GB2407462A

GB2407462A - Multi-channel coded orthogonal frequency division modulation system

Info

Publication number: GB2407462A
Application number: GB0422287A
Authority: GB
Inventors: Les Sabel
Original assignee: RadioScape Ltd
Current assignee: RadioScape Ltd
Priority date: 2003-10-07
Filing date: 2004-10-07
Publication date: 2005-04-27
Anticipated expiration: 2024-10-07
Also published as: EP1678838A1; GB0323438D0; WO2005036763A1; GB2407462B; GB0422287D0

Abstract

A method which allows multiple DAB transmission signals in the form of COFDM signal streams to be modulated, up-converted, filtered and combined prior to a High Power Amplifier (HPA), thus reducing the multiple up-conversion and HPA to a single unit and eliminating the need for a high power combiner. This significantly reduces the equipment needed for multi-channel transmissions and the consequent cost. In addition the techniques described provide signal timing alignment as required in Single Frequency Networks (SFN).

Description

- 2407462

MULTI-CHANNEL CODED ORTHOGONAL FREQUENCY DIVISION

MODULATION SYSTEM

FIELD OF THE INVENTION s

The present invention provides a method of processing multiple Coded Orthogonal Frequency Division Modulation (COFDM) signals.

DESCRIPTION OF THE PRIOR ART

Traditionally, multiple DAB system channels which are a transmitted from a single antenna are individually modulated, up-converted to the transmission frequency, amplified using a high power transmitter, filtered and then combined usmg a passive combiner network. The modulation may, or may not, be performed using Digital Signal Processing (DSP) techniques (including algorithms, hardware and software). The output of the modulator is in the form of an analog signal in either a baseband I/Q format or at an Intermediate Frequency (IF).

More specifically, different channels are individual/ amplified to high power, filteecd using cavity (or other) filters and then either combined using a power divider for transmission over a single antenna or are transmitted using multiple antenna systems. Such a construction is expensive due to the power involved for combining equipment and filters. Further, in the case of multiple antennas, the cost of duplicating and erecting the antenna systems can be considerable. This approach has been necessary due to the poor linearity of power amplifiers: this has limited the amplification of multiple signals due to inter-modulation products which cause the transmission signal to violate the spectral mask that must be adhered to according to legal requirements. Indeed even at low powers (e.g. exciter power levels) the ability to provide high linearity has been expensive.

SUMMARY OF THE PRESENT INVENTION

The first aspect of the invention is a method of processing COFDM signals, comprising the steps of: (a) receiving multiple COFDM signal streams as an input; (b) processing each of the signal streams in the digital domain using digital signal processing and then combining them into a single digital stream; (c) converting the single digital stream to analog and then sending the analog signal to a high power amplifier and then to an antenna.

The invention uses digital signal processing methods to provide multichannel capabilities and hence reduce the need for external RF power combiners or multiple antenna systems.

In one implementation, this invention allows multiple COFDM transmission signals to be modulated, up-converted, filtered and combined prior to the High Power Amplifier (HPA), thus reducing the multiple up-conversion and HPAs to a single unit and eliminating the need for a high power combiner. This significantly reduces the equipment needed for multi channel transmissions and the consequent cost. In addition the invention provides signal timing alignment as required in Single Frequency Networks (SFN).

The invention is useful for a range of systems which use COFDM signals and therefore includes without limitation such systems as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Digital Multimedia Broadcasting (DMB) (which is based on DAB) and Wireless Local Area Networks (WLANs), irrespective of the exact details of the COFDM signal format used (e.g. symbol length, number of sub-carriers, FEC code employed etc).

Another aspect is an apparatus for processing COFDM signals, comprising: (a) means for receiving multiple COFDM signal streams as an input; (b) means for processing each of the signals in the digital domain using digital signal processing and then combining them into a single digital stream; (c) means for converting the single digital stream to analog and then sending the analog signal to a high power amplifier and then to an antenna.

A final aspect is a method of transmitting multiple COFDM signals in which the transmitted signals are frequency aligned such that subcarriers for adjacent channels are orthogonal to a wanted signal and hence do not interfere.

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DETAILED DESCRIPTION

While the DAB system is used to aid the description below, the techniques are equally applicable to other systems based on COFDM signals. s

An implementation of the invention uses digital signal processing techniques to convert multiple input DAB F,TI streams (or for other systems the digital input signal) into a single analog output signal suitable for power amplification and RF transmission. This involves signal formatting, coding and interleaving, modulation, frequency shifting and combining in l 0 the digital domain prior to a single frequency up-conversion stage which shifts the combined signal to the wanted frequency band.

A block diagram of the signal processing is shown in Figure 1. For each input ETI stream, the received signal is buffered and then the DAB formatting, coding, interleaving, and l S control channel (FIC), and synchronization insertion, is then performed. The resulting signal is then applied to the OFDM modulator. The resultant baseband signal is then frequency shifted to the frequency selected for that channel with the result being placed in a time alignment buffer. The process is repeated for each input 12TI channel. The frequencies selected for each channel are generally exclusive as required by the associated channel frequency plan. Each of the input time aligned channels are then summed in the digital domain and an output shaping filter is applied to ensure that the overall spectral shaping is correct. This signal is then applied to the DAC for conversion to the analog domain. The output signal is filtered to remove unwanted frequency domain images and then the result is applied as the input to the final frequency conversion stage which shifts the signal to the wanted final RF frequency. This signal is then applied to the high power amplifier, the final output filter and finally the antenna.

