GB2372181A

GB2372181A - Coherent demodulation of OFDM radio signals

Info

Publication number: GB2372181A
Application number: GB0128346A
Authority: GB
Inventors: Frank Hofmann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2000-12-06
Filing date: 2001-11-27
Publication date: 2002-08-14
Anticipated expiration: 2021-11-27
Also published as: FR2817689A1; DE10060569A1; GB0128346D0; GB2372181B; DE10060569B4; JP2002208905A; FR2817689B1

Abstract

A method of coherent demodulation of radio signals, wherein OFDM-symbols which are used for the transmission of additional data and are not required for channel-encoding are buffered, in order to be used for determining replacement pilots during a switch to an alternative frequency.

Description

l DESCRIPTION

Method of Coherent Demodulation of Radio Signals The invention relates to a method of coherent demodulation of radio signals of the type wherein the radio signals are received in the orthogonal frequency multiplex (OFDM) in OFDM- symbols, wherein the OFDM-symbols are divided into frames, a first part of the OFDM-symbols of a frame is used for the transmission of additional data and a second part is used for the transmission of useful data, in the first part and in the second part pilots for the purpose of channel assessment are received and wherein the channel assessment is used for coherent demodulation of the radio signals.

In the coherent demodulation of radio signals, it is already known to provide a channel assessment for the demodulation. Amplitudes and phase distortions of the radio signals, which have occurred by reason of the radio channel, are determined in the channel assessment and can therefore also be compensated for. In the case of digital radio transmission methods, pilots, i.e. training sequences which are known to the receiver, are especially used for the purpose of determining the channel transmission function. In the case of DRM (Digital Radio Mondiale), it is possible to transmit the same audio-service on different frequencies. Therefore, it is also possible to perform a switch to an alternative frequency, if the received audio-service is received less effectively on a set frequency than on an alternative frequency. The various transmitters which transmit this audio-service on the different frequencies, comprise mutual propagation time differences caused by reason of the transmitter

intervals with respect to a receiver, so that the radio signals which are transmitted on the different frequencies and which are transmitted in frames are received with their respective frame beginning at different times at the receiver.

It is an object of the invention to render it possible to improve channel assessment during a switch to an alternative frequency.

In accordance with the present invention on the reception-side of the first part of the OFDM-symbols is buffered as comparative data; upon switching the frequency on the reception-side to an alternative frequency of a set audio-service, replacement pilots are generated by means of a comparison of the comparative data and the first part of the OFDM-symbols which have been received on the alternative frequency; and by interpolation of the replacement pilots and the received pilots in a channel assessment is performed on the alternative frequency.

In contrast to the known arrangements described hereinbefore, the method in accordance with the invention has the advantage that it is possible to improve channel assessment for the received radio signals during a switch to an alternative frequency, since replacement pilots are generated and used for channel assessment by means of a comparison of OFDM-symbols which are used for the purpose of transmitting additional data. This then produces a smaller bit-error rates as the phase and amplitude distortions can be compensated for, and thus also an improved audio quality for the set audio-service.

It is particularly advantageous that the OFDM-symbols which are used by the originally set audio-service for the purpose of determining the replacement pilots are

initially channel-decoded, buffered and then channel-encoded once again, in order to be ready for the comparison.

Furthermore, it is advantageous that the replacement pilots are added to specified OFDM-carriers, so that it is then possible to interpolate the channel assessment in a time direction for each OFDM-carrier and then on the basis of these OFDM-carriers to perform frequency-interpolation for all remaining OFDM-carriers which do not comprise any pilots. It is then possible to perform channel assessment for all OFDM-cells.

It is also advantageous that, if a switch is made back from the alternative frequency to the original frequency, replacement pilots are likewise generated, in order to be able to perform the channel assessment for all OFDM-cells.

Finally, a receiver can be provided which comprises means for implementing the method in accordance with the invention.

The invention is described further hereinafter by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram illustrating one embodiment of a receiver in accordance with the present invention; Figure 2 is a flow diagram illustrating one embodiment of a method in accordance with the present invention; Figure 3 shows a distribution of the pilots in the OFDM-multiplex, Figure 4 shows a svvitch-over between frames of an audio-service on two different frequencies, and

Figure 5 shows the replacement pilots in the SDC-symbols.

