WO2006098639A1

WO2006098639A1 - Method for cancelling interference

Info

Publication number: WO2006098639A1
Application number: PCT/NO2006/000100
Authority: WO
Inventors: Ole GRØNDALEN
Original assignee: Telenor Asa
Priority date: 2005-03-16
Filing date: 2006-03-16
Publication date: 2006-09-21
Also published as: NO20051366D0; NO20051366L

Abstract

A method for cancelling interference between base stations in a Time Division Duplex radio access system, by transmitting and receiving signals in a first base station (2) to and from a number of client terminals with a main antenna (3) covering a sector around said first base station (2), receiving a signal from a second interfering base station (1) with an auxiliary narrow beam antenna (4), filtering said signal in an adaptive filter and determining filter coefficients in said adaptive filter from the signals received by the main and auxiliary antennas, and subtracting the signal received by the auxiliary antenna (4) from the signal received by the main antenna (3).

Description

METHOD FOR CANCELLING INTERFERENCE

Field of the invention

The present invention relates to broadband radio access systems for wireless transfer of data, and in particular a method and system for reducing base station to base station interference in TDD based radio access systems.

Technical background

Broadband radio access is expected to both complement and become an important competitor to broadband access systems based on copper, fibre and cable. The most common architecture of such systems is so called point-to- multipoint (P-MP) , in which there is a network of base stations, each base station servicing a number of clients. The client terminals are equipped with directive antennas (narrow opening angle) that transmit (and receive) the major part of its energy in a narrow beam pointing towards the base stations. The base stations, on the other hand, are equipped with antennas that transmit (and receive) signals in a wide sector (e.g. sectors of 60 or 90 degrees) for reaching all client terminals in the sector. Commonly, one antenna is used both for transmitting and receiving signals, both at the base station and the client terminal.

A prerequisite for the success of broadband access is that the solution is sufficiently cost-effective. As spectrum has soared in cost, the frequencies must be exploited efficiently. If the spectral efficiency can be improved, the number of simultaneous clients may be increased and the cost per client reduced.

Using time division duplex (TDD) instead of frequency division duplex (FDD) is one of the most effective ways of increasing the spectral efficiency in P-MP radio networks. In frequency division duplex, signals are transmitted downstream (from base station towards clients) and upstream (from clients towards base station) on different frequencies, while in time division duplex signals are transmitted on one frequency but in different time slots. The biggest advantage of using TDD is that the share of spectral resources used for upstream and downstream traffic, respectively, can easily be changed by changing the length of the time slots. Thus, the system may optimize the distribution of spectral resources to maximize the total capacity. In FDD systems it is normally not possibly to change the spectral resources for upstream and downstream traffic.

A major challenge in TDD based systems is to control interference between base stations. In FDD based systems, interference between base stations can not occur, as the base stations are transmitting and receiving on different frequencies. In TDD based systems on the other hand, the base stations are transmitting and receiving on the same frequencies. The signals from the base stations will cover large areas due to the large opening angles of the antennas, and the stations will also receive signals from the same large areas. Thus, there is a large probability of a transmitting base station interfering with an adjacent base station receiving signals. The interference in question is both co-channel and adjacent channel interference .

Existing solutions for preventing interference is mainly based on allocating different frequencies to base stations that may possibly interfere. Another solution is to synchronize the network such that all base stations switch to the same modus (receiving or transmitting) simultaneously. However, then it is only possible to adjust the share between downstream and upstream capacity collectively, and not individually for each base station. This will severely reduce the capacity and flexibility of the system. Thus, there exists a need for a method of controlling the interference level between base stations in a TDD network while preserving the flexibility of adjusting the spectrum use in individual base stations.

From adaptive filter theory, it is well known that interference might be reduced if it is possible to obtain an auxiliary signal that is correlated with the interference [Simon Haykin, "Adaptive Filter Theory", 2^nd ed. 1991 Prentice-Hall, Englewood Cliffs, NJ 07632] . This method is for example used for echo cancellation in telephony systems .

