GB2370476A - Combined demodulation of OFDM and CDMA signals using reduced area on an ASIC by sharing FFT hardware - Google Patents

Abstract

A receiver capable of successfully receiving both DAB or DVB digital audio/video signals modulated using OFDM, and CDMA telecommunications signals, is based around an ASIC containing FFT hardware. The FFT hardware is shared in the demodulation of data from signals in either format, the demodulation of the CDMA signals using an overlap-save FFT technique rather than a matrix inversion technique. This arrangement rrquires less area on the ASIC.

Description

COMBINED DEMODULATION The orthogonal frequency division multiplexing (OFDM) modulation scheme is used, amongst other purposes, to transmit digital audio and digital video signals. Both digital video broadcasts (DVB) and digital audio broadcasts (DAB) are transmitted using this scheme. A receiver for OFDM signals uses a fast fourier transform (FFT) method to demodulate the digital data from the received transmission. The demodulated digital data is then passed to a decoder for subsequent decoding.

In one mode of operation, a CDMA telephone system transmits signals in a time division (TD) format, the UMTS-TDD (time division duplex) format for example. The UMTS-TDD mode is typically recovered in a receiver using a matrix inversion for the demodulation, meaning that there is little synergy with a OFDM receiver. However, it is known that the UMTS-TDD mode can be implemented using the overlap-save FFT method.

According to one aspect, the invention relates to a receiver comprising fourier transform means for use in demodulating both OFDM and TD-CDMA signals.

Thus, the invention provides a device which can recover efficiently both TD-CDMA and OFDM signals in that the fourier transform means is used for demodulating both kinds of signals. Where the receiver is implemented as integrated circuit, such as an application specific integrated circuit (ASIC), the sharing of the fourier transform means may result in a valuable saving of silicon area within the device.

In one embodiment, the fourier transform means implements an overlap-save FFT technique for demodulating the TD-CDMA signals.

The invention is useful in that its simplifies the production of a device which is capable of receiving both broadcast signals destined for many subscribers (DAB and DVB signals) and subscriber-specific CDMA signals. The capability to receive such signals will be of use in providing, for example, wireless digital television units wherein the TV subscriber is capable of interacting with the content supplier using the CDMA capability (e. g. to order movies). The sharing of the fourier transform means complements the sharing of other devices (such as a decoder for the demodulated received signals, a screen and a keypad for interacting with the user) which one would normally find present in each of a OFDM device and a CDMA device.

By way of example only, an embodiment of the invention will now be described with reference to the accompanying figures, in which: Figure I illustrates a prior art ASIC designed to perform TDD demodulation and DVB demodulation; and Figure 2 illustrates an ASIC according to the invention for performing TDD and DVB modulation.

Figures I and 2 illustrate, respectively, application specific integrated circuits (ASICs) 10 and 12. Each of the ASICs 10 and 12 forms the basis of a demodulator for a receiver.

Each of the ASICs 10 and 12 is capable of demodulating UMTS-TDD and DVB signals.

The prior art ASIC 10 comprises discrete sections for demodulating DVB and UMTS-TDD signals respectively, as indicated by the two vertical bars at the left hand side of Figure 1. The DVB demodulation section of ASIC 10 comprises FFT hardware for demodulating the received OFDM-type DVB signals. Whilst the FFT hardware constitutes the heart of the OFDM demodulation process, the OFDM control section provides control circuitry for, inter alia, frequency correction, timing estimation and channel decoding. The UMTS demodulation section of ASIC 10 comprises matrix inversion hardware for performing a matrix inversion on the received data to achieve demodulation of UMTS-TDD signals. The TDD control section is for, inter alia, channel estimation, FFT engine loading and channel decoding in relation to received UMTS-TDD signals. Effectively, the DVB demodulation and UMTS-TDD demodulation sections function independently of one another. In the ASIC 12 of Figure 2 the DVB demodulation section is substantially the same as that employed in ASIC 10. The UMTS-TDD demodulation section, however, is different. The UMTS-TDD demodulation section uses an overlap-save FFT routine to demodulate the UMTS-TDD signals. The UMTS-TDD demodulation section comprises a discrete TDD control section for controlling the FFT process used in the UMTS-TDD demodulation, and the TDD control section utilises a portion of the FFT hardware of the DVB demodulation section to perform the overlap-save FFT routine. This sharing of a portion of the ASIC, namely of a part of the FFT hardware, is indicated by the overlapping bars at the righthand side of Figure 2 denoting the extent of the two demodulation sections.

The design of ASIC 12 achieves a hardware saving over the design of ASIC 10 since it occupies less silicon area by omitting the matrix inversion hardware. The compact nature of ASIC 12 lends itself to systems wherein UMTS-TDD and DVB signals are used together, e. g. in interactive wireless television. By combining the abilities to receive UMTS-TDD and DVB signals in the same ASIC 12, and hence in the same user device, it is possible to make further hardware savings by providing a single decoder for operating on both types of demodulated signals (the decoder exploits redunancy within the UMTS and DVB/DAB signals to remove errors). Furthermore, DVB and UMTS-TDD signals would normally be handled in entirely separate devices each with their own user input (keypad) and output (screen) devices, whereas a user device based around the ASIC 12 of the invention requires a single set of such i/o devices.

Claims (7)

1. CLAIMS 1. A receiver comprising fourier transform means for use in demodulating both OFDM and TD-CDMA signals.
2. 2. A receiver according to claim 1 wherein the fourier transform means implements an overlap-save FFT technique for demodulating the TD-CDMA signals.
3. 3. A receiver according to claim 1 or 2, further comprising a decoder for use in decoding both OFDM and TD-CDMA signals.
4. 4. A receiver according to any preceding claim wherein the fourier transform means is implemented on an application specific integrated circuit.
5. 5. A receiver according to any preceding claim wherein the OFDM signals are DAB or DVB signals.
6. 6. A receiver according to any preceding claim, wherein the TD-CDMA signals are UMTS-TDD signals.
7. 7. A receiver, substantially as hereinbefore described with reference to Figure 2.