PhD Summary (isprateno)

Ss. Cyril and Methоdious University
Faculty of Electrical Engineering and Information
Technology
Institute of Telecommunications
Skopje, Macedonia
Jovan Stosic
Cooperative Communications over
Wireless Relay Channels
- Abstract of Ph.D. Thesis -
Supervisor: Prof. Dr. Zoran Hadzi-Velkov
Skopje, 2014

Ss. Cyril and Methоdious University
Faculty of Electrical Engineering and Information Technology
Institute of Telecommunications
Skopje, Macedonia
Jovan Stosic
Cooperative Communications over Wireless Relay
Channels
- Ph. D. Thesis -
Abstract
Cooperative (relay) communications is a very important research area of the wireless com-
munications [1]. Its concept is based upon the properties of the wireless channel, i.e. the
multi-path propagation which can be utilized to increase the efficiency and the robustness of
the communications system. Adjacent wireless nodes (also known as relays or partners) are
helping each other in the process of communication by dedicating part of their resources for
transmission of a part or all data from the partner.
In the thesis we analyze the cooperative relay channel from information-theoretic and
communication-theoretic perspective, including both the single-input single-output (SISO) re-
lay channel and the multiple-input multiple-output (MIMO) relay channel. For the SISO relay
channel, we study the capacity upper bound of the relay channel, as well as bounds on the
achievable rates of the main relaying schemes ([2], [3]).
For the MIMO relay channel, we focus on the two-hop MIMO relay channels with orthog-
onal space-time block coding (OSTBC) in Rayleigh fading environment. We assume that the
relay is using a special type of amplify-and-forward (AF) scheme [4], called the Decouple-and-
Forward (DCF) [5]. The full channel state information (CSI) is available to both the relay and
destination. In AF relay, the input signal is decoupled, amplified and forwarded toward the
destination. For such setup, we derive tight and loose error probability (EP) approximations
for the entire signal-to-noise (SNR) region of practical interest. In the high SNR regions, the
tight approximations are simplified into simple asymptotic EP expressions. The obtained EP
approximations are compared with the EP obtained by applying the moment generating func-
tion (MGF) method, as well as by computer Monte Carlo simulations. We also derive similar
approximations for the outage probability (OP) for the considered AF relaying scheme. The
OP approximations are compared with the exact results obtained from the numerical integra-
tion based on the MGF and from computer Monte Carlo simulations. Additionally we derive
closed form OP approximation of AF MIMO relay channel with direct path to the destina-
tion. Finally, we derive expressions for the ergodic capacity and outage capacity of the AF
MIMO relay channel with and without direct path to the destination. The systems with direct
path significantly outperform the systems without direct path in terms of ergodic capacity and
outage capacity.
1 Capacity of Cooperative Relay Channels
In this chapter, we present the derivations for the bounds of the achievable rates for the most
important relaying schemes: Decode-and-Forward (DF) Gaussian relay channel, Compress-and-
Forward (CF) Gaussian relay channel and Amplify-and-Forward (AF) Gaussian relay channel.
1

These expressions are already known in the literature, however we re-derive these capacity
expressions by including our own original derivation steps. These derivations are fitted to
the communication-theoretic analysis in the successive chapters. We analyze the coopera-
tive relay system with three nodes [6] which is represented with Fig. 1. The source (node
1) wants to communicate the message W with the destination (node 3) with help of the re-
lay (node 2). We analyze discrete memoryless relay channel (DMRC) which is denoted with:
(X1 × X2, p(y2, y3|x1, x2), Y2 × Y3), consisted of four finite sets X1, X2, Y2, Y3, and conditional
probability p(y2, y3|x1, x2) of set Y2xY3 for each (x1, x2) ∈ XxX, where x1 is a transmitted
signal at the source, y2 is the received signal at the relay, y3 is the received signal at the des-
tination, and x2 is the transmitted signal at the relay. The code (2nR
, n) for DMRC consist
of: set of messages 1 : 2nR
, encoder which assigns the codeword xn
1 (w) to each message
w ∈ [1 : 2nR
], encoder in the relay which assigns symbol x2i(yi−1
2 ) to each previously received
sequence yi−1
2 ∈ Y i−1
2 for each time i ∈ [1 : n] and a decoder in the destination that assigns
an estimate ˆw or an error e to each received sequence yn
3 ∈ Y n
3 . The channel is memoryless
in the sense that the current received symbols (Y i
2 , Y i
3 ) and the messages and past symbols
(w, Xi−1
1 , Xi−1
2 , Y i−1
2 , Y i−1
3 ) are conditionally independent given the current transmitted sym-
bols (Xi
1, Xi
2). Although the capacity of the general relay channel is unknown, there are several
special cases for which the capacity has been established: (a) the physically degraded and the
reversely degraded relay channels [2], (b) the semi-deterministic relay channel [7], (c) the or-
thogonal relay channel [8], and (d) a class of deterministic relay channels [9]. Therefore, in the
systems where the capacity cannot be exactly found we analyze the upper and lower capacity
bounds. As one of the most important and practical cases of relay channel, the Gaussian relay
channel has been thoroughly analyzed with emphasis on the DF, CF and AF relaying schemes.
Figure 1: Point to point communication system with relay
1.1 Capacity of main relay processing schemes
In general the upper capacity bound of DMRC is:
C ≤ max
p(x1x2)
min {I (X1, X2; Y3) , I (X1; Y2, Y3|X2)} . (1)
This bound is called cutset upper bound (CUB) since the elements of the minimum may be
interpreted as cooperative combination of multiple-access channel [10, ch.(15.3)] and broadcast
channel [10, ch.(15.6)] which is illustrated on Fig. 2. CUB is information-theoretic generalization
of the maximal flow - minimal cut theorem [11].
One simple scheme for coding of the relay channel is direct transmission for which the lower
capacity bound is [3, ch.(16.3)]:
CDT ≥ max
x2∈X
max
p(x1)
I(X1; Y3|x2) . (2)
2

