CN101141235A - Interference canceling method for co-frequency co-time slot duplexing - Google Patents
Interference canceling method for co-frequency co-time slot duplexing Download PDFInfo
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Abstract
The utility model provides a interference clearance technique, so that the up-down-going link of the wireless communication can run via the same frequency and the time-slot, so as to improve the rate of spectrum utilization. When the up-down-going channel is under the common frequency and the time-slot, the down link will interfere the up-link, namely, the base station receiver in the sub-zone is interfered by the base station transmitter of the sub-zone and the base station transmitter of the adjoining sub-zone. The utility model delivers the undesired signals to the receiver through the wired communication. The receiver is arranged with a signal preprocessing unit, so as to remove the transmitter signal interference in the radio frequency. Such signal interference is from the air interface. In addition, the utility model provides an aerial distribution rule to reduce the direct interference of the transmitter to the receiver. The interference clearance method is suitable to the duplex system with the common frequency and time-slot, so as to greatly reduce the frequency resources and time resources.
Description
Technical Field
The invention relates to a novel interference elimination method, which belongs to the field of wireless communication.
Background
A basic task of a wireless communication system is to establish a communication link between a wireless terminal and a base station. The transmission of data is divided into two directions, namely: the link from the base station to the direction of the wireless terminal, called downlink; the link from the wireless terminal to the base station is called the uplink. A system that can implement uplink and downlink data transmission is called a duplex system. Existing duplex systems include frequency division duplex and time division duplex systems. The frequency division duplex system adopts different frequencies to transmit uplink and downlink data so as to isolate the interference between the uplink and the downlink. Time division duplex systems use the same frequency but different time slots for transmission to isolate interference between the uplink and downlink. The two duplexing methods sacrifice frequency resources and time resources in isolating the uplink and downlink, respectively.
If the uplink channel and the downlink channel adopt the same frequency and the same time slot for transmission, the frequency spectrum efficiency can be greatly improved. To achieve this, it is necessary to establish an effective interference cancellation technique.
Disclosure of Invention
The invention aims to provide an interference elimination technology between an uplink and a downlink which are suitable for common-frequency and simultaneous-slot duplexing and an antenna arrangement method for avoiding direct interference.
The basic idea of the invention is to use a wired connection, e.g. a separate microwave channel, to send the interfering signal to the receiver, which cancels the interfering signal in the air interface. The technical scheme is as follows:
1. uplink interference cancellation method
In the system using the same frequency and the same time slot duplex, the uplink transmits the signal of the wireless terminal to the base station, and the interference at this time mainly comes from the interference of the signal of the transmitter of the base station of the local cell and the transmitter of the base station of the adjacent cell to the receiver of the local cell.
Ignoring thermal noise interference, the signal received by the base station receiver of the cell is the sum of the signal transmitted by the wireless terminal (uplink) and the signals of the base station transmitter of the cell and the base station transmitters of the neighboring cells:
in the above formula, X (t) is the received signal of the base station, X i (t) is the signal transmitted by the ith wireless terminal in the cell, y d And (t) is a signal transmitted by the d interference transmitter. The first item on the right of the equal sign is a wireless terminal transmitting signal; second term is base station interferenceA transmitter signal; "\58366" represents convolution in mathematical operations; h is i And h d Respectively representing the channel response functions of the air interface of the ith handset and the d-th base station transmitter. The second term on the right in equation (1) is the interference from the air interface. The base station transmitter is defined herein as an interfering transmitter.
To eliminate the above-mentioned interference from the air interface, we provide a signal preprocessing unit at the conventional receiver (the system is shown in fig. 1). An independent signal connection channel, i.e. a wired connection, is established between the signal preprocessing unit and the interference transmitter. The interference transmitter signal is fed directly to the signal pre-processing unit using this wired connection to cancel out the interference signal from the air interface.
