Cognitive waveform design for spectral coexistence
3.6 Appendix
3.6.2.1 Feasibility of P 3
Problem P3 is strictly feasible due to the strict feasibility of P. In fact, let us assume that cs is a strictly feasible solution of P, i.e., cyscs¼1, cysRIcs<EI
and jcysc0j2 ðReðcysc0ÞÞ2>de. Then there are u1;u2;. . .;uN1 such that
5In fact, takinga1¼a3¼a4¼0 anda2¼1, thena1Rþa2Iþa3RIþa4C0:¼I 0.
U¼½cs;u1;u2;. . .;uN1is a unitary matrix [67], and for any 0<h<1 the matrix Cs¼ ð1hÞUe1ey1Uyþ h
N1U I e1ey1
Uy 0 (3.47)
withe1¼ ½1;0;. . .;0T is a strictly feasible solution of the SDP problem EQPR.
In fact,
trðCsÞ ¼1 (3.48)
trðCsRIÞ ¼ ð1hÞcysRIcsþ h N1
X
N1
n¼1
uynRIun (3.49)
trðCsC0Þ ¼ ð1hÞcysC0csþ h N1
X
N1
n¼1uynC0un (3.50)
which highlight that ifhis suitable chosen, thenCs is a strictly feasible solution, i.e., trðCsÞ ¼1; trðCsR1Þ>EIand trðCsC0Þ>de.
3.6.3 Waveform design algorithm for signal-dependent scenario In this section, it is described the polynomial time procedure employed to obtain an optimized solution to the non-convex optimization problem P in the presence of signal-dependent interference. In this case, the design problem is given by
P1 maxc;w
jaTj2 wyc2 wyM cð Þw s:t: kck2¼1
cyRIcEI kcc0k2e 8>
>>
>>
>>
><
>>
>>
>>
>>
:
(3.51)
Notice that problemP1 is a non-convex optimization problem, as the objective function is a non-convex function and the constraintkck2 ¼1 defines a non-convex set. Therefore, following the guidelines in [57], the goal is to derive optimized solutions toP1 via a alternating maximization procedure. The idea is to iteratively improve the SINR, controlling, at the same time, the total amount of energy injected in the licensed bandwidth, as well as radar waveform features. Specifically, given wðn1Þ, an admissible radar codecðnÞat stepnimproving the SINR corresponding to the receive filterwðn1Þand the transmitted signalcðn1Þis searched. WhenevercðnÞ is found, the filterwðnÞthat improves the SINR corresponding to the radar codecðnÞ and the receive filterwðn1Þis searched, and so on. Otherwise stated,wðnÞis used as starting point at stepnþ1. To trigger the procedure, the optimal receive filterwð0Þ, for an admissible code cð0Þ, is considered. Notice that the proposed optimization procedure requires a condition to stop the iterations; to this end, an iteration gain constraint can be forced, namelyjSINRðnÞSINRðn1Þj z; where z0 is the desired precision.
From an analytical point of view,wðnÞ can be computed solving the optimi- zation problem
PðnÞw max
w
jaTj2wycðnÞ2 wyMðcðnÞÞw (
(3.52) whose optimal solution, for any fixedcðnÞ, is given by
wðnÞ¼ MðcðnÞÞ1cðnÞ
cðnÞyMðcðnÞÞ1cðnÞ (3.53)
On the other hand,cðnÞ is an optimal solution to the following non-convex opti- mization problem
PðcnÞ maxc
jaTj2jwðn1Þycj2 wðn1ÞyMðcÞwðn1Þ s:t: kck2¼1
cyRIcEI
kcc0k2e 8>
>>
>>
><
>>
>>
>>
:
(3.54)
It is possible to show that problem PðnÞc is a hidden-convex optimization pro- blem. Precisely, its optimal solution can be computed in polynomial time (resorting to the rank-one matrix decomposition theorem [55, Theorem 2.3]), starting from an optimal solution to the SDP problem
P2
maxC;t trðQCÞ s:t: trM1ðwðnÞÞC
¼1 trð Þ ¼C t
trðRICÞ tEI
trðC0SÞ tde
C≽0 t0 8>
>>
>>
>>
>>
>>
>>
<
>>
>>
>>
>>
>>
>>
>:
(3.55)
witht an auxiliary variable,C0 ¼c0cy0,C2HN,Q¼wðn1Þwðn1Þy,M1ðwðnÞÞ ¼ PN1
k¼Nþ1;k6¼0 bk JTk wðn1Þ wðn1Þy Jk þ wðn1ÞyM w ðn1ÞI and de ¼ ð1e=2Þ2. Algorithms 2 describes the procedure leading to an optimal solution PðnÞc .
Algorithm 2:Algorithm for Radar Code Optimization Input:M1ðwðnÞÞ;Q;RI;c0;de;EI.
Output:An optimal solutioncðnÞ toPðnÞc .
1: solve SDPP2finding an optimal solutionðC;tÞand the optimal valuev; 2: letC:¼C=t;
3: ifrankðCÞ ¼1then
4: perform an eigen-decompositionC¼cðcÞyand getc 5: else
6: apply the rank-one decomposition theorem [55, Theorem 2.3] to the set of matricesðC;QvM1ðwðnÞÞ;c0cy0;I;RIÞand getc;
7: end
8: outputcðnÞ:¼cejargðcyc0Þand terminate.
Finally, Algorithm 3 summarizes the devised alternating optimization proce- dure. To trigger the recursion, an initial radar codecð0Þ from which obtaining the optimal filterwð0Þ is required; a natural choice iscð0Þ¼c0.
Algorithm 3:Algorithm for Transmit-Receive System Design in the Presence of Signal-Dependent Interference
Input:f gbk ,M,RI,EI,e,c0. Output:A solutionðc;wÞto P.
1: setn¼0,cðnÞ¼c0, wðnÞ¼ MðcðnÞÞ1cðnÞ
cðnÞyMðcðnÞÞ1cðnÞ;
and the value of the SINR for the pairðcðnÞ;wðnÞÞ; 2: do
3: n¼nþ1;
4: construct the matrixM1ðwðnÞÞ;
5: solve problem PðcnÞfinding an optimal radar codecðnÞ, through the use of Algorithm 2;
6: construct the matrixMðcðnÞÞ;
7: solve problem PðwnÞfinding an optimal receive filter wðnÞ¼ MðcðnÞÞ1cðnÞ
cðnÞyMðcðnÞÞ1cðnÞ;
and the value of the SINR for the paircðnÞ;wðnÞ
; 8: let SINR(n)¼SINR;
9: untiljSINRðnÞSINRðn1Þj z 10: outputc¼cðnÞandw¼wðnÞ.
As to the computational complexity of Algorithm 3, it is linear with respect to the number of iterationsN, whereas in each iteration, it includes the computation of the inverse ofSccðnÞ
þRindand the complexity effort of Algorithm 2. The former is in the order of OðN3Þ. The latter is connected with the complexity of SDP solution, i.e.,OðN4:5Þ.
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