Figure 2: Interpretation of the cutset upper bound with maximal ﬂow-minimal cut theorem
In the two-hop cascade relay channel, the relay is reconstructing the message received form
the source in each block and retransmitting it in the next block (Fig. 3). Since the codeword
sent by the relay is statistically dependent form the message sent in the previous block, this
type of coding is called block Markov coding (BMC) [12]. The lower bound of the capacity of
this system is:
C ≥ max
p(x1)p(x2)
min {I (X2; Y3) , I (X1; Y2|X2)} = min max
p(x2)
{I (X2; Y3)} , max
p(x1)
{I (X1; Y2)} .
(3)
The rate achieved in the two-hop cascade relay channel may be improved if we allow the
Figure 3: Set of transmission blocks used in two-hop cascade relay channel
source and the relay to coherently collaborate in the sending of their codewords. With this
improvement we get the lower bound of the capacity of the coherent cascade relay channel:
C ≥ max
p(x1,x2)
min {I (X2; Y3) , I (X1; Y2|X2)} . (4)
We again use BMC in which the (b − 1) i.i.d messages Wj, j ∈ [1 : b − 1] are sent over b blocks
where each block is transmitted by n accesses to the channel.
The capacity performance of the cascade relay channel may be improved if the destination is
simultaneously decoding the messages sent from the source and the relay. With this improve-
ment we get Decode-and-Forward relay channel with lower capacity bound:
C ≥ max
p(x1x2)
min {I (X1X2; Y3) , I (X1; Y2|X2)} , (5)
where (5) can be proved by usage of random binning ([13],[3]).
Degraded relay channel is DMRC where the received signal in the relay y2 is better than the
signal in the destination y3. The relay channel (XxX,p(y, y|x, x), Y 2xY) is called degraded
if p(y3, y2|x1, x2) may be written in following form:
p(y3, y2|x1, x2) = p(y2|x1, x2) · p(y3|y2, x2) . (6)
From (6) follows that relay channel is degraded if p(y3|x1, x2, y2) = p(y3|x2, y2) i.е., X1 →
(X2, Y2) → Y3 form Markov chain [10, eq.(2.117)]. The capacity of the degraded relay channel
is:
3

C = max
p(x1,x2)
min {I(X1, X2; Y3), I(X1; Y2|X2)} . (7)
In the DF relaying scheme, the relay recovers the entire message. If the channel from the
source to the relay is weaker than the direct channel to the destination, this requirement can
reduce the rate below that for direct transmission in which the relay is not used at all. In the
Compress-and-Forward relaying scheme the relay helps communication by sending a description
of its received sequence to the receiver. Since this description is correlated with the received
sequence, Wyner-Ziv coding [14] is used to reduce the rate needed to communicate it to the
destination. This scheme achieves the following lower bound:
C ≥ max min I (X1, X2; Y3) − I Y2; ˆY2|X1X2Y3 , I X1; ˆY2, Y3|X2 . (8)
Subtype of the CF relay channel is the DMRC with orthogonal receiver components (ORC)
depicted in Fig. 4:
Figure 4: Relay channel with orthogonal receiver components
Here Y3 = (Y3, Y3 ) and p (y2, y3|x1x2) = p (y3, y2|x1) p (y3 |x2), decoupling the broadcast channel
form the source to the relay and the destination form the direct channel from the relay to the
destination. The capacity of the DMRC with orthogonal receiver components is not known in
general. The CUB (1) for ORC relay channel simpliﬁes to:
C ≤ max
p(x1)p(x2)
min {I (X1; Y3) + I (X2; Y3 ) , I (X1; Y2, Y3)} . (9)
1.2 Capacity of Gaussian relay channel
Consider the Gaussian relay channel depicted on Fig. 5, which is a simple model for wireless
point-to-point communication with a relay [3]. The channel outputs corresponding to the inputs
X1 and X2 are:
Y2 = g21X1 + Z2, Y3 = g31X1 + g32X2 + Z3, (10)
where g21, g31, and g32 are channel gains, and Z2 ∼ N (0, 1) and Z3 ∼ N (0, 1) are independent
noise components. Assume average power constraint P on each of X1 and X2. Since the relay
can both send X2 and receive Y2 at the same time, this model is sometimes referred to as the
full-duplex Gaussian relay channel.
We denote the SNR of the direct channel by γ31 = g2
31 · P, the SNR of the channel form
the source to the relay by γ21 = g2
21 · P, and the SNR of the channel from the relay to the
destination by γ32 = g2
32 · P. In fact, for this model the capacity is not known for arbitrary
γ21, γ31, γ32 > 0.
The cutset bound is attained by jointly Gaussian probability distribution function (PDF) of
(X1, X2) [10, eq.(8.35)]:
C ≤ max
0≤ρ≤1
min C (γ31 + γ32 + 2ρ
√
γ31γ32) , C 1 − ρ2
(γ31 + γ21) . (11)
4