Since the wired channel and the air interface channel of the interference transmitter generally have a large difference, a channel simulator needs to be arranged in the signal preprocessing unit. The channel simulator may first estimate the inverse of the channel parameters between the local receiver and the interfering transmitter, which may use any existing channel estimation method, such as: pilot-assisted Minimum Mean Square Error (MMSE) estimation, etc. The channel simulator recovers the inverse number of the interference signal received by the receiver end by using the estimated value of the inverse number of the channel parameter and the signal sent by the interference transmitter transmitted by the wired channel, and then adds the inverse number of the interference signal with the received signal in a subsequent adding unit, thereby eliminating the interference of the interference transmitter. The channel simulator can also estimate the channel parameters between the local receiver and the interference transmitter, simulate the signal of the interference transmitter transmitted by the wired connection by using the estimated value of the channel parameters to obtain the air interference signal received by the receiver, and subtract the output signal of the channel simulator from the signal received by the receiver, namely eliminate the interference of the interference transmitter.
The estimation of the inverse of the interference signal of the channel simulator described above can be implemented by an adaptive FIR filter (see fig. 2). When the air interface channel is a single path channel, the FIR filter may be replaced with a single tap multiplier.
Considering the limitations of the Automatic Gain Control (AGC) performance of the receiver and the number of analog-to-digital conversion bits, the signal preprocessing unit should be first provided in the rf circuit. If the elimination is not complete, a digital signal preprocessing unit with the same working principle is arranged to further eliminate the baseband signal.
2. Antenna arrangement method
The arrangement of the interference transmitter and the receiver antenna is another key point of the invention, and in an actual system, the receiver antenna is placed at the position of the radiation minimum point of the interference transmitter antenna as far as possible, so that the receiving power of the interference transmitter signal is reduced.
Fig. 3 shows an example of a situation when the interfering transmitter is very close to the receiver, i.e. the transmitting and receiving antennas in the same cell. In the two whip antennas shown in fig. 3, the transmitting and receiving antennas are arranged vertically, the upper is the transmitter antenna, and the lower is the receiver antenna. According to the radiation theorem, the following results are obtained: the receiver antenna is at an azimuth of zero radiation from the transmitter antenna. Therefore, the transmitter has zero interference with the receiver under the assumption that no reflected waves are present. In the case of non-negligible reflection, due to the arrangement, interference of direct waves is avoided, so that interference power can still be effectively reduced, and workload of the signal preprocessing unit in the previous section is reduced.
When the transmitting and receiving antennas of a base station operate simultaneously, the transmitting antenna constitutes a strong interference source for the receiver of the base station, usually because of the close distance between the two antennas. As described above, by using the antenna placement method of the present invention, such interference can be effectively reduced, and even if there is remaining interference, the remaining interference can be further removed by the interference removal method described above. The interference elimination method is suitable for a same-frequency and same-time-slot duplex system, and can greatly save frequency resources and time resources.
Drawings
Fig. 1 is a schematic diagram of an inter-base station interference cancellation method;
FIG. 2 is a schematic diagram of a FIR filter structure;
fig. 3 is a schematic diagram of a method for placing transmitting and receiving antennas in the same cell.
Detailed Description
The invention is described in further detail below with reference to the drawings, without in any way limiting the scope of the invention.
The key to implementing the above interference cancellation technique is to accurately obtain the filter parameters in the channel simulator, as shown in fig. 1, and convert the interference transmitter signal transmitted through the wired connection into a signal approximately consistent with the interference signal on the air interface through the channel simulator, so as to completely cancel the interference signal in the receiver signal as much as possible by subtraction. The method for obtaining the channel parameters may be: the method comprises the steps of firstly establishing special narrow time slots with equal intervals on a time domain through a system protocol, stopping signal transmission of all wireless terminals in a cell in the special time slots, stopping signal transmission of adjacent interference transmitters except the interference transmitter needing channel estimation, estimating channel parameters by using signals received by a receiving antenna and interference transmission signals transmitted by a wired channel, and then carrying out channel estimation on other interference transmitters by using the same method.
Without loss of generality, we first describe a method of canceling the interference transmitter of the 1 st base station, i.e., canceling the interference term when d =1 in the second term on the right side of equation (1). First, the receiver estimates the 1 st interference transmitter to the local receiver by using an existing channel estimation methodThe inverse of the channel parameter between the earth receivers, i.e. the estimate h 1 ' (t) is p-h 1 (t) estimation.
