Pseudo-random sequence padding in an OFDM modulation system
View Patent ↗A method of estimating timing of at least one of the beginning and the end of a transmitted signal segment in the presence of time delay in a signal transmission channel. Each of a sequence of signal frames is provided with a pseudo-noise (PN) m-sequences, where the PN sequences satisfy selected orthogonality and closures relations. A convolution signal is formed between a received signal and the sequence of PN segments and is subtracted from the received signal to identify the beginning and/or end of a PN segment within the received signal. PN sequences are used for timing recovery, for carrier frequency recovery, for estimation of transmission channel characteristics, for synchronization of received signal frames, and as a replacement for guard intervals in an OFDM context.
1. A method of estimating timing of at least one of the beginning and the end of a transmitted signal segment in the presence of time delay in a signal transmission channel in an OFDM system, the method comprising:
providing a set of pseudo-random signal m-sequences PN(t;k) (k=1, . . . , K; K≧1) for which a convolution signal formed from any two sequences satisfies PN(t;i)*PN(t+Δt;j)=δ(Δt)·δ(i,j), where i and j are index numbers identifying the two sequences, t is a time variable, δ(Δt) is a modified delta function with infinitesimal width Δt1 (δ(Δt)=0 for |Δt|>Δt1) and δ(i,j)=0 unless i=j;
appending a selected sequence PN(t;k) from the set of pseudo-random signal m-sequences PN(t;k) to at least one signal frame to be transmitted to form a padded signal frame;
transmitting at least one padded signal frame as the transmitted signal through the signal transmission channel in which the transmitted signal may be received with an uncontrollable time delay Δt(delay);
receiving a received signal Rc(t) of the transmitted signal associated with the at least one padded signal frame being transmitted and forming a composite signal, denoted as Rc(t; Δt;comp), given as:
R
c
(
t
;
Δ
t
;
comp
)
=
∑
k
=
k
1
k
2
PN
(
t
+
Δ
t
;
k
)
*
R
c
(
t
)
,
where Δt is a selected time increment and k1 and k2 satisfy 1≦k1≦k2≦K;
forming a remainder signal, denoted as Rc(t;rem), where Rc(t;rem)=Rc(t)−Rc(t;Δt;comp); and
determining from the remainder signal at least one time at which said selected sequence PN(t;k) (k=k1, k1+1, . . . , k2) associated with said at least one padded signal frame begins in the received signal Rc(t).
2. The method of claim 1 , further comprising determining a carrier frequency associated with said selected sequence PN(t;k) of the at least one padded signal frame being transmitted.
3. The method of claim 1 , further comprising using at least one of the selected sequences PN(t;k) associated with the padded signal frames being transmitted to estimate at least one parameter associated with said signal transmission channe.
4. The method of claim 1 , further comprising replacing at least one guard interval associated with at least one of said signal frames to be transmitted with a selected one of the m-sequences PN(t;k).
5. The method of claim 1 , further comprising using at least one of the selected sequences PN(t;k), associated with one of said padded signal frames being transmitted, to provide time synchronization for said associated padded signal frame.