Figure 5: Gaussian Relay Channel
where ρ = E (X1X2) / E (X2
1 ) · E (X2
2 ).
Direct-transmission lower bound is:
C ≥ C (γ31) . (12)
For the cascade relay channel the distributions on the inputs X1 and X2 that optimize the
bound are not known in general. Assuming X1 and X2 to be Gaussian we obtain the lower
bound:
C ≥ min {C (γ21) , C (γ32/ (γ31 + 1))} . (13)
Maximizing the decode-and-forward lower bound in (5) subject to constraints yields:
C ≥ max
0≤ρ≤1
min C (γ31 + γ32 + 2ρ
√
γ31γ32) , C 1 − ρ2
· γ21 . (14)
Since implementing coherent communication is diﬃcult in wireless systems, one may consider
a non-coherent decode-and-forward relaying scheme, where X1 and X2 are independent. In this
model the lower bound is:
C ≥ min {C (γ31 + γ32) , C (γ21)} . (15)
In general, the capacity of compress-and-forward scheme for Gaussian relay channel is not
known. If we assume X1 ∼ N (0, P), X2 ∼ N (0, P), and Z ∼ N (0, N) are jointly indepen-
dent and ˆY2 = Y2 + Z (Fig. 6), and if we introduce this assumption in (8) and optimize by N
the lower capacity bound for CF relay channel is:
C ≥ C γ31 +
γ21 · γ32
γ31 + γ21 + γ32 + 1
. (16)
CF scheme outperforms the DF scheme when the channel from the source to the destination
Figure 6: Compress-and-forward for Gaussian relay channel
is worse than the channel from the source to destination i.e. when γ21 < γ31, or when the
channel from relay to destination is not good. DF scheme shows better results then CF in
other scenarios. In general, for both schemes it may be shown that the achievable rates are
within half bit of CUB [3, ch.(16.7.2)].
5

Figure 7: Gaussian channel with frequency division in the destination
The relay channel with receiver frequency division (RFD) at the receiver of destination shown
on Fig. 7 is subtype of the ORC relay channel.
In this half-duplex system, the channel from the relay to the destination is using different
frequency band from the broadcast channel from the source to the relay and destination. More
precisely for this model: Y3 = (Y3, Y3 ) and
Y2 = g21X1 + Z2, Y3 = g31X1 + Z3 Y3 = g32X2 + Z3 , (17)
where g21, g31, and g32 are channel gains, and Z2 ∼ N (0, 1) and Z3 ∼ N (0, 1) are independent
noise components and we assume average power constraint P on X1 and X2.
The capacity of this channel is not known in general. Under the power constraints the CUB in
(1) simplifies to:
C ≤
C (γ31) + C (γ32) if γ21 ≥ γ32 (γ31 + 1)
C (γ21 + γ31) otherwise
. (18)
The decode-and-forward lower bound in (5) simplifies to:
C ≥
C (γ31) + C (γ32) if γ21 ≥ γ32 (γ31 + 1)
C (γ21) otherwise
. (19)
The compress-and-forward lower bound in (8) with X1 ∼ N (0, P) , X2 ∼ N (0, P) and
Z ∼ N (0, N), independent of each other, and ˆY2 = Y2 + Z, simplifies (after optimizing over
N) to:
C ≥ C γ31 +
γ21 · γ32 · (γ31 + 1)
γ21 + (γ31 + 1) · (γ32 + 1)
. (20)
In case of RFD Gaussian relay channel with linear relaying the relaying functions are re-
stricted to being linear combinations of past received symbols. Note that under the ORC
assumption, we can eliminate the delay in relay encoding simply by relabeling the transmission
time for the channel from X2 to Y3 . Hence, we equivalently consider relaying functions of the
form x2i = i
j=1 aijy2j i ∈ [1 : n] or in vector notation of the form Xn
2 = A · Y n
2 where A is and
nxn lower triangular matrix.