The transmission system Digital Radio Mondiale (DRM) for the transmission bands below 30 Megahertz uses a coherent modulation, either 1 6-QAM (Quadrature Amplitude Modulation) or 64-QAM in combination with OFDM (Orthogonal Frequency Division Multiplex), in order to achieve high transmission efficiency. A channel assessment is required for the purpose of coherent demodulation in the receiver. The channel assessment determines the amplitude and phase distortion of the transmission channel, in this case the radio channel. In order to be able to estimate the radio channel, signal parts which are known to the receiver are added to the signal in the transmitter and are referred to hereinunder as pilots. In the case of incoherent methods such as DAB (Digital Audio Broadcasting) , this is not required, since in this case it is the difference in phases between the consecutive symbols which is evaluated.

The OFDM-transmission method is a multiple carrier method having a specified symbol duration and a corresponding carrier number. The OFDMcells are the signal parts having a symbol duration on the different carriers, whereas an OFDM-symbol is made up in each case of the sum of the OFDM-cells for a symbol duration. A method of coherent demodulation of radio signals is proposed herein, wherein known OFDM-syrnbols are used for the purpose performing a comparison, if frequency switching occurs for the same audio-service, in order to generate replacement pilots and thus to compensate for the loss of pilots which could not be

received by reason of the propagation time differences. These replacement pilots then render it possible to perform a channel assessment for all OFDM-Symbols, so as to improve the bit-error rate and thus the audio quality.

The term coherent demodulation refers to demodulation with the aid of the carrier having frequency- and phase-accuracy.

Figure 3 shows a section of an OFDM-signal in the time-frequency-plane.

The X-axis represents the frequency and the Y-axis represents time. Each circuit corresponds to an OFDM-cell. The OFDM-carrier numbers are provided in the X-

axis direction. By way of example, this section of the OFDM-signal shows 16 OFDM-carriers in this case. As shown in Figure 3, the frequency spacing between two carriers is M: The OFDM-symbol numbers are plotted in the ordinate direction, wherein the first symbol has the number -6. Figure 3 shows 13 consecutive OFDM-

symbols, wherein each black-filled circle corresponds to an OFDM-cell with a known phase and known amplitude and can thus be used as a pilot. The white-filled circles correspond to modulated OFDM-cells which are used for data transmission purposes and are allocated to the so-called MSC (Main Service Channel). This data is either multimedia data or audio data. The gray-filled circles correspond to modulated OFDM-cells which are used for data transmission of the SDC (Service Description

Channel) which transmit [sic] so-called additional data. The additional data does not contain any audio data as in MSC, but rather multiplex information.

The pilots can be used for the purpose of sub-sampling the channel transmission function. The channel transmission function characterises the radio

channel. In this case, the term sub-sampling means that not all points of the channel transmission function are established directly by pilots. This is then achieved by means of interpolation. The influence of the radio channel upon the OFDM-data cells is then ascertained.

Such interpolation filters are normally used one-dimensionally in a time and frequency direction, since this constitutes a substantially smaller computation effort than e.g. a two-dimensional filter. In the case of the diagram as shown in Figure 3, interpolation is initially performed on the carriers 0, 2, 4..., K in a time direction over a number of pilots, wherein this number is 3 in this case. In order, for example, to obtain the interpolation value for the data cell at the point (0,2), the pilot cells at (0,-

3), (0,0) and (0,3) are required. Alternatively, it is also possible to use the pilot cells (0,0), (0,3) and (0,6) to perform interpolation. With respect to the values for the cells, the carrier number is stated initially followed by the symbol number. In a second step, the interpolation is performed in the frequency direction, in order to obtain estimated values for the data cells of the carriers 1, 3, 5,., K- 1. These carriers do not contain any pilots. Accordingly, pilots are provided in the even carrier numbers but not in the uneven carrier numbers.

Figure 4 illustrates the switch-over from an alternative frequency in the case of an audio-service. The reception frequency F 1, which receives the current audio-

service, is initially set. Therefore, the top part of the Figure schematically illustrates for the reception frequency F1 a frame in which the data is transmitted At the beginning of the frame, there are located two OFDM-symbols which are carriers of

