JP2009212835A - Ofdm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM (Orthogonal Frequency Division Multiplexing) receiver having the structure of antenna diversity high in diversity effect. <P>SOLUTION: The OFDM receiver is composed of: AD converters 13, 14; a delayer 15; OFDM demodulation circuits 16, 17; and a synthesis and decision unit 18. Signals received by a plurality of antennas 11, 12 are converted into baseband signals, sampled and made into digital signals by the AD converters 13, 14 to be driven by being shifted by a fraction interval of a sample period Ts. In each antenna branch, OFDM signals are demodulated by the OFDM demodulation circuits 16, 17, and decided by performing synthesis such as the maximum ratio synthesis for each signal of each subcarrier by the synthesis and decision unit 18. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) receiver, and more particularly to an OFDM receiver having a configuration of antenna diversity with a high diversity effect.

  FIG. 4 is a diagram illustrating a configuration of an OFDM receiving apparatus according to Prior Art 1. This OFDM receiver comprises antennas 21 and 22, AD converters 23 and 24, OFDM demodulation circuits 25 and 26, and a combiner / determinator 27. Signals received by the plurality of antennas 21 and 22 are converted into baseband signals, which are sampled and converted into digital signals by AD converters 23 and 24 driven by the same clock. In each antenna branch, the OFDM signal is demodulated by the OFDM demodulation circuits 25 and 26, and is synthesized and determined for each subcarrier signal by the synthesizer / determinator 27. This configuration is a general antenna diversity configuration.

  FIG. 5 is a diagram illustrating a configuration of an OFDM receiving apparatus according to prior art 2. The OFDM receiver includes antennas 31 and 32, AD converters 33 and 34, a cyclic delay or delay circuit 35, a combiner 36, an OFDM demodulator circuit 37, and a determiner 38. In this configuration, signals received by different antennas 31 and 32 are synthesized by cyclic delay or delay on the time axis by a cyclic delay or delay circuit 35 (see, for example, Non-Patent Document 1).

FIG. 6 is a diagram showing the concept of cyclic delay synthesis and delay synthesis. The delay synthesis is performed by giving an appropriate delay amount to other signals within a range that falls within the guard interval. Cyclic delay synthesis is performed by removing a guard interval GI of another signal and then giving an appropriate cyclic delay amount. The response in each subcarrier is changed by cyclic delay synthesis or delay synthesis, and the effect of frequency diversity is improved together with the error correction code.
Armin Dammann, et al., “Standard conformable antenna diversity techniques for OFDM and its application to the DVB-T system,” IEEE GLOBECOM '01, Nov. 2001, vol. 5, pp. 3100-3105.

  However, in both prior arts 1 and 2, since the sampling timing is the same for both antennas, the correlation between antennas is high, that is, the correlation between signals received at different antennas at the same timing is high. In some cases, the received signal level of all antennas may be lowered, and the effect of diversity is limited.

  Further, the combining method of the prior art 2 is limited to equal gain combining in the time domain. In general, a combining method such as maximum ratio combining in each subcarrier has higher diversity effect than equal gain combining.

  In view of the above problems, an object of the present invention is to provide an OFDM receiver having an antenna diversity configuration with a high diversity effect.

  The OFDM receiver of the present invention is different from the first AD converter for the first AD converter that converts the received analog signal from the first antenna into a digital signal and the received analog signal from the second antenna that is different from one antenna. A second AD converter for sampling at a phase and converting it to a digital signal, a first OFDM demodulator for OFDM demodulating the digital signal from the first AD converter, and a second OFDM for OFDM demodulating the digital signal from the second AD converter A demodulator and a combiner that combines the signals demodulated by the first and second OFDM demodulators are provided.

  In addition, since the sample timings of the first and second AD converters are equally spaced, the diversity effect can be further enhanced.