x21
x22
...
x2n



 =




a11 0 0 0
a21 a22 0 0
... ... ... ...
an1 an2 ... ann



 ·




y21
y22
...
y23



 . (21)
This scheme reduces the relay channel to a point-to-point Gaussian channel with input Xn
1
and output Y n
3 = (Y n
3 , Y n
3 ). Note that linear relaying is considerably simpler to implement
in practice than DF and CF. It turns out that its performance also compares well with these
6

more complex schemes under certain high SNR conditions. The capacity with linear relaying,
CL, is characterized by the multi-letter expression:
CL = lim
k→∞
C
(k)
L C
(k)
L = sup
F(xk
1 ),A
1
k
· I Xk
1 ; Y k
3 . (22)
The supremum is over all cumulative distribution functions (CDFs) F xk
1 and lower triangular
matrices A that satisfy the source and the relay power constraint P. It can be shown that C
(k)
L
is attained by a Gaussian input Xk
1 that satisfies the power constraint [3].
If in (22) we consider C
(1)
L , we obtain the maximum rate achievable via a simple AF relaying
scheme. By observing the figure Fig. 8 and taking C
(1)
L = I (X1; Y31Y31), it can be shown that
C
(1)
L is attained by X1 ∼ N (0, P) and X2 = Y2 P/ (γ21 + 1).Therefore:
C
(1)
L = C γ31 +
γ21γ32
γ21 + γ32 + 1
. (23)
The model of the AF cascade relay channel is based on the AF Gaussian relay channel given
Figure 8: Amplify-and-forward relay channel
on Fig. 8 but without existence of the direct path component from the source to the destination
i.e. g31 = 0. The capacity of this type of channel may be obtained if we take C
(1)
L = I (X; Y3 )
or if in (23) we let γ31 = 0:
C
(1)
L = C
γ21γ32
γ21 + γ32 + 1
. (24)
2 Performance analysis of AF MIMO relay chan-
nels with multiple antennas per node
There is abundance of research papers related to AF MIMO relaying systems. Some focus
on single-antenna relays ([15]-[19]) and some on multiple-antenna relays ([5], [20]-[25]). Most
of the papers in the literature for the implementation of the MIMO AF relaying system ([5],
[17], [18], [21]-[25]) use a specific amplify-and-forward (AF) relaying scheme called Decouple-
and-Forward (DCF) relaying that has recently been proposed by Lee et al. in [5]. DCF is
linear processing technique by which the relay converts multiple spatial streams of the received
OSTBC signal into a single spatial stream signal without symbol decoding. As a result of
this linear decoupling process, if the additive channel noise is neglected, the estimate of the
transmitted symbol can be mathematically expressed as product of the transmitted symbol and
the sum of the squared modulus of the MIMO channel coefficients. After the relay decouples
the OSTBC signal received from the source it re-encodes the decoupled symbols by usage of
OSTBC, amplifies each of them separately and transmits them over the relay-destination hop.
7

In this chapter we study the approximations of the error and outage performance of a dual
hop DCF relaying system, consisted of a source, a DCF half-duplex relay and a destination,
each equipped with multiple antennas and utilizing an OSTBC transmission technique in a
Rayleigh fading environment. We have arrived at a simple universal approximation for the
error and outage probability of those systems, which proves to be extremely accurate in the
SNR of a practical interest.
2.1 System Model
We analyze the error and outage performance of a dual-hop relay channel (consisted of a
source, a relay and a destination), with multiple antennas at the nodes that utilize OSTBC
transmission (Fig. 9). We consider two system configurations: N x1xN configuration, where
the source and the destination are equipped with NT = NR = N antennas and the relay with
single antenna and N xN xN configuration, where the source, relay and destination are each
equipped with N antennas. We assume that there is no spatial correlation between the signals
transmitted or received in different antennas. The AF relay applies variable-gain amplification
of its input signal, which requires the instantaneous channel state information (CSI) of the
source-relay hop being available to the relay [26]. The destination is also assumed to have a
full CSI of the relay-destination hop for coherent demodulation. In Fig. 9 we present a dual-
hop MIMO relay channel (as the most general configuration that incorporates the two system
configurations as special cases), where source, relay and destination are denoted by S, R and
D, respectively. The relay channel is designated as N x1xN in the case when the relay has
one antenna and N xN xN when the relay has N antennas. The S-R hop and the R-D hop
are modeled as the independent MIMO Rayleigh channels with respective channel matrices H
and G. The elements hij and gij of these matrices are the channel coefficients between the i-
th transmit antenna and j-th receive antenna, considered as independent circularly-symmetric
complex Gaussian random processes with zero mean and unit variance. Therefore, the squared
envelope of signal transmitted over channel hij (gij) follows the exponentially decaying PDF
[27] with same mean squared values E |hij|2
= E |gij|2
= 1. The source average transmit
power is P and the direct communication between the source and destination is unavailable.
The utilized OSTBC codes are designated as three digit codes, NKL, where N is the number of
antennas, K is the number of code symbols transmitted in a code block, and L represents the
number of required time slots to transmit a single codeword [28]. For the particular OSTBC
the average power per symbol is calculated as:
E = P · c, c =
L
K · N
. (25)
We assume that the communication system operates in half-duplex mode, divided in two
phases (phase 1 and phase 2). The source transmits towards the relay during phase 1, then
the relay transmits towards the destination during phase 2. Since we assume that the source
employs the orthogonal space-time block (OSTB) encoding in the phase 1, group of K in-
formation symbols X = [x1, x2, ..xK]T
are transmitted over the N transmit antennas in L
successive time slots. During the phase 2 the N x1xN system’s relay decouples, amplifies and
transmits the K received symbols to the destination. In case of the N xN xN system, during the
phase 2 the relay decouples, amplifies, OSTB encodes and transmits the K received symbols
to the destination. The received signal in the single relay antenna at the end of the phase 1 is
([29],[30]):
Y =
√
E · C H + N , Y = [y1, y2, . . . yL]T
, (26)
where C is LxN codeword matrix of the OSTB code ([28], [31] and [32, eq.(32)]), H is Nx1
channel vector of the S-R hop H = [h1, h2, ..hN ]T
and N = [n1, n2, ..nL]T
is S-R hop’s Lx1
8