But this method can not be employed directly in the P-MP radio access case due to the temporal characteristics of the physical channel between the antennas of two base stations .

Summary of the invention

The present invention provides a new method for overcoming the problems discussed above by exploiting the fact that both the signal and the interference sources are under the control of the network operator (s).

In particular, the invention relates to a method for cancelling interference between base stations in a Time Division Duplex radio access system, said method including the following steps: transmitting and receiving signals in a first base station to and from a number of client terminals with a main antenna covering a sector around said first base station, receiving a reference signal from a second interfering base station over an auxiliary channel, and subtracting the reference signal from the signal received by the main antenna, as claimed in the appended claim 1. The invention also relates to a base station in a Time Division Duplex radio access system, said base station including a main antenna, the base station further including an auxiliary antenna, said base station being arranged to subtract a reference signal received from an interfering base station by the auxiliary antenna from a signal received by the main antenna, as covered in the appended claim 6.

Advantageous embodiments of the invention appear from the associated dependent claims.

Brief description of the drawings

The invention will now be described in detail in reference to the appended drawings, in which:

Fig. 1 is an overview of the inventive system for interference cancellation,

Fig. 2 shows some more detail of a receiver setup that may be used in a system according to the invention,

Fig. 3 illustrates the principle of an adaptive filter as used in the invention.

Detailed description

In the following description the term "interferer" will be used for the interfering base station and the term "victim" for the base station suffering from the interference.

The adaptive interference cancellation is performed by the victim 2 receiving an additional signal that is correlated with the received interference. This may be done by sending a copy of the signal emitted by the interferer 1 to the victim 2 over a copper cable, an optical fibre or by using a free space optic system. A more feasible alternative is the victim using an auxiliary antenna with a narrow lobe (high directivity) for receiving a high quality version of the signal emitted by the interferer. The high quality version of the signal emitted by the interferer is then used by the victim for estimating and reducing the interference component in the signal received by the main antenna. This solution is illustrated in Fig. 1.

The signal received by the victim' s main antenna 3 will include a desired upstream signal from a client terminal, an undesired and distorted (mainly due to multipath transmission) signal from the interfering base station 1, as well as thermal noise and other interference.

The signal received on the auxiliary antenna 4 will include a practically undisturbed signal from the interfering base station, thermal noise and other interfering signals. As this antenna has a small opening angle (high directivity) , the interfering signal will be received nearly undisturbed (negligible multipath) and with a good signal-to-noise ratio .

The signal from the auxiliary antenna 4 is input to an adaptive filter 5. The output from this filter should be an estimate of the signal component in the signal received in the main antenna 3 originated by the interfering base station 1. The taps (coefficients) in the adaptive filter are determined from the signal received by the two antennas. The tap weights may be determined continuously or based on particular pilot symbols (or both) .

Fig. 2 shows the receiver system in more detail. The signals received by the main antenna 3 and the auxiliary antenna 4 are processed in identical receivers. The receivers shown in the figure are direct conversion receivers, even though any type of receiver may be used. The signals received by the antennas 3, 4 are filtered in bandpass filters 11, 17, converted to baseband in mixers 12, 18, again filtered in lowpass filters 13, 19 and digitized in A/D converters 14, 20. The mixers 12, 18 share a common local oscillator 16. The signal in the auxiliary antenna path is filtered in a filter 21 with n taps, whereupon the signal is subtracted from the main antenna signal in subtractor 15. The filter taps w^H(n) for the filter 21 is determined in optimizer 22 by comparing the signals d(n) in the main path and u(n) in the auxiliary path.

The filter 21 is illustrated in Fig. 3. In principle, the filter includes a series of delay elements Z^"1. Different weights w(n), n=0 - M-I are applied to the input signal u(n) and each delayed signal before the weighted signals are summed into output signal y(n) .

