Apparatus and method for improved performance in MC-CDMA radio telecommunication systems that use pulse-shaping filters

• US 20050018598A1
• Filed: 12/22/2003
• Published: 01/27/2005
• Est. Priority Date: 07/25/2002
• Status: Active Grant

• Current Assignee: Hannu Vilpponen, Mohammad J. Borran, Panayiotis Papadimitriou, Prabodh Varshney
• Original Assignee: Nokia Corporation
• Inventors: Borran, Mohammad J., Varshney, Prabodh, Vilpponen, Hannu, Papadimitriou, Panayiotis
• Granted Patent: US 7,400,686 B2
• Time in Patent Office: Days
• US Class Current: 370/208
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/0212 of impulse response
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 25/0224 using sounding signals
• CPC Class Codes: H04L 25/0246 with factorisation
• CPC Class Codes: H04L 25/025 using least-mean-square [LMS] method
• CPC Class Codes: H04L 25/03834 using pulse shaping
• CPC Class Codes: H04L 27/2602 Signal structure
• CPC Class Codes: H04L 27/2644 with oversampling
• CPC Class Codes: H04L 27/265 Fourier transform demodulators, e.g. fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators H04L27/26524 takes precedence
• CPC Class Codes: H04L 5/026 using code division

Abstract

A apparatus and method for improved performance in radio telecommunications systems, and in particular multi-carrier code division multiple access (MC-CDMA) networks that employ pulse-shaping filters on the transmit side of a radio link. In order to more accurately transmit a radio signal bearing a symbol sequence, the modulated and spread information stream is upsampled using a technique that involves inserting zeros in the frequency domain. A corresponding downsampling procedure on the receive side permits reconstruction of the transmitted symbols. A new channel estimation algorithm may also be used, and the improved channel estimation advantageously employed to obtain more faithful symbol detection.

Claims

1. An apparatus for use in a multi-carrier code division multiple access (MC-CDMA) telecommunication network, said apparatus comprising:

a transmitter for transmitting a radio signal, wherein the transmitter comprises;

an orthogonal frequency division multiplexing (OFDM) modulator that divides an input vector X of length N_b into two parts, inserts N_x zeros between them, and takes an inverse fast Fourier transform (IFFT) of size N_bN_x to obtain a time-domain signal; and

a pulse-shaping filter coupled to the OFDM modulator for processing the time-domain signal prior to transmission; and

a receiver for receiving the transmitted time-domain signal, wherein the receiver comprises an OFDM demodulator for taking a fast Fourier transform (FFT) of size N_bN_x, of the received signal and removes the zeros inserted in the transmitter from the resulting frequency-domain signal.

2. The apparatus of claim 1, wherein the time-domain signal has a sampling rate of Ns samples per chip, and wherein N_x, =N_b(N_s−

1).

3. The apparatus of claim 1, wherein the pulse-shaping filter is a raised-cosine filter.

4. The apparatus of claim 1, wherein the inserted zeros are higher frequency components than the input vector X.

5. The apparatus of claim 1, wherein the receiver further comprises a filter matched to the pulse-shaping filter of the transmitter for filtering the received signal and providing the filtered signal to the OFDM demodulator.

6. The apparatus of claim 1, wherein the transmitter further comprises:

an encoder for encoding information to be transmitted; and

a modulator for modulating the encoded information.

7. The apparatus of claim 1, wherein the OFDM demodulator reconstructs as output vector Z the transmitted signal by concatenating the remaining portions of the frequency-domain signal after removing the zeros.

8. The apparatus of claim 7, wherein the receiver further comprises a detector for detecting received symbols after the transmitted signal has been converted to the frequency domain.

9. The apparatus of claim 8, wherein the receiver further performs channel estimation by analyzing received pilot symbols that were inserted in the frequency domain by the transmitter.

10. The apparatus of claim 9, wherein the frequency response of the pulse-shaping filter is known, and wherein performing the channel estimation applies the known pulse-shaping filter frequency response.

11. The apparatus of claim 10, wherein the receiver performs the channel estimation using a least squares (LS) analysis.

12. The apparatus of claim 9, wherein the detector applies the channel estimation to the detection of the received symbols.

13. For use in a telecommunication system operable according to an MC-CDMA protocol, an improved receiver for processing a received time-domain signal that was upsampled by taking the IFFT of a divided input signal into which zeros have been inserted in the frequency domain, said receiver comprising:

an OFDM demodulator for taking an FFT of the received time-domain signal and removing the inserted zeros.

14. The receiver of claim 13, wherein the OFDM demodulator comprises a lowpass filter for removing the zeros.

15. A method for processing an encoded and modulated information-bearing signal to be transmitted over an air interface in an MC-CDMA communication network;

said method comprising the steps of;

dividing the modulated information-bearing signal into a plurality of streams;

spreading each of the plurality of streams with a spreading code;

summing the spread streams into a symbol sequence of length N_b;

dividing the symbol sequence into two portions;

inserting zeros between the two portions;

taking an IFFT of the resulting stream to obtain a time-domain signal;

providing the time-domain signal to a pulse-shaping filter; and

transmitting the signal.

16. The method of claim 15, wherein the zeros are inserted as high frequency components.

17. The method of claim 15, further comprising the steps of:

receiving the transmitted time-domain signal;

taking an FFT of the received signal to obtain a frequency-domain signal; and

removing the previously inserted zeros from the frequency-domain signal.

18. The method of claim 17, further comprising the step of estimating the frequency response of the transmission channel.

19. The method of claim 18, wherein the transmitted signal is transmitted by a transmitter having a pulse-shaping filter with a known frequency response, and wherein the step of estimating comprises determining a maximum likelihood estimate by solving a LS problem that applies the known pulse-shaping filter frequency response.

20. The method of claim 18, further comprising the step of detecting the received symbols using the channel estimate.