Interpolation IIR filter for OFDM baseband processing
A transmit filter for a stream of OFDM symbols has a remapper, Infinite Impulse Response (IIR) filter and a controller, the transmit filter operating on a stream of OFDM symbols. The transmit filter accepts symbols to be transmitted, the re-mapper re-orders them, the IIR filters the re-ordered stream, and a controller provides an output by rearranging the filtered symbols. The incoming symbol stream contains a series of symbols, each followed by a guard interval, where each guard interval has a first Tg symbol interval, and a second Tg symbol interval, the remapper generating a re-ordered stream having a first Tg symbol interval, a second Tg symbol interval and the symbol, the output of the IIR filter thereby generating a filtered first Tg symbol, a filtered second Tg symbol, and a filtered symbol, and the controller forms the transmit output by discarding the filtered first Tg symbol and outputting, in sequence, the filtered second Tg symbol, the filtered symbol, and a copy of the filtered second Tg symbol. The filtered second Tg symbol may be saved into a local buffer at the time it is initially output for use following the current symbol.
1. A process for filtering a stream of symbols prior to transmission, each symbol having, in sequence, a symbol value followed by a guard interval having a first Tg value followed by a second Tg value, the process comprising the steps:
remapping said symbol value, said first Tg value, and said second Tg value into, in sequence, said first Tg value, said second Tg value, and said symbol value;
upsampling, by an upsampler, said first Tg value, said second Tg value, and said symbol value by inserting one or more zero values between sampled values of said first Tg value, said second Tg value, and said symbol value;
applying said upsampled values to an IIR filter which has a reset signal for initializing at least one of a plurality of registers of said IIR filter, said IIR filter thereby generating, in sequence, a filtered first Tg value, a filtered second Tg value, and a filtered symbol value, asserting the reset signal of said IIR filter during said first Tg value input to said IIR filter; and
outputting said filtered second Tg value, said filtered symbol value, and a copy of said filtered second Tg value.
2. The process of claim 1 where said outputting includes saving said filtered second Tg value which is first output and subsequently outputting said saved filtered second Tg value after said filtered symbol value as the copy of said filtered second Tg value.
3. The process of claim 1 where said upsampling includes inserting a zero between the sample values of said first Tg value, said second Tg value, and said symbol value, and outputting resultant data at a rate twice the incoming data rate of the upsampler.
4. The process of claim 1 where said filtered second Tg value, said filtered symbol value, and the copy of said filtered second Tg value are transmitted to the antenna of a wireless LAN.
5. The process of claim 1 where said reset signal is asserted at the start of or during said guard interval of a symbol.
6. The process of claim 1 where said IIR filter is a BIQUAD filter.
7. The process of claim 1 where said stream of symbols is OFDM symbols.
8. A process for outputting values from an IIR filter having a stream of symbols, each symbol including a filtered first gap value, a filtered second gap value, and a filtered symbol value, the process comprising the steps:
resetting said IIR filter upon application of said filtered first gap value;
outputting said filtered second gap value and saving said filtered second gap value in a memory;
outputting said filtered symbol value; and
outputting said saved second gap value from said memory.
9. The process of claim 8 where said filtered second gap value, said filtered symbol value and said saved second gap value are transmitted to the antenna of a wireless LAN.
10. The process of claim 9 where said wireless LAN is compatible with one of the IEEE standards IEEE 802.11a or IEEE 802.11g.