  According to the present invention, the effect of antenna diversity in the OFDM receiver can be enhanced with a simple configuration.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram illustrating a configuration of an OFDM receiver according to an embodiment of the present invention. The OFDM receiving apparatus according to this embodiment includes AD converters 13 and 14, a delay unit 15, OFDM demodulation circuits 16 and 17, and a combiner / determiner 18. The antennas 11 and 12 are not the object of the present invention, but are shown for explanation of the present invention. Signals received by the plurality of antennas 11 and 12 are converted into baseband signals, sampled by the AD converters 13 and 14 that are driven by a fractional interval of the sample period Ts, and converted into digital signals. Here, since there are two systems of antennas, the delay is Ts / 2. In each antenna branch, the OFDM signal is demodulated by the OFDM demodulating circuits 16 and 17, and the combining & determining unit 18 performs a combination such as maximum ratio combining for each subcarrier signal and determines.

  FIG. 2 is a diagram for explaining the effect of the present invention. When the correlation between the antenna 11 and the antenna 12 is high, a similar response is shown in the delay region. If the antenna 11 and the antenna 12 are sampled at the same timing (● in the figure), it is conceivable that the reception levels at the two antennas simultaneously decrease. On the other hand, when sampling is performed with the antenna 12 shifted by a half of the sample period (Ts / 2) (indicated by a triangle in the figure), the correlation between the received signals at the antenna 11 and the antenna 12 decreases, and a diversity effect can be expected.

  FIG. 3 is a diagram illustrating a result of simulating BER characteristics. As a channel model, a two-wave model with a delay amount Ts / 2 is assumed, and the correlation between the direct wave and the interference wave received with a delay is substantially zero. “Model 1” assumes that radio waves arrive from a direction orthogonal to the straight line connecting the antennas, and “Model 2” assumes that radio waves arrive from a direction of 60 ° with respect to Model 1. “Invention” indicates the case where the time shift sampling of the present invention is used, and “Conventional” indicates the case where the time shift sampling is not used. However, as a synthesis method, the maximum ratio synthesis is assumed in both cases. In any model, the correlation between antennas is close to 1.

  FIG. 3 shows that the error rate is improved by using the time shift sampling of the present invention.

  In addition, this invention is not limited to the said Example.

  The number of antennas is not limited to two and may be two or more. At this time, it is desirable that the sample timing is shifted evenly by a fractional interval of the sample period, but it is not necessarily equal.

  The combining is preferably the maximum ratio combining described in the embodiment (synthesizing with the phase of the received signal being matched), but may be selective combining that selects a large level signal, or equal gain combining that is simply added.

It is a figure which shows the structure of the OFDM receiver by one Example of this invention. It is a figure explaining the effect of this invention. It is a figure which shows the result of having simulated the BER characteristic. It is a figure which shows the structure of the OFDM receiver of the prior art 1. FIG. It is a figure which shows the structure of the OFDM receiver of the prior art 2. FIG. It is a figure which shows the view of cyclic delay synthesis and delay synthesis.

Explanation of symbols

11, 12, 21, 22, 31, 32 Antenna 13, 14, 23, 24, 33, 34 AD converter 15 Delay device 16, 17, 25, 26, 37 OFDM demodulation circuit 18, 27 Synthesizer & determiner 35 Size Click delay or delay device 36 Synthesizer 38 Judger

Claims (2)

A first AD converter for converting a received analog signal from the first antenna into a digital signal;
A second AD converter that samples a received analog signal from a second antenna different from the first antenna at a phase different from that of the first AD converter and converts it into a digital signal;
A first OFDM demodulator for OFDM demodulating the digital signal from the first AD converter;
A second OFDM demodulator for OFDM demodulating the digital signal from the second AD converter;
An OFDM receiving apparatus comprising: a synthesizer that synthesizes the signals demodulated by the first and second OFDM demodulators. 2. The OFDM receiving apparatus according to claim 1, wherein sample timings of the first and second AD converters are equally spaced.

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