The tap weights may be determined using various algorithms. An example is the Least Mean Square (LMS) adaptation algorithm:

w(0)=0

y (n) =w^H (n) u (n)

e(n)=d(n) -y (n)

w (n+1) =w (n)+μ*u(n)^»e*(n)

where underscore refers to a vector variable, * means complex conjugated, the superscript H means "Hermitian transpose" and the parameter μ is the step size (1/ μ representing the memory of the LMS algorithm) .

However, the signal emitted by the interferer 1 will be reflected by various objects and pass through vegetation before being received by the victim's main antenna 3. The received interfering signal will be a sum of a number of copies of the emitted interfering signal in which each copy has different amplitude, time delay and phase. The number of "copies", their amplitude, time delay and phase will vary with time as both the vegetation and reflecting objects may move.

When the variations are rapid, the filter coefficients in the adaptive filter must be updated often. Each set of filter coefficients must then be computed based on short observation times (few samples) , and the accuracy will turn out low. In such cases, in order for the interference cancellation to work, the filter coefficients must be determined in a more efficient way.

The filter coefficients may be calculated in a more efficient way by using one or both of the following methods :

• The victim instructing ^ΛXits" client terminals not to send in certain time slots. Then, the victim will receive only the interfering signal (and some noise from other sources) . The filter coefficients may then be determined with great accuracy from a small number of samples. How frequently the victim should add such time slots depends on how rapidly the channel between interferer and victim is changing.

• The interferer adds reference signals in certain time slots of the emitted signal. The reference signals must be feasible for channel estimation. The victim knows which reference signals that are emitted and in which time slots they are sent. As the reference signals are designed for channel estimation, they may provide much better information on the channel over the bandwidth of interest than an arbitrary signal of identical length (duration) . The outcome is that the filter coefficients of the adaptive filter can be determined with greater efficiency.

The interferer may add reference signals according to a fixed pattern, or he can arrange the frequency by which the reference signals are emitted and which reference signals to be utilized, dependent on how rapidly the channel between interferer and victim is changing.

Claims

C l a i m s

1. A method for cancelling interference between base stations in a Time Division Duplex based radio access system, said method including the following steps:

transmitting and receiving signals in a first base station (2) to and from a number of client terminals with a main antenna (3) covering a sector around said first base station (2) ,

characterized in receiving an auxiliary signal from a second interfering base station (1) over an auxiliary channel, and

subtracting the auxiliary signal from the signal received by the main antenna (3) .

2. A method as claimed in claim 1, in which said auxiliary signal is received over a cable link.

3. A method as claimed in claim 1, in which said auxiliary signal is received over an optical fibre link.

4. A method as claimed in claim 1, in which said auxiliary signal is received over a free space optical link.

5. A method as claimed in claim 1, in which said auxiliary signal is received with an auxiliary narrow beam antenna (4 ) .

6. A method as claimed in claim 5, said method including the additional steps of filtering the auxiliary signal received by the auxiliary antenna (4) in an adaptive filter (5) and determining filter coefficients in said adaptive filter from the signals received by the main (3) and auxiliary (4) antennas.

7. A method as claimed in claim 6, said method including the step of the first base station (2) instructing said client terminals not to send signals in some time slots.

8. A method as claimed in claim 6 or 7, said method including the step of said second base station (2) including reference signals in some time slots when transmitting .

9. A method as claimed in claim 8, in which the reference signals are transmitted according to a predefined pattern.

10. A base station in a Time Division Duplex radio access system, said base station (2) including a main antenna (3),

characterized in the base station further including an auxiliary antenna (4), said base station being arranged to subtract an auxiliary signal received by the auxiliary antenna (4) from an interfering base station (1) from a signal received by the main antenna (3) .

11. A base station as claimed in claim 10, said base station (2) further including an adaptive filter (5) arranged to filter the auxiliary signal received by the auxiliary antenna (4) before it is subtracted from the signal received on the main antenna (3) .