FREQUENCY DIVISION MULTIPLEXING SYSTEM WITH SELECTABLE RATE

摘要

An OFDM system uses a normal mode which has a symbol length T, a guard time TG and a set of N sub-carriers, which are orthogonal over the time T, and one or more fallback modes which have symbol lengths KT and guard times KTG where K is an integer greater than unity. The same set of N sub-carriers is used for the fallback modes as for the normal mode. Since the same set of sub-carriers is used, the overall bandwidth is substantially constant, so alias filtering does not need to be adaptive. The Fourier transform operations are the same as for the normal mode. Thus fallback modes are provided with little hardware cost. In the fallback modes the increased guard time provides better delay spread tolerance and the increased symbol length provides improved signal to noise performance, and thus increased range, at the cost of reduced data rate.

主权项

1. A method comprising:
receiving a first orthogonal frequency division multiplex (OFDM) symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the duration of the first OFDM symbol is T and the first OFDM symbol is associated with a first guard time having a length of TG; and receiving a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein the duration of the second OFDM symbol is KT and the second OFDM symbol is associated with a second guard time having a length of KTG, where K is a positive integer greater than one.