Service manual

WLAN TX Measurements

R&S

FSV-K91/91n/91ac/91p

27Operating Manual 1176.7649.02 ─ 04

Referring to the IEEE 802.11a measurement standard [6], the compensation of the

gain g

is not part of the requirements. Therefore the "gain tracking"' (Tracking Gain) is

not activated as the default setting of the R&S FSV-K91/91n.

How can the parameters above be calculated? In this application the optimum maxi-

mum likelihood algorithm is used. In the first estimation step the symbol-independent

parameters Δ f

rest

and ξ are estimated. The symbol dependent parameters can be

neglected in this step, i.e. the parameters are set to g

= 1 and dγ = 0. Referring to

Equation (10), the log likelihood function L must be calculated as a function of the trial

parameters

and . (The tilde generally describes a trial parameter. Example: is

the trial parameter of x.)

(13a) (3 - 4)

The trial parameters leading to the minimum of the log likelihood function are used as

estimates and . In (13a) the known pilot symbols a

l,k

are read from a table.

In the second step, the log likelihood function is calculated for every symbol l as a func-

tion of the trial parameters and :

(13b) (3 - 5)

Finally, the trial parameters leading to the minimum of the log likelihood function are

used as estimates ĝ

and .

This robust algorithm works well even at low signal to noise ratios with the Cramer Rao

Bound being reached.

After estimation of the parameters, the sequence r

l,k

is compensated in the compensa-

tion blocks.

In the upper analyzing branch the compensation is user-defined i.e. the user deter-

mines which of the parameters are compensated. This is useful in order to extract the

influence of these parameters. The resulting output sequence is described by: γ

δ,k

In the lower compensation branch the full compensation is always performed. This

separate compensation is necessary in order to avoid symbol errors. After the full com-

pensation the secure estimation of the data symbols â

l,k

is performed. From Equation

Signal Processing of the IEEE 802.11a Application