Interference estimation for LTE receiver

Abstract

A method of estimating interference in a received signal is disclosed. The method includes receiving a plurality of subcarriers from a remote transmitter. Each of the subcarriers is multiplied by a control signal. At least two of the subcarriers are compared to produce a differential signal. Interference is estimated in response to the differential signal.

Claims

1. A method of suppressing interference in a received signal, comprising: receiving a plurality of subcarriers from a remote transmitter circuit by a receive antenna circuit; comparing at least two of the subcarriers to produce a plurality of differential signals; multiplying each differential signal by its Hermitian transpose to produce a plurality of products; adding the plurality of products to produce a sum of products in a covariance matrix; estimating the interference in response to the covariance matrix; and suppressing the interference in response to the step of estimating.

2. A method as in claim 1, wherein the at least two subcarriers are adjacent.

3. A method as in claim 1, comprising multiplying the plurality of subcarriers by a control signal.

4. A method as in claim 3, wherein the control signal is a pilot signal.

5. A method as in claim 3, wherein the control signal is a demodulation reference signal.

6. A method as in claim 1, comprising equalizing the received signal in response to the covariance matrix.

7. An interference suppression circuit, comprising: an input circuit arranged to receive a data stream comprising a plurality of subcarriers and to extract at least one control signal; an estimate circuit arranged to calculate a channel estimate in response to the at least one control signal; a differential circuit arranged to compare at least two of the subcarriers to produce a differential signal; a matrix circuit arranged to calculate a covariance matrix in response to the differential signal; a weight calculation circuit arranged to calculate interference suppression weights in response to the covariance matrix and the channel estimate; and an equalizer circuit arranged to suppress interference in the received data stream in response to the interference suppression weights.

8. A circuit as in claim 7, wherein the at least one control signal is a pilot signal.

9. A circuit as in claim 7, wherein the at least one control signal is a demodulation reference signal.

10. A circuit as in claim 7, comprising a multiplication circuit arranged to multiply each of the plurality of subcarriers by a respective control signal.

11. A method of suppressing interference in a wireless receiver circuit, comprising: receiving a data stream comprising a plurality of subcarriers from a remote transmitter circuit; extracting a plurality of control signals from the data stream with an extraction circuit; multiplying each of the plurality of subcarriers by a respective control signal to produce a plurality of products; comparing at least two of the products to produce a plurality of differential signals; producing a sum of products in a covariance matrix in response to the plurality of differential signals; calculating equalizer weights in response to the covariance matrix; and suppressing the interference in response to the equalizer weights.

12. A method as in claim 11, wherein at least two subcarriers of the plurality of subcarriers are adjacent.

13. A method as in claim 11, wherein the control signal is one of a pilot signal and a demodulation reference signal.

14. A method as in claim 11, comprising equalizing the data stream in response to the covariance matrix.

15. A method as in claim 11, comprising: estimating a channel in response to the plurality of control signals; and equalizing the data stream by in response to the channel estimate and the covariance matrix.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a diagram of a wireless communication system of the prior art;

(2) FIG. 2 is a diagram of a pair of Physical Resource Blocks of the prior art;

(3) FIG. 3 is a block diagram of a wireless receiver of the present invention; and

(4) FIG. 4 is a flow diagram showing calculation of equalizer weights based on a differential between adjacent subcarriers.

DETAILED DESCRIPTION OF THE INVENTION

(5) Inter-cell interference is a significant problem and a major source of performance degradation in both uplink and downlink LTE wireless communication systems. This problem is especially significant in cell areas with dense deployment. An accurate estimate of interference information is necessary to effectively suppress inter-cell interference.

(6) Referring to FIG. 3, there is a block diagram of a Long Term Evolution (LTE) diversity receiver of the present invention. The receiver includes receive antennas 300-304, however, receivers of the present invention may include as few as two receive antennas and as many as N receive antennas, where N is an integer. Circuit 306 is coupled to the receive antennas and extracts pilot signals from a received data stream as is known in the art. The data stream is then applied to receiver equalizer circuit 308. The equalized data stream is subsequently applied to circuit 310 for demapping, deinterleaving, and decoding. The decoded data stream is then applied to a baseband processor (not shown).

(7) Extracted pilot signals are applied to circuit 312 to estimate the wireless channel. Circuit 312 is coupled to equalizer weight calculation circuit 322. The data stream and extracted pilot signals are also applied to circuit 320 according to the present invention. Circuit 320 may be realized in software, hardware, or a combination of hardware and software. The Long Term Evolution (LTE) data stream comprises a data frame as shown at FIG. 2. Circuit 320 includes multiplication circuit 314, differential circuit 316, and covariance matrix circuit 318. Circuit 320 is also coupled to equalizer weight calculation circuit 322.

