlink frequency band.