the additional information, i.e. associated with the SDC. The same audioservice is also received on the reception frequency F2. This frequency F2 is known to a receiver. It can be contained, for example, in the additional data SDC, i.e. a list of alternative frequencies for a set program. If the receiver now switches to the frequency F2, it must calculate as rapidly as possible a channel assessment for the new frequency, in order to be able to commence with the channel-decoding of the channel. As illustrated in Figure 4, the two signals in the case of the reception frequencies F 1 and F2 are, however, displaced with respect to each other by reason of the propagation time differences between the transmitters on the receiver. The propagation time differences are determined in advance by the receiver by means of a comparison of frame beginnings. However, other methods are also possible. The switch-over time from frequency F1 to frequency F2 is marked in this case by an arrow. The switch-over takes place during the SDC, since these SDC-symbols do not contain any data which is required for the purpose of decoding the audio-service and which therefore does not always have to be decoded. As illustrated in this case, the SDC-symbols are located in each case at the frame beginning after e.g. 75 OFDM-

symbols. It is evident in Figure 4 that a receiver which switches over from frequency F 1 to frequency F2, cannot completely receive the SDCsymbols by reason of the propagation time differences. This has consequences for the channel assessment.

For this purpose, it is necessary to refer back to Figure 3. The middle part of the

diagram in Figure 3 illustrates which OFDM-cells are used to fill the SDCsymbols.

Since the SDC-symbols also contain pilots and if the SDC-symbols cannot then be received, the pilots are lost and are not available for the purpose of channel assessment. If the OFDM-cell at (0,2) in Figure 3 is considered and it is not possible to receive the first SDC-cell at (0,0), then the pilot is not available at (0,0) for the purpose of channel assessment and no channel assessment is provided for the OFDM-

cell (0,2). This can lead to an increased bit-error rate and thus reduce the audio quality. In accordance with the present method, the receiver then uses the remaining SDC-cell, in order to perform a channel assessment. The SDC-symbols at the beginning of the frame have the same data content over a fixed period of time, i.e. Over several transmission frames, and furthermore for both frequencies F 1 and F2.

If the receiver has received the SDC-symbols on the frequency F1 and switches to frequency F2, it consequently knows the SDC-data. It stores this SDC-data which has been received on frequency F1 and generates the SDC-symbols itself in the same way a transmitter would. That is to say, the receiver uses the known SDC-data initially to perform channelencoding, followed by a corresponding modulation and finally the OFDMcell allocation. The calculation must be as precise as that performed in the transmitter. After a switch has been made to an alternative frequency, the receiver receives one or both SDC-symbols on frequency F2. By means of a comparison of the received and stored SDC-symbols, it is possible to calculate the channel transmission function, and furthermore by means of a simple

quotient formation. Therefore, the receiver obtains a replacement pilot at the beginning of the transmission frame on frequency F2. The receiver is then able to perform a time interpolation for all OFDM-cells on the relevant carrier and then perform a corresponding frequency interpolation, in order to implement the channel assessment for each OFDM-cell.

Figure 5 illustrates how the replacement pilot cells have been added for the channel assessment, since the first SDC-symbol could not be received. It is evident that for the carrier numbers 0, 6 and 12 replacement pilots have been generated in each case, as indicated by means of the crossed circle, in order to be able to perform an interpolation for these carriers in the time direction.

Figure 1 illustrates a block diagram of a receiver in accordance with the invention. An antenna 1 is connected to an input of a high frequency receiver 2. The antenna 1 is used for the purpose of receiving the radio signals, in order then to be filtered in the high frequency receiver 2, amplified and converted to an intermediate frequency. Furthermore, the OFDM-demodulation is performed here. The signal stream thus produced is then transmitted to a digital part 3 which performs the digitization process, so that it is subsequently possible to perform the demodulation and the channel-encoding and, where appropriate, the source-decoding on a processor 4. A data output of the digital part 3 is therefore connected to a data input of the processor 4. The processor 4 performs the coherent demodulation of the quadrature amplitude modulation, in order to perform the channel-decoding with the aid of the demodulated signals. However, prior to the demodulation process, the processor 4

performs a channel assessment, wherein the processor 4 utilizes the pilots which are illustrated in Figures 3 to 5. The processor 4 then performs the interpolation in the time and frequency direction for the OFDM-data cells.

The processor 4 uses a memory 8 which is connected to a data input/output to store the aforementioned SDC-data for a possible switch to be made to an alternative frequency. If the processor 4 recognises that during a switch to an alternative frequency not all of the SDC-symbols have been received on the new frequency, the processor 4 utilizes the stored SDCsymbols, in order to generate replacement pilots in each case. Said processor compares the stored SDC-symbols with the currently received symbols and forms a quotient, in order to calculate the channel transmission function of the radio channel and thus to obtain a channel assessment. The processor 4 is then able to perform an interpolation in the time and frequency direction for the data received on the alternative frequency.

