Joint Adaptive Transmission and Switched Diversity Reception - Adaptive Signal Processing in Wireless Communications - page 161

and



()

L −1

   

F γ p (), γ γγ < ;

 

γ γ

T T

SEC

p () γ = (6.12)



γ c

∑

L − 1




j

 

()

 

F γ p γ γγγ (), ≥ ,

γ T T



j = 0

respectively, where F γ (·) and p γ (·) are the common CDF and PDF of received SNR per
path. Note that when L = 2, (6.11) and (6.12) reduce to the CDF and PDF for dual-branch
SSC, respectively.

6.4.1.2 SECps
We can further improve the performance of SSC/SEC by better exploiting the available
channel estimates in the worst-case scenario for which no acceptable diversity path is
found. Since all available diversity paths have been examined in this case, the receiver
may use the strongest one among all these unacceptable paths, instead of randomly
choosing the last unacceptable path, for data reception. We term the resulting scheme
SECps [29]. In particular, the receiver with SECps tries to use an acceptable path by
examining as many diversity paths as necessary, as is the case with SEC. When no
acceptable path is found after examining all diversity paths, unlike conventional SEC,
the receiver with SECps selects the best unacceptable path, i.e., the one with the highest
SNR, for data reception.
More specifically, the receiver first estimates the SNR of the currently used diversity
path, denoted by γ 1 , without loss of generality, and compares it with the threshold γ T . If
the current diversity path is acceptable (i.e., γ 1 ≥ γ T ), then the receiver continues to use
it for data reception (i.e., γ c = γ 1 ). Otherwise (i.e., γ 1 < γ T ), the receiver tries to find an
acceptable path by sequentially examining the other L - 1 diversity paths. This process
is continued until either an acceptable path is found or all available diversity paths have
been estimated. In the latter case, since all path SNRs are known, the receiver compares
the estimate path SNRs and selects the one with the largest SNR for data reception.
Compared to conventional SEC, SECps requires the additional complexity for the occa-
sional selection of the best path among all unacceptable paths. However, SECps takes
full advantage of the available path estimates. In particular, by using the best unaccept-
able path instead of a randomly chosen one (usually the last one examined) as in SEC,
SECps can deliver better performance than conventional SEC.
Based on the mode of operation of SECps, it can be shown that the CDF and PDF of
the output SNR with L -SECps are given by [29]


 


L − 1

∑

i

 

()

 

   

1 − F γ 1 − F (), γ γ ≥ γ ;

γ T γ T

SEC

F () γ = (6.13)

i = 0

γ c



L

   

F () , γ γ γ < .

γ T