Block time domain channel estimation in OFDM system
An OFDM receiver receives OFDM symbols in the frequency domain and comb filters and then punctures the OFDM symbols to leave symbols with actual pilot information and with null values at the data symbols. The receiver provides the punctured OFDM symbols to an OFDM symbol queue. A virtual pilot interpolator is coupled to the punctured OFDM symbol storage to generate virtual pilot information introduced to OFDM symbols. The interpolator may be a two dimensional Wiener filter. The receiver also includes a time domain channel estimator that processes a first OFDM symbol including virtual pilot information to generate a channel impulse response for the first OFDM symbol. A frequency equalizer equalizes the OFDM symbol in response to the channel impulse response for the first OFDM symbol.
1. An OFDM receiver that determines a time domain channel impulse response, the receiver comprising:
a filter that receives OFDM symbols, the filter comb filtering and puncturing OFDM symbols to provide punctured OFDM symbols having pilot information and having at least some data information removed;
punctured OFDM symbol storage to receive and store a number of punctured OFDM symbols;
a virtual pilot generator coupled to the punctured OFDM symbol storage to generate virtual pilot information for a plurality of OFDM symbols based on one or more of the punctured OFDM symbols, the virtual pilot information based at least in part on the pilot information;
a time domain channel estimator that processes a first punctured OFDM symbol including virtual pilot information to generate a channel impulse response for the first OFDM symbol; and
a frequency equalizer that equalizes the first OFDM symbol in response to the channel impulse response for the first OFDM symbol.
2. The receiver of claim 1 , wherein the virtual pilot generator performs interpolation from actual pilots to determine at least one virtual pilot value.
3. The receiver of claim 1 , wherein the virtual pilot generator performs interpolation from three or more nearest neighbor actual pilots to determine a virtual pilot value.
4. The receiver of claim 1 , further comprising a CIR selector responsive to an initial channel impulse response to select a channel impulse response span.
5. The receiver of claim 4 , wherein the time domain channel estimator receives a shortened channel impulse response span and generates a time domain channel estimate in response to the shortened channel impulse response.
6. The receiver of claim 1 , wherein the virtual pilot generator is responsive to a plurality of punctured OFDM symbols to generate virtual pilots for at least all OFDM symbols that do not have actual pilots.
7. The receiver of claim 1 , wherein the first OFDM symbol is part of a block of OFDM symbols and wherein the channel impulse response is an initial channel impulse response determined over a plurality of actual and virtual pilot locations.
8. The receiver of claim 7 , wherein the block of OFDM symbols includes subframes of OFDM symbols and wherein the channel impulse response is generated based on virtual pilot or actual pilot information within each subframe.
9. The receiver of claim 8 , wherein each subframe includes fourteen OFDM symbols and wherein the virtual pilot generator generates virtual pilot values for each OFDM symbol within the block.
10. The receiver of claim 1 , wherein the punctured symbol storage stores a predetermined number of symbols from a block of transmitted OFDM symbols before the virtual pilot generator determines virtual pilot information for the block.
11. The receiver of claim 10 , wherein the virtual pilot generator identifies virtual pilot locations based on data carrier locations in the block and generates virtual pilot values based on received pilot information.
12. The receiver of claim 11 , wherein the virtual pilot generator generates virtual pilot values using interpolation.
13. The receiver of claim 1 , wherein the virtual pilot generator comprises a Wiener filter that generates virtual pilot values based on values of channel samples associated with pilot locations.
14. The receiver of claim 13 , wherein the Wiener filter uses doppler information to generate virtual pilot values.
15. The receiver of claim 13 , wherein the Wiener filter uses signal to noise ratio information to generate virtual pilot values.
16. The receiver of claim 13 , wherein the Wiener filter uses doppler information and signal to noise information to generate virtual pilot values.
17. The receiver of claim 16 , wherein the Wiener filter implements nearest neighbor interpolation.
18. The receiver of claim 1 , wherein the virtual pilot generator generates virtual pilot values for data carrier locations using values of channel samples associated with pilot locations, the receiver further comprising a CIR selector responsive to an initial channel impulse response to select a channel impulse response interval, wherein the time domain channel estimator is responsive to the channel impulse response interval to generate a further channel impulse response.
19. The receiver of claim 18 , wherein the CIR selector identifies channel paths to include within the channel impulse response interval, and the CIR selector ends a process of identifying channel paths based on a normalized peak value associated with a last identified channel path.
20. The receiver of claim 1 , further comprising a CIR selector responsive to an initial channel impulse response to select a channel impulse response interval, wherein the time domain channel estimator is responsive to the channel impulse response interval to generate a further channel impulse response, wherein the CIR selector ends a process of identifying channel paths based on a normalized peak value associated with a last identified channel path.