Apparatus and method for improved performance in radio telecommunication systems that use pulse-shaping filters
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.
1. An apparatus comprising:
a transmitter for transmitting a time domain signal, wherein the transmitter comprises:
an orthogonal frequency division multiplexing (OFDM) modulator that divides a modulated information-bearing signal into a plurality of streams, spreads each of the plurality of streams with a spreading code, sums the spread streams into an input vector X of length N b , divides the input vector X of length N b into two parts, inserts N x zeros between the two parts, and takes an inverse fast Fourier transform (IFFT) of size N b N 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 b N 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. An apparatus comprising:
a receiver configured to receive a time domain signal, wherein the time domain signal is generated by dividing a 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 an input vector X of length N b , dividing the input vector X of length N b into two parts, inserting N x zeros between the two parts, and taking an inverse fast Fourier transform (IFFT) of size N b N x to obtain the time-domain signal, said receiver comprising:
an orthogonal frequency division multiplexing (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 performs a lowpass filtering operation to remove the zeros.
15. A method comprising:
dividing a 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 to generate a resulting stream;
taking an inverse fast Fourier transform (IFFT) of the resulting stream to obtain a time-domain signal;
providing the time-domain signal to a pulse-shaping filter; and
transmitting the time-domain signal over a transmission channel.
16. The method of claim 15 , wherein the zeros are inserted as high frequency components.
17. The method of claim 15 , further comprising:
receiving the transmitted time-domain signal;
taking a fast Fourier transform (FFT) of the received time domain 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 estimating the frequency response of the transmission channel.
19. The method of claim 18 , wherein the transmitted time domain signal is transmitted by a transmitter having the pulse-shaping filter with a known frequency response, and wherein the estimating comprises determining a maximum likelihood estimate by solving a least squares problem that applies the known pulse-shaping filter frequency response.
20. The method of claim 18 , further comprising detecting received symbols using the estimated frequency response of the transmission channel.
21. A transmitter comprising:
an orthogonal frequency division multiplexing (OFDM) modulator that divides a modulated information-bearing signal into a plurality of streams, spreads each of the plurality of streams with a spreading code, sums the spread streams into an input vector X of length Nb, divides the an input vector X of length Nb into two parts, inserts Nx zeros between the two parts, and takes an inverse fast Fourier transform (IFFT) of size NbNx 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.