The processor 4 then transmits the channel-encoded data, if this is audio data, to a source decoder 7 which is itself either a processor or an algorithm executed in hardware. The source decoder 7 is then connected to a digital-analogue converter 9 which then outputs analogue signals to an audio amplifier 10, wherein the audio amplifier 10 is then connected to a loudspeaker 1 1 which ultimately reproduces the audio signals.

The processor 4 is connected via a second data output to a signal processing device 5, in order to reproduce multimedia data, i.e. text, image and graphics data on a display 6 which is connected to a data output of the signal processing device 5.

Figure 2 illustrates a flow diagram of a method of coherent demodulation of radio signals in accordance with the invention. In method step 12, the radio signals are received by means of the antenna l, in order then to be filtered by the high frequency receiver 2, amplified and converted into an intermediate frequency and in order at this point to perform an OFDMdemodulation of the received radio signals.

In method step 13, the signal stream which is produced in this manner is digitized. In method step 14, a channel estimation is performed, as illustrated above, with the aid of pilots contained in the signal stream. Therefore, the coherent demodulation can be performed in method step 14 and the channel-decoding can be performed in method step 15. In method step 16, the SDC-symbols are buffered. In method step 17, a decision is reached as to whether a switch is to be made over to an alternative frequency. This is determined with the aid of signal parameters which are determined for both frequencies, the currently set frequency and the alternative frequency. Such signal parameters include e.g. the reception field strength or a bit-

error rate. These signal parameters are determined, for example, during part of or during a whole transmission frame. The loss of a transmission frame can be compensated for by the methods of channel-encoding, i.e. error correction, or error compensation or error masking, without causing significant reductions in audio quality. If it has been ascertained in method step 17 that a switch is to be made to an alternative frequency, then a check is carried out in method step 18 as to whether all SDCsymbols have been received. If this is the case, then in method step 19 a

channel assessment is performed for the reception frequency F2. If this is not the case, then in method step 24 replacement pilots are generated, as described above, for the specified carriers with the aid of the SDC- data, in order then to be able to perform the corresponding channel assessment in method step 19.

If it has been ascertained in method step 17 that it is not necessary to switch to an alternative frequency, then a check is carried out in method step 20 as to whether the channel-encoded data is audio data or different multimedia data. If the data is multimedia data of this type, then in method step 21 this data is reproduced by the display 6. If the data is not multimedia data, then it is audio data and this is then supplied to the source decoder 7 in method step 22, in order then in method step 23 to use the source-decoded audio data and the audio data converted into analogue signals for the purpose of audio amplification and finally acoustic reproduction by means of the loudspeaker 11.

Claims

1. A method of coherent demodulation of radio signals, in which the radio signals are received in the orthogonal frequency multiplex (OFDM) in OFDM-

symbols, the OFDM-symbols being divided into frames; a first part of the OFDM-

symbols of a frame is used for the transmission of additional data and a second part is used for the transmission of useful data; in the first part and in the second part pilots for the purpose of channel assessment are received; and the channel assessment is used for coherent demodulation of the radio signals; and wherein on the reception-

side the first part of the OFDM-symbols is buffered as comparative data; upon switching the frequency on the reception-side to an alternative frequency of a set audio-service, replacement pilots are generated by means of a comparison of the comparative data and the first part of the OFDM-symbols which have been received on the alternative frequency; and by interpolation of the replacement pilots and the received pilots a channel assessment is performed on the alternative frequency.

2. A method according to claim 1, wherein the OFDM-symbols of the first part are decoded, buffered and then encoded, in order to be used as comparative data.

3. A method according to claim 1, wherein the replacement pilots are added to specified OFDM-carriers, wherein the interpolation is performed in the time direction for the specified carriers.

4. A method according to claim 1, 2 or 3, wherein an interpolation is performed in the frequency direction on the basis of the interpolation performed in the time direction.

5. A method according to any of the preceding claims, wherein replacement pilots are generated as a switch is made back from the alternative frequency.

6. A receiver for the purpose of implementing the method according to any of the preceding claims 1 to 5, wherein the receiver comprises a demodulator for coherent demodulation, a processor for generating the replacement pilots, and a memory for buffering the comparative data.

7. A method of coherent demodulation of radio signals substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.