Then h is mixed 1 ' (t) and a transmission signal y of a first interfering transmitter transmitted via a wired channel 1 And (t) performing convolution to obtain an output signal of the channel simulator. The sum of the output of the channel simulator and the output obtained by inputting the receiver signal into the adding unit is:
we split the second term on the right of equation (1) into two terms d ≠ 1 and d =1, and the second term on the right of the above equation is the channel simulator output signal. To make the sum of the last two terms in the right side of equation (2) zero, we can use the following method.
Designing a system protocol, setting special narrow time slots with equal intervals on a time domain, wherein in the special time slots, all wireless terminals in a local cell stop transmitting signals, adjacent interference transmitters also stop transmitting signals except a first interference transmitter, adding signals received by a receiver at the time with output signals of a channel simulator, and solving the following equation:
h 1 (t)y 1 (t)+h 1 ′(t)y 1 (t)＝0(3)
obtaining: h is 1 ′(t)＝-h 1 (t).
And substituting the h' (t) solved in the step (3) into the formula (2), and finishing the interference elimination. The output signal of the adding unit is:
in practical situations, due to noise interference and randomness of channel variation, we can only obtain an approximate solution of equation (3) in general. Namely:
h 1 (t)y(t)+h 1 ′(t)y(t)＝δ 1 (t)(5)
in the above formula, delta 1 (t) is a random function that varies with time t, and is referred to herein as residual interference.
Thus, the addition unit output signal is:
by analogy, we can eliminate the interference signals of 2 nd and 3 rd interference transmitters \8230;, until the interference of all adjacent interference transmitters is eliminated, and finally obtain:
to suppress the effect of the remaining interference, one can use CDMA spreading technique for uplink and downlink signal modulation (called code division duplex), that is: x is the number of i (t) and y d And (t) adopting spread spectrum code modulation.
The despreading of the formula (7) can obtain the signal-to-interference ratio gain of 12-15dB at most, thereby effectively overcoming the influence of the residual interference.
In addition, the invention takes the elimination of the interference of a base station as an example to briefly describe the basic working principle. The method is suitable for the condition of multi-base station interference. Finally, it is pointed out that the above principle is completely applicable to the interference cancellation of the uplink and downlink channels of the handset itself. And thus will not be repeated.
Claims (9)
1. An interference elimination method for co-frequency and time-slot duplex system features that a signal pre-processing unit is arranged at receiver, and the signals of interference transmitter are transmitted to the pre-processing unit for signal processing by wired connection, so cancelling out the signals of interference transmitter from air interface.
2. The interference cancellation method of claim 1, wherein said wired connection is a stand-alone microwave connection.
3. The interference cancellation method of claim 1 or 2, wherein the signal preprocessing unit first estimates the channel parameters or the inverse values of the channel parameters between the local receiver and the interference transmitter by using a channel simulator, then recovers the interference signals or the inverse values of the interference signals received by the receiver by using the estimated values of the channel parameters or the inverse values thereof and the signals of the interference transmitter transmitted through the cable channel, and then subtracts or adds the received signals to cancel the interference of the interference transmitter.
4. The interference cancellation method of claim 3, wherein said channel simulator performs channel estimation using a pilot-assisted minimum mean square error estimation method.
5. The interference cancellation method of claim 3, wherein said channel simulator performs channel estimation by: the method comprises the steps of firstly establishing special narrow time slots with equal intervals on a time domain through a system protocol, stopping signal transmission of all wireless terminals in a local cell in the special time slots, stopping signal transmission of adjacent interference transmitters except the interference transmitter of a channel to be estimated, estimating channel parameters by using signals received by a receiving antenna and interference transmitting signals transmitted by a wired channel, and then estimating channels of other interference transmitters by using the same method.
6. The interference cancellation method of claim 3, wherein said channel simulator is an adaptive FIR filter or a single tap multiplier.
7. The interference cancellation method of claim 1, wherein said signal preprocessing unit is disposed on a radio frequency circuit.
8. The interference cancellation method of claim 1, wherein the receiver antenna is placed as far as possible at the location of the interference transmitter antenna radiation minima.
9. The interference cancellation method of claim 8, wherein the transmit and receive antennas of the same base station are vertically arranged, with the transmitter antenna above and the receiver antenna below.
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