additive white Gaussian noise (AWGN) vector whose elements have zero mean and variance
N0.
Figure 9: Dual-hop MIMO relay channel
The superscript operator T denotes matrix transpose operation, and ES is the average trans-
mitted power per symbol. In [33] it is shown that the particular decoupled symbol at the single
antenna at the relay is given by:
˜xk = ES H 2
F xk+ξk, k = 1, 2, ..K , (27)
where H 2
F = N
i=1 |hi|2
is the squared Frobenius norm of the matrix H, ξ is complex-valued
AWGN random variable with zero mean and variance H 2
F N0. In case where the relay
has N antennas (N xN xN system), the decoupled symbol at the relay is again given by (27)
where H 2
F = N
i=1
N
j=1 |hij|2
. We have chosen to amplify the decoupled symbol ˜xk with the
following amplification factor ([4, eq.(9)], [34], [32, eq.(4)]):
A ≈
1
√
b H 2
F
, (28)
where b is the power normalization factor. Namely, in (28) we select b = c for the N x1xN and
b = 1 for the N xN xN system. The decision variable for each of the decoupled symbols at the
destination is expressed as:
ˆxk = A G 2
F ˜xk + µk, k = 1, 2, . . . K, (29)
where G 2
F = N
i=1 |gi|2
and G 2
F = N
i=1
N
j=1 |gij|2
are the corresponding squared Frobe-
nius norms of the channel matrix G for the N x1xN and N xN xN systems, and µ is complex-
valued AWGN random variable with zero mean and variance G · N0.
In destination the received signal [27] is passed through match filter [35], [36] (since the
destination knows the channel coefficients for the R-D hop), after which the destination detects
the sent symbols.
2.2 Error Probability
Given that instantaneous channel power for channel hij follows exponential distribution, the
instantaneous SNR of the MIMO channel’s hop follows the gamma probability distribution
function:
fΓ (x) =
xα−1
θα · Γ (α)
e−x
θ . (30)
9