(8) Turning now to FIG. 4, there is a flow diagram that will be used to explain operation of the receiver of FIG. 3. A data stream of symbols is initially received by N receive antennas 300-304 at step 400. The LTE data stream for N receive antennas is given by equation [1].

(9) $\begin{array}{cc}\stackrel{.fwdarw.}{y}=\left[\begin{array}{c}{y}_1\\ y_2\\ \mathrm{.Math.}\\ y_N\end{array}\right]=\stackrel{.fwdarw.}{H}s+\stackrel{.fwdarw.}{I}+\stackrel{.fwdarw.}{v}=\left[\begin{array}{c}{H}_1\\ H_2\\ \mathrm{.Math.}\\ H_N\end{array}\right]s+\left[\begin{array}{c}{I}_1\\ I_2\\ \mathrm{.Math.}\\ I_N\end{array}\right]+\left[\begin{array}{c}{v}_1\\ v_2\\ \mathrm{.Math.}\\ v_N\end{array}\right]& \left[1\right]\end{array}$

(10) Here, vector {right arrow over (y)} is the received data or pilot signal from all N receive antennas, s is the transmitted signal or data stream, H is the channel between a remote transmitter and each respective receive antenna, and {right arrow over (H)} are respective interference and noise components associated with each channel. At step 402 the pilot signals are extracted from the data stream by circuit 306. The pilot signals are applied to circuit 312 at step 404 to estimate the wireless channel between a remote transmitter and the N receive antennas. The channel estimate is then applied to equalizer weight calculation circuit 322.

(11) At step 406, each subcarrier from the multiple receive antennas of the received signal is multiplied by a corresponding control signal or known pilot signal s* by circuit 314. The products are stored in vector {right arrow over (z)}.sub.n as in equation [2], where n is the index of each subcarrier.
{right arrow over (z)}.sub.n={right arrow over (y)}.sub.ns.sub.n*[2]

(12) Circuit 316 calculates a differential {right arrow over (q)}.sub.n between any two adjacent subcarriers n and n+1 at step 408 as in equation [3].
{right arrow over (q)}.sub.n={right arrow over (z)}.sub.n{right arrow over (z)}.sub.n+1[3]

(13) At step 410, circuit 318 calculates a covariance matrix R of interference for each subcarrier group as in equation [4]. Here, n.sub.0 and n.sub.1 are preferably lower and upper indices of a column of subcarriers of the data frame of FIG. 2, and H denotes a Hermitian transpose.

(14) $\begin{array}{cc}R=\underset{n=n_0}{\overset{n_1}{.Math.}}{\stackrel{.fwdarw.}{q}}_n{\stackrel{.fwdarw.}{q}}_n^H& \left[4\right]\end{array}$

(15) At step 412, the channel estimate from step 404 and the covariance matrix R from step 410 are applied to equalizer weight circuit 322. Equalizer weights W for the data stream are calculated by weight circuit 322 in response to the channel estimate and covariance matrix R. These weights are applied to receiver equalizer circuit 308 at step 414 to correct received data symbols and suppress interference in the received signal. In general, covariance matrix R can be used in any equalizer weight calculation method to suppress interference energy in the received signal. In a preferred embodiment of the present invention, the channel estimate from circuit 312 is used together with covariance matrix R in a linear minimum mean squared error (LMMSE) method according to equation [5] to produce equalizer weights W.
W=.sup.H(.sup.H+R).sup.1[5]
The corrected data symbols less interference are then applied to circuit 310 for demapping, deinterleaving, and decoding. The decoded symbols are then applied to a baseband processor.

(16) There are several advantages of the present invention over interference suppression methods of the prior art. First, interference suppression of the present invention does not depend on the channel estimate. Thus, errors in the channel estimate do not negatively impact interference suppression. This is especially important in high density areas where signal quality is degraded. Second, the present invention advantageously employs the LTE wireless characteristic that there is little difference in channels for adjacent or closely spaced subcarriers. Thus, a difference in signals on adjacent subcarriers is primarily due to interference. Third, adjacent LTE subcarriers are typically separated by 15 KHz or 7.5 KHz. This is much less than the coherence bandwidth of the channels. For example, the coherence bandwidth for 0.9 correlation for the extended pedestrian A specification is approximately 460 Hz and for the extended vehicular A specification is approximately 60 Hz. Consequently, it is not strictly necessary to compare signals on adjacent subcarriers as long as the subcarriers are closely spaced. Moreover, multiple comparisons such as with upper and lower adjacent subcarriers are possible for confirmation of the covariance matrix.

(17) Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling with the inventive scope as defined by the following claims. Other combinations will be readily apparent to one of ordinary skill in the art having access to the instant specification.