Channel estimation and synchronization with preamble using polyphase code

• US 20050128938A1
• Filed: 12/16/2003
• Published: 06/16/2005
• Est. Priority Date: 12/16/2003
• Status: Active Grant

• Current Assignee: University of Florida Research Foundation, Incorporated (State University System of Florida)
• Original Assignee: University of Florida Research Foundation, Incorporated (State University System of Florida)
• Inventors: Fang, Yuguang, Kim, Byung-Seo
• Granted Patent: US 7,453,794 B2
• Time in Patent Office: Days
• US Class Current: 370/210
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/0226 sounding signals per se
• CPC Class Codes: H04L 27/2605 Symbol extensions, e.g. Zero Tail, Unique Word [UW]
• CPC Class Codes: H04L 27/2613 Structure of the reference signals
• CPC Class Codes: H04L 27/2656 Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• 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

A preamble for an OFDM signal synchronizes (104) and estimates (106) the sub-channels with only one code. One polyphase code sequence is used repeatedly for the preamble. The preamble is spread out over the bandwidth, which is the same as an OFDM symbol in the frequency domain and has good autocorrelation characteristics in the time domain. All OFDM signals are added with this preamble at the beginning of the OFDM signal and transmitted on the channel at a transmitter (50). At the receiving end, the receiver (100) first does the autocorrelation process to find out a peak value for synchronization in the time domain. Then, since the polyphase code is known at the receiver, the signal to noise ratio for each sub-carrier is calculated in the frequency domain and smoothed using the normal (Gaussian) distribution to provide the channel estimation. Since the synchronization and channel estimation are processed with a single preamble, the overhead for these two functions is significantly reduced.

Claims

1. A method of generating a preamble in a OFDM communication system, comprises the steps of:

inverse Fast Fourier-transforming polyphase code sequences whose number is the same as half of an IFFT size using Hermitian symmetry;

replicating output signals from the transforming step for a predetermined number of times to provide replicated signals; and

placing the replicated signals in serial.

2. The method of claim 1, wherein the predetermined number of times is 4.

3. The method of claim 2, wherein the preamble is further used for synchronization by performing the steps comprising:

convolving a received signal with a polyphase code sequence which is same as a transmitted polyphase code sequence; and

maintaining the convolving step until four peaks are found.

4. The method of claim 3, wherein if four peaks are found, the synchronization is done successfully.

5. The method of claim 3, wherein synchronization is unsuccessful if four peaks are not found.

6. The method of claim 4, wherein the threshold for deciding whether a peak is found is a number which is a magnitude of a first peak times a constant that is a number between 0.7 and 1.0.

7. The method of claim 1, wherein the preamble is used for channel estimation comprising the steps of:

taking samples of an IFFT size in advance from a subsequent sample of a sample of each peak;

generating four sample blocks;

Fourier-transforming each of the four sample block to provide Fourier-transformed signals;

taking the Fourier-transformed signals from a first output to an output signal having half of an FFT size;

squaring each of the output signals having the half of the FFT size for calculating magnitudes of the signals;

averaging each of the output signals which are in the same position in the output from the FFT; and

dividing each average of the output signals by a respective magnitude of the polyphase code sequence transmitted originally.

8. A method of generating a training packet for a signal-to-noise ratio calculation and bit loading, comprising the steps of:

using polyphase code sequences whose number is same as the half of an IFFT size;

inverse-Fourier-transforming the polyphase code sequence using Hermitian symmetry and IFFT;

replicating output signals of the IFFT six times; and

placing replicated signals in serial.

9. The method of claim 8, wherein the method further comprises the step of synchronizing using the training packet.

10. A method of calculating a signal-to-noise ratio using polyphase code sequences whose number is the same as a half of an IFFT size, comprising the steps of:

taking samples of an IFFT size in advance from a subsequent sample of a sample of each peak;

generating six sample blocks;

Fourier-transforming each block to provide Fourier-transformed signals;

taking the Fourier-transformed signals from a first output to an output signal having half of an FFT size; and

calculating the signal-to-noise ratio for each sub-carrier with six signals from six Fourier-transformed blocks for a same sub-carrier.

11. The method of claim 10, wherein the method further comprises the step of smoothing a signal-to-noise ratio distribution by convolving the signal-to-noise ratio distribution with 7 sample points of a normal distribution.

12. The method of claim 11, wherein the method further comprises the step of bit allocating by selecting a modulation type for each sub-carrier according to the signal-to-noise ratio distribution.

13. The method of claim 10, wherein the method further comprises the step of generating a bitmap and storing the bitmap.

14. The method of claim 13, wherein the method further comprises the step of transmitting the bitmap.