Patent Application
20080002798
Other features, objects and advantages of the invention will be made clear when reading the following description and drawings, only given by way of nonrestrictive examples. In the accompanying drawings:
The detection method according to the invention will now be described. The method is particularly adapted to realize a wireless communication technique such as the one known as 802.11.
Generally, a frame comprises a repetition of periodic elements B having a length of n symbols belonging to a given alphabet.
Let us consider a vector u (also represented by Uk in the figure) that corresponds to a group of periodic elements B and v (also represented by Uk+L in the figure), which is the element shifted by a whole number of periods. It should be noted that u and v can constitute any regrouping of a plurality of the periodic sub-sets noted as B in drawing 5.
Generally, the length of the periodic window (B on
In the traditional approach, the detection operation is based on calculation of a correlation between vectors u and v, which calculation is based on the scalar product <u, v>. In the end the detection operation is followed by a comparison with the value of the preset threshold K.
In theory, the scalar product <u, v> has a maximum value when the vectors are suitably synchronized but, in practice, it is necessary to take into account the disturbances introduced by the channel and the effect resulting from the automatic gain control (AGC), these two factors being the reason why, in the receiver, vectors u and v do not have the same module and, consequently, their scalar product does not reach a maximum value when both vectors are aligned.
In the approach proposed by the invention, carrying out the detection operation and finally the receiver control operation is done by using a correlation calculation adjusted by taking into account the modules of both vectors u and v in order to take into account the disturbance from the channel.
In a first approach, the correlation is calculated from normalized values of both vectors, according to the formula:
This first approach however presents the disadvantage of introducing division operations that turn out to be expensive to implement.
In a second approach, which is the preferred embodiment, a less complex calculation in term of architecture is done, which is based on a first mean cost function F1 that is the linear combination of a correlation calculation based on the scalar product <u, v> and a mean quadratic error calculation for vectors u and v. Such calculation now becomes directly feasible without it being necessary to resort to division operations.
Preferably, the following calculation is done:
F1=<u,v>−K||e−jΔf2πLu−v||2 (2)
with ΔF being the phase correction to introduce in order to take into account the frequency shift on transmission and reception, and L being the number of samples separating vectors u and v;
with Δf=αarg (<u, v>) (3)
where arg is the operator giving the argument of the scalar product and with, for example, α=½πnT with T being the clock period
Detection is based on determining the sign of the result of the first cost function, as defined in formula (2) noted above. If the result is positive, a signal is detected and the estimation of the beginning of frame can be started, using a second cost function F2 without it being necessary to precisely set a threshold K as in previously known techniques. Indeed, it can be proved that the threshold K that is in formula (2) must be approximately equal to the ratio signal to the weakest noise one wants to be able to detect. For example K=1 can be set, which is a good choice.
In step 10, a correlation calculation is done, based on the scalar product <u, v>.
In step 20, the phase correction to be made is calculated according to formula (3).
In step 30, the mean quadratic error between periodic element u and shifted element v is calculated and then corrected by phase correction according to the formula:
||e−jΔf2πLu−v||2
In step 40, a first cost function F1 that is the linear combination of the results of the first two functions evaluated in the previous steps 10 and 30 is calculated. Preferably, cost function F1 is calculated according to formula (2) while using a constant K that, as can be noted, does not present the critical problem of threshold determination common to all known techniques. Indeed, in the new approach as provided, the cost function is not compared with parameter K that is only used to carry out a linear combination between the correlation function and the phase-corrected mean quadratic error.
In step 50, the result of cost function F1 is compared with 0 and, when the result is positive, the process proceeds to step 60. When the value is negative, the process returns to step 10 to treat a new group of vectors u and v.
In step 60, the frame beginning is estimated using a second cost function F2. A particularly interesting possibility consists in re-using the previously calculated expression
||e−jΔf2πLu−v||2
as a cost function.
The minimum index of this cost function represents the beginning of frame and then the receiver can be started in step 70.
An advantage is observed that is particularly interesting to estimate the phase representing the modulation and demodulation clock shift between the transmitter and the receiver (Δf). In fact, what is remarkable is that the frame beginning time corresponds to the best estimate for the frequency shift (Δf). Knowing that it is calculated all the time (as required for the detection and estimation algorithm) it is then directly exploitable once the frame start time is found.
This shift is then used for rough phase correction by the system for the rest of the communication. It can be materialized by the presence of a rotor in which each sample of the received signal is phase-shifted by a value proportional to ΔF.
In practice, the frame beginning estimation can be based on two mechanisms that can be used either alternatively or jointly.
In a first mechanism, the mean quadratic error between deshifted vector u and vector v is calculated on a finished group of samples and the minimal value of these samples then makes it possible to determine the frame beginning.
In a second mechanism, the mean quadratic error between deshifted u and v is calculated as long as the first cost function F1 remains positive and the change of sign of F1 is used to determine the beginning of frame.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03/10808 | Sep 2003 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/12184 | 9/15/2004 | WO | 00 | 7/9/2007 |