There are two levels of time synchronization applied. The first is local, or stream level, where each of the signal streams to be transmitted is aligned in time. This should be an accurate alignment with the symbols in the transmission being aligned to a small fraction of the symbol guard interval. Figure 1 shows the use of Time Alignment buffers on each input ETI stream to provide such stream level time alignment. The second level of synchronization is at network level. Here all transmissions within the network (e.g. the network of computers that performs the required digital signal processing) are also Lime aligned to within a small fraction of a symbol guard interval as required for Single Frequency Network (SFN) operation. An external master clock is required at the network level to provide the timing reference as shown in Figure 2. A preferred embodiment of the time synchonsanon uses the GPS system for the dime reference signals.

Each of the transmitted signals needs to be frequency aligned. As shown in Figure 3, in a typical radio transmission system each Transmission has a specified occupation channel, BCh, and then is flanked by guard bands, Bg, which provide a buffer between signals to ensure that the interference between the adjacent signals is lower enough to ensure suitably high quality service. Clearly this principal also applies to interference from far off (several channel, or lS tens of channels distant) interference. The apparatus provides the ability to position the wanted signals within the appropriate channel spacing e.g. for DAB the channel spacing is 1.712MHz. For COFDM signals the fact that the subcarriers have a spacing which Is the inverse of the symbol period provides the orthogonality which is exploited to provide interference free sub-carrier reception.

This principle can be extended to the individual transmissions within the total signal. In this case, in order to minimise inter-signal interference the signals are aligned on a common frequency grid. In general, as shown in Figure 3, the subcarrier frequency spacing f,c and the channel frequency spacing fc should be related as fc = n fee where n is an integer. For example, if the subcarriers within a transmission have a 1 kHz spacing then the transmissions themselves should also have a 1 kHz spacing, or if necessary n x 1 kHz. Indeed if this principal is adhered to then the need for guard bands between transmissions is no longer required as the subcarriers in an adjacent channel are orthogonal to the wanted signal and hence will not interfere. Note that this is particularly the case when the transmissions are locked together within a common single signal generation system as described here as the frequency spacing can be tightly controlled. If the signals in adjacent channels are

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transmitted by different sites and hence have different and independcut transmission equipment then the use of this principle will require tight frequency synchronization between sites. Note that for SI;N operation it is required that the individual transmitters have a degree of frequency synchronization. This is generally specified to be less than a few percent of the smallest sub-carrier frequency spacing.

A preferred embodiment would use the GPS system to provide an accurate and stable refercoce across physically independent sites. In this case the frequency accuracy must be kept to less than a few percent of the subcarner spacing, e.g. for a lkHz subcarrier spacing a frequency difference between transmitters of less than 10Hz would typically result in a receiver interference loss of less than ldB.

Claims

1. A method of processing COFDM signals, comprising the steps of: (a) receiving multiple COFDM signal streams as an input; (b) processing each of the signal streams In the digital domain using digital signal processing and then combining them into a single digital stream; (c) converting the single digital stream to analog and then sending the analog signal to a high power amplifier and then to an antenna.

2. The method of Claim 1 in which the step of processing the signal streams includes the steps of: (a) signal formatting, coding and interleaving, modulation and frequency shifting each stream separately using digital signal processing.

3. The method of Claim 2 in which each signal stream Is shifted to a different pre defined frequency.

4. The method of any preceding Claim comprising the step of sending the single digital stream to an upconverter prior to the high power amplifier.

5. The method of any preceding claim In which each signal stream is time aligned with all other signal streams with an accuracy better than a small fraction of a symbol guard interval.

6. The method of any preceding claim in which transmission of the signal streams through a network of components that performs the digital signal processing are time aligned with an accuracy better than a small fraction of a symbol guard interval to achieve a single frequency network.

7. The method of any preceding Claim 5 or 6 in which an external clock provides the timing reference nccdcd to ensure time alignment.

8. The method of any preceding claim in which the high power amplifier sends an amplified signal to an antenna for transmission of transmitted signals, the transmitted signals corresponding to each of the input multiple COFDM signal streams, the transmitted signals being frequency aligned such that subcarriers for adjacent channels are orthogonal to a wanted signal and hence do not interfere.

9. The method of Claim 8 in which a subcarrier frequency spacing fsc and a channel frequency spacing fc are related as fc = n f,c where n is an integer.

10. The method of Claim 8 or 9 in which, if adjacent channels are transmitted by physically separate sites, then a GPS system provides an accurate and stable time reference across each site for accurate frequency alignment of the signals from each site.

11. The method of any preceding Claim in which the COFDM signals are DAB ETI streams.

12. An apparatus for processing COFDM signals, comprising: (a) means for receiving multiple COFDM signal streams as an input; (b) means for processing each of the signals in the digital domain using digital signal processing and then combining them into a single digital stream; (c) means for converting the single digital stream to analog and then sending the analog signal to a high power amplifier and then to an antenna.

13. A method of transmitting multiple COFDM signals in which the transmitted signals are frequency aligned such that subcarriers for adjacent channels are orthogonal to a wanted signal and hence do not interfere.

14. The method of Claim 13 in which a subcarner frequency spacing fsc and a channel frequency spacing fc are related as fc = n fsc where n is an integer.

15. The method of Claim 13 or 14 in which, if adjacent channels are transmitted by physically separate sites, then a GPS system provides an accurate and stable time reference across each site for accurate frequency alignment of the signals from each S site.