1 Introduction
1.1 Background and related work
1.2 Our contributions
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In decoding the mixed signals using SIC, we define an event to describe whether a specific message can be successfully recovered. In order to illustrate the correlation between the successive events in decoding mixed signals, we introduce a graphical representation by which each event can be represented by the corresponding region in a 2-dimensional (or 3-dimensional) graph. On this basis, by integrating over the respective regions of events, accurate closed-form expressions of the end-to-end outage probability can be derived for both primary and secondary users under the proposed protocol.
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Without requiring non-causal knowledge, CR attempts to decode both primary and secondary messages after a first transmission phase in the proposed protocol. For the case where both messages are successfully recovered at CR after the first transmission phase, in order to further mitigate the mutual interference, we propose using DPC at CR to pre-cancel the interference seen at PR or SR in the subsequent relaying phase. Numerical results demonstrate a performance upper bound for the primary (or secondary) user, without affecting the performance of the other user.
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To guarantee that no harm is caused to the primary system, besides the power allocation performed at CR to forward the primary and secondary messages respectively, we find that a power control at ST is also needed to facilitate the SIC decoding at CR as well as to limit the interference caused to PR. Numerical results demonstrate that with a proper design of the power allocation at CR and the transmit power at ST, the secondary user is allowed to access the licensed spectrum and at the same time performance gains can be achieved for both primary and secondary systems.
2 System model and protocol description
SIC | Successive interference cancellation |
---|---|
IFC-CR | Interference channel with a cognitive relay |
DPC | Dirty paper coding |
AF/DF | Amplify-and-forward/decode-and-forward |
PT/PR | Primary transmitter/primary receiver |
ST/SR | Secondary transmitter/secondary receiver |
MRC | Maximal-ratio combining |
AWGN | Additive white Gaussian noise |
x
p
| Signal transmitted from PT |
x
s
| Signal transmitted from ST |
x
r
| Signal transmitted from CR |
y
p
| Signal received at PR |
y
s
| Signal received at SR |
y
r
| Signal received at CR |
n
0
| AWGN that is with unitary variance |
h
ij
| Channel coefficient of link i→j
|
γ
ij
=|h
ij
|2
| Channel power gain of link i→j
|
exp(δ) | An exponential distribution with mean δ
−1
|
P
P
| Transmit power at PT |
P
S
| Transmit power at ST |
P
R
| Transmit power at CR |
R
pt
| Target rate at PT |
R
st
| Target rate at ST |
\(\mathcal {E}\), \(\overline {\mathcal {E}}\)
| An event and its complementary event |
\(\Pr \{\mathcal {E}\}\)
| Probability of event \(\mathcal {E}\)
|
O
P
| End-to-end outage probability of the primary system |
O
S
| End-to-end outage probability of the secondary system |
α
| Power allocation factor at CR |
θ
| Ratio between P
S
and P
P
|
τ
| Ratio between \(\delta _{\textit {sr}}^{-1}\) and \(\delta _{\textit {pr}}^{-1}\)
|
φ
| Ratio between \(\delta _{\textit {sp}}^{-1}\) and \(\delta _{\textit {pp}}^{-1}\)
|