In (30) the scale parameter of the gamma PDF is equal to the average SNR per symbol i.e.
θ = γ = E/N0 = c · ρ = L
N·K
· ρ, where ρ is the total average transmit SNR per symbol. The
shape parameter of the gamma PDF given in (30) is α = m = N for the N x1xN system and
α = m = N2
for the N xN xN system.
If we introduce (27) in (29) we obtain the expression that represents the decoupled symbols
for both the N x1xN and N xN xN configurations at the destination:
ˆxk = Es A H 2
F G 2
F xk+A G 2
F ξk + µk . (31)
From (31) it is straightforward to express the end-to-end instantaneous signal-to-noise ratio
γ. For finding the closed form expression of CDF and PDF for γ it is necessary to calculate
the inverse Laplace transform of the MGF for 1/γ which contain a product of m-th order
modified Bessel function of second kind (Km) ([37, eq.(8.432.1)], [38, eq.(9.6.24)]) with different
arguments ([32, eq.(15)], [39], [40]). For larger values of N it is very difficult to find close form
expression for the CDF and PDF and finding the average EP with MGF approach given in
[27] is even more difficult. Therefore we approximated the function Km using its power series
expansion [37, eq.(8.446)]. For small arguments z → 0, the infinite sum in [37, eq.(8.446)] can
be neglected and kept only the finite sum i.e.:
Km(z) ≈
1
2
·
2
z
m
·
m−1
k=0
(−1)k
·
(m − k − 1)!
k!
·
z
2
2 k
. (32)
By usage of approximation (32) we obtained following closed form expressions of approximate
CDF for the N x1xN and N xN xN systems:
FΓa (γ) ≈ 1 +
1
Γ (m)
·
m−1
k=0
m−1
n=0
(−1)m+k+n
Γ (m − k) · (2 k + n − m + 1)m−n−1
Γ(m − n)
·
(b + 1)n
bk
Γ (k + 1) Γ(n + 1)
·
γ
γ
n+2 k
· exp −
(b + 1)γ
γ
.
, (33)
where (...)... denotes the Pochhammer symbol. The expression for PDF can easily be found by
taking derivate of (33). Based on the approach presented in [41], we used expression (33) to
obtain the tight approximation of the average EP:
Pea ≈
1
2
+
√
d γ
2
√
π Γ (m)
·
m−1
k=0
m−1
n=0
(−1)m+k+n
2n+2 k
Γ (m − k)
Γ (m − n)
·
Γ n + 2 k + 1
2
(2 k + n − m + 1)m−n−1 (b + 1)n
bk
Γ (k + 1) Γ (n + 1) (d γ + 2 b + 2)n+2 k+1
2
,
(34)
where d is a constant determined by the modulation and the demodulation scheme (e.g. for
BPSK with coherent demodulation d = 2).
Based on the approach presented in [42] and by usage of initial value theorem given in [43,
eq.(19.1.2.1.9)] the asymptotic approximation of average EP for high value SNR range can be
presented in the following form:
Peas ≈
Γ (2 m + 1) · (bm
+1)
2m+1 · Γ2 (m + 1) · dm · γm . (35)
In order to simplify the mathematical analysis we let k = 0 in equation (34) , hence we get
the loose approximation of the average EP for the both system configurations:
Pel ≈
1
2
−
√
d γ
2
√
π
·
m−1
n=0
2n
Γ n + 1
2
(b + 1)n
Γ (n + 1) (d γ + 2 b + 2)n+1
2
. (36)
10

Expressions (33), (34), (35) and (36) can be used for the N x1xN system by usage of the
substitutions: m = N and b = c and for the N xN xN system by usage of the substitutions:
m = N2
and b = 1.
0 5 10 15 20 25
10
−4
10
−3
10
−2
10
−1
10
0
ρ [dB]
BEP
2x1x2 sim
2x1x2 num
2x1x2 app
2x1x2 asy
3x1x3 sim
3x1x3 num
3x1x3 app
3x1x3 asy
4x1x4 sim
4x1x4 num
4x1x4 app
4x1x4 asy
−5 0 5 10 15 20
10
−4
10
−3
10
−2
10
−1
10
0
ρ [dB]
BEP
2x2x2 sim
2x2x2 num
2x2x2 app
2x2x2 asy
3x3x3 sim
3x3x3 num
3x3x3 app
3x3x3 asy
4x4x4 sim
4x4x4 num
4x4x4 app
4x4x4 asy
Figure 10: BEP for N x1xN and N xN xN AF relay channel with 222/334/434 OSTBC
On Fig. 10 (left) we present the bit error probability (BEP) (d = 2) for 2x1x2 system with
Alamouti’s coding [44], 3x1x3 with 334 OSTBC and 4x1x4 system with 434 OSTBC ([28],
[31] and [32, eq.(32)]). We have compared the results obtained by Monte Carlo simulation,
the approximation results obtained by (34) and (35) for m = N and b = c and exact results
obtained by numeric integration of [27, eq.(9.15)] by using the expressions for MGF obtained
by approach presented in [17]. The comparison has shown close match of the results obtained
by approximation (34) and exact result obtained by numerical integration and simulation.
On Fig. 10 (right) we present the BEP for 2x2x2, 3x3x3 and 4x4x4 systems using 222, 334
and 434 OSTBC ([28], [31] and [32, eq.(32)]). On the figure we present a comparison of the
results obtained by means of simulation, results obtained by usage of tight EP approximation
(34), results obtained by asymptotic EP approximation (35) and results obtained by numeric
integration of MGF given in [22, eq.(15)] by usage of [27, eq.(9.15)]. We have chosen to use MGF
presented in [22] due to better numerical computational tractability. Again, the comparison
has shown close match of the results obtained by approximation (34), numerical integration
and simulation.
Henceforth, we are going to illustrate the accuracy of the loose EP approximation (36). We
validate its accuracy for different number of antennas N with comparison with: (a) accurate
results obtained by numerical integration of appropriate MGF functions, and (b) accurate
values obtained with Monte Carlo simulations. We focus on 222, 334 and 434 OSTBC schemes
[32, eq.(32)] for the system given in Fig. 9. In the figures we use abbreviation ”sim” if the curve
is obtained by Monte Carlo simulation, the abbreviation ”num” if the curve is obtained by
numerical integration of the MGF function, abbreviation ”ap1” if curve is obtained by the tight
approximation (34) and abbreviation ”ap2” if the curve is obtained with loose approximation
(36).
On Fig. 11 (left) we present the BEP for 2x1x2 system with Alamouti’s coding [44], 3x1x3
with 334 OSTBC and 4x1x4 system with 434 OSTBC. We have compared the results obtained
by Monte Carlo simulation, the tight approximation (34), the loose approximation (36) and
exact results obtained by numeric integration of [27, eq.(9.15)] by using the expressions for
MGF obtained by approach presented in [17]. The comparison shows close match of the results
obtained by tight approximation (34), exact result obtained by the numerical integration and
11

0 5 10 15 20 25
10
−4
10
−3
10
−2
10
−1
10
0
ρ [dB]
BEP
2x1x2 sim
2x1x2 ap1
2x1x2 num
2x1x2 ap2
3x1x3 sim
3x1x3 ap1
3x1x3 num
3x1x3 ap2
4x1x4 sim
4x1x4 ap1
4x1x4 num
4x1x4 ap2
−5 0 5 10 15 20
10
−4
10
−3
10
−2
10
−1
10
0
ρ [dB]
BEP
2x2x2 sim
2x2x2 num
2x2x2 ap1
2x2x2 ap2
3x3x3 sim
3x3x3 num
3x3x3 ap1
3x3x3 ap2
4x4x4 sim
4x4x4 num
4x4x4 ap1
4x4x4 ap2
Figure 11: Tight and loose BEP for N x1xN and N xN xN AF MIMO relay channel with
222/334/434 OSTBC
simulation. Moreover, the comparison shows good matching of the loose approximation (36)
and the exact results. On Fig. 11 (right) we present the BEP for 2x2x2, 3x3x3 and 4x4x4
systems using 222, 333 and 434 OSTBC. On the figure we present a comparison of the results
obtained by means of simulation, results obtained by usage of tight EP approximation (34),
loose EP approximation (36) and results obtained by numeric integration of MGF given in [22,
eq.(15)] by usage of [27, eq.(9.15)]. We have chosen to use MGF presented in [22] due to better
numerical computational tractability. Again, the comparison has shown close match of the
results obtained by tight approximation (34), exact result obtained by the numerical integration
and simulation. Moreover, the comparison shows good matching of the loose approximation
(36) and the exact results.
With the comparison of the results on Fig. 11 we have shown that the results obtained with
the loose EP approximation match very well with the accurate values obtained by simulation,
numerical integration and the tight approximation (34). The matching of the results is better
for lower values of the SNR range.
2.3 Outage probability
We derived universal OP approximation for the relay channel given on Fig. 9, which is very
accurate for entire SNR range of practical interest. Moreover, we simplify the tight approxima-
tion in loose form which is more appropriate for further theoretical analysis [45]. OP is defined
as probability that instantaneous SNR falls below the threshold γth (33):
Pout = P (γ < γth) = FΓ (γ)|γ=γth
≈ FΓa (γ)|γ=γth
. (37)
By introducing of [32, eq.(15)] in [32, eq.(16)] we obtain the accurate values of OP by usage
of numeric inversion of Laplace transform and by usage of (37) we obtain the tight OP ap-
proximation for the two system configurations (N x1xN и N xN xN ). In order to get the loose
approximation we simplified (33) by letting k = 0:
Pout ≈ FΓa (γ)|γ=γth
= 1 −
m−1
n=0
(b + 1)n
n!
·
γth
γ
n
· exp −
(b + 1)γth
γ
. (38)
We use (37) and (38) for calculation of tight and loose approximation of OP for N x1xN (m = N
and b = c) and N xN xN (m = N2
and b = 1) system .
12

2 4 6 8 10 12 14 16 18 20
10
−4
10
−3
10
−2
10
−1
10
0
ρ [dB]
Pout
2x1x2 sim
2x1x2 num
2x1x2 ap1
2x1x2 ap2
3x1x3 sim
3x1x3 num
3x1x3 ap1
3x1x3 ap2
4x1x4 sim
4x1x4 num
4x1x4 ap1
4x1x4 ap2
2 4 6 8 10 12 14 16 18 20
10
−4
10
−3
10
−2
10
−1
10
0
ρ [dB]
Pout
2x2x2 sim
2x2x2 num
2x2x2 ap1
2x2x2 ap2
3x3x3 sim
3x3x3 num
3x3x3 ap1
3x3x3 ap2
4x4x4 sim
4x4x4 num
4x4x4 ap1
4x4x4 ap2
Figure 12: OP for AF MIMO relay channel with single and multiple antenna relay(γth = 5dB).
The results obtained for tight and loose approximation are compared with the accurate
results obtained with numerical inversion of Laplace transform of the appropriate MGF and
with the results obtained by Monte Carlo simulation. For inverse Laplace transform we used
Euler’s numerical technique ([46, eq.(12)], [46, eq.(13)]). We focus on 222, 334 и 434 OSТBC
schemes. In the ﬁgures we use abbreviation ”sim” if the curve is obtained by usage of Monte
Carlo simulations, the abbreviation ”num” if the curve is obtained by numerical integration
of the appropriate MGF function, the abbreviation ”ap1” if the curve is obtained with tight
approximation (37) and abbreviation ”ap2“ if the curve is obtained by loose approximation (38).
On Fig. 12 (left) we present OP performance results for the N x1xN system with up to 4
antennas at the source and the destination. The solid line curves represent the OP results
obtained by Monte Carlo simulations, the triangle markers represent the exact OP results ob-
tained by numerical inversion of Laplace transform, the dashed line curves represent the results
obtained by loose OP approximation (38) and remaining markers (o, x and +) represents the
results obtained by the expression for tight OP approximation (37). Similarly, on Fig. 12 (right)
we present the OP results for the N xN xN system with up to 4 antennas at the source, relay
and destination. The solid line curves represent the OP results obtained by Monte Carlo simu-
lations, the triangle markers represent the exact OP results obtained by numerical inversion of
Laplace transform, the dashed line curves represent the results obtained by loose approximation
(38) and remaining markers (o, x and +) represents the results obtained by the expression for
tight approximation of the outage probability (37). Both comparisons show close match of the
results obtained by approximation (37), exact results obtained by the numerical inversion of
the Laplace transform and the results obtained by the Monte Carlo simulation. Moreover, the
comparison shows good matching of the loose OP approximation (38) and the exact results.
2.4 AF MIMO relay channel: inclusion of direct path in
the analysis
In sections 2.2 and 2.3 we have analyzed the performance of the AF MIMO relay channels
which don’t have direct path from the source to the destination. This typе of relay channels
are usually called cascade relay channels. Henceforth we are going to expand the analysis by
including the direct path from the source to the destination.
From the information-theoretic analysis in chapter 1 it is obvious that usage of the relay
which is helping the communication may increase the capacity of the system. If we closely
13

Figure 13: Relay channel with direct path to the destination
analyze the expression for capacity of AF relay channel (23) we see that total instantaneous
end-to-end SNR is a sum of two random variables: the instantaneous SNR on the hop from
the source to the destination - γ1 (Fig. 13) and the equivalent instantaneous SNR on the hop
which is including the relay - γ2 (Fig. 13). Henceforth we are going to analyze the performance
of this relay channel with assumption that the relay is using AF relaying scheme and that the
γ1 is statistically independent from γ2.
For the sake of simplified theoretical analysis we are going to simplify the expressions for
CDF (33). Accuracy of this approximation is tested for the cascade AF MIMO relay channel
through the analysis of the loose BEP approximation in section 2.2 and loose OP approximation
in section 2.3. Following the same approach as for obtaining (36) and (38) for simplification of
(33) we let k = 0. By usage of certain mathematical operations as shown in [47, eq.(16)-eq.(18)]
we obtain the loose approximation of the CDF:
FΓl
(γ) ≈ 1 −
Γ m, (b+1)γ
γ
Γ (m)
=
γ m, (b+1)γ
γ
Γ (m)
, (39)
where Γ (.) is Gamma function [37, eq. (8.310.1)], Γ (., .) is upper incomplete Gamma function
[37, eq. (8.350.2)], γ (...) is lower incomplete Gamma function [37, eq. (8.350.1)]. CDF function
given with (39) is CDF of random variable which follows the Gamma PDF with parameter of
shape m and scale parameter θ = γ/ (b + 1), hence the end-to-end instantaneous SNR for the
two-hop cascade AF MIMO relay channel may loosely be approximated by random variable
which follows Gamma PDF:
f (γ) =
1
θm · Γ (m)
γm−1
e−γ
θ where θ =
γ
b + 1
and γ =
L
NT · K
· ρ. (40)
If we assume Rayleigh fading in the direct hop from the source to the destination, the
instantaneous SNR of this hop is following the gamma PDF (30) with scale parameter θ = γ
and shape parameter α = m and the instantaneous SNR of the hop passing via the relay is
following (40). The total SNR is a sum of two random variables which follow Gamma PDF
with different shape parameters. In the thesis by usage of [48] it is shown that resulting PDF
is:
fΓ (γ) =
1
(b + 1)m ·
∞
k=0
(m)k
k!
·
b
b + 1
k
·
γ2m+k−1
(b + 1)2m+k
Γ (2m + k) · γ2m+k
· e−
(b+1)·γ
γ . (41)
If we replace (41) in (37) we obtain the OP of the AF MIMO relay channel with direct path to
the destination:
Pout = 1 −
1
(b + 1)m ·
∞
k=0
(m)k
k!
·
b
b + 1
k
·
Γ 2m + k, γth(b+1)
γ
Γ (2m + k)
. (42)
14

Figure 14: OP for N xN