FFT SPREADING AMONG SELECTED OFDM SUB-CARRIERS

• US 20100061474A1
• Filed: 05/03/2008
• Published: 03/11/2010
• Est. Priority Date: 05/04/2007
• Status: Abandoned Application

• Current Assignee: NXP B.V. (NXP Semiconductors NV)
• Original Assignee: NXP B.V. (NXP Semiconductors NV)
• Inventors: Razzell, Charles
• Application Number:
  US12/597,469
• Publication Number:
  US 20100061474A1
• Time in Patent Office: Days
• US Class Current: 375/260
• CPC Class Codes: H04L 27/2602 Signal structure
• CPC Class Codes: H04L 5/0007 the frequencies being orthogonal, e.g. OFDM(A), DMT
• CPC Class Codes: H04L 5/0048 Allocation of pilot signals, i.e. of signals known to the receiver

Abstract

An improved communications system for frequency-selective fading channels spreads the data-bearing modulation symbols across many OFDM sub-carriers using a discrete Fourier transform (FFT) at the transmitter, and despreads them using an inverse discrete Fourier transform (IFFT) at the receiver. An IFFT is included at the transmitter to form the OFDM symbols in the usual way, and a corresponding FFT is used at the receiver for the frequency-domain processing of the received signal. The difference is, the size of the FFT used for spreading at the transmitter is smaller than that of an IFFT used just after the FFT to create the OFDM symbols for transmission. This results in improved bit-error-rate, similar to single carrier block transmission (SCBT), while retaining the frequency domain benefits of conventional OFDM, e.g., low out of band emissions and the ability to control the power spectral density using active interference cancellation and other frequency-domain techniques. The improved frequency diversity is especially useful in the higher data rate modes of WLAN or WiMedia Physical Layer Standards, and in other cases where low redundancy forward error correction is employed.

Claims

1. An improved orthogonal frequency division multiplexing (OFDM) communications system, including an inverse fast Fourier transform (IFFT) device in a transmitter to form OFDM symbols, and a corresponding fast Fourier transform (FFT) device at a receiver for frequency-domain processing of a received signal, the improvement comprising:

a transmitter circuit in which data-bearing modulation symbols are spread across many OFDM sub-carriers using a discrete Fourier transform (FFT); and

a receiver circuit that despreads said data-bearing modulation symbols using an inverse discrete Fourier transform (IFFT);

wherein, bit-error-rate (BER) performance overall is improved.

2. The improved OFDM communications system of claim 1, wherein the size of an FFT used for spreading at the transmitter is smaller than that of an IFFT used just after the FFT to create the OFDM symbols for transmission.

3. The improved OFDM communications system of claim 1, wherein low redundancy forward error correction is enabled.

4. A transmitter method for improving bit-error-rate (BER) performance in an orthogonal frequency division multiplexing (OFDM) communications system such that low redundancy forward error correction in high data rate applications, comprising:

using an M-point complex inverse fast Fourier transform (IFFT) device to transform M-complex input symbols into M-complex values;

sub-carrier mapping said M-complex values and N-M additional pilots and null subcarriers, where N>

M; and

processing a N-point complex with an inverse fast Fourier transform (IFFT) device for OFDM wireless transmission.

5. A receiver method for improving bit-error-rate (BER) performance in an orthogonal frequency division multiplexing (OFDM) communications system such that low redundancy forward error correction in high data rate applications, comprising:

decoding N-number of OFDM subcarriers that include M-number of data subcarriers and N-M number of pilot and null subcarriers, where N>

M;

using an N-point complex fast Fourier transform (FFT) device to despread N-complex values;

sub-carrier de-mapping said N-complex values to remove said N-M additional pilots and null subcarriers; and

processing a N-point complex with an M-point complex inverse fast Fourier transform (IFFT) device for recovery of M-complex symbols.

6. A communications system method for improving bit-error-rate (BER) performance in an orthogonal frequency division multiplexing (OFDM) communications system such that low redundancy forward error correction in high data rate applications, comprising:

using an M-point complex inverse fast Fourier transform (IFFT) device to transform M-complex input symbols into M-complex values;

sub-carrier mapping said M-complex values and N-M additional pilots and null subcarriers, where N>

M;

processing a N-point complex with an inverse fast Fourier transform (IFFT) device for OFDM wireless transmission;

with a wireless receiver, decoding N-number of OFDM subcarriers that include M-number of data subcarriers and N-M number of pilot and null subcarriers, where N>

M;

using an N-point complex fast Fourier transform (FFT) device to despread N-complex values;

sub-carrier de-mapping said N-complex values to remove said N-M additional pilots and null subcarriers; and

processing a N-point complex with an M-point complex inverse fast Fourier transform (IFFT) device for recovery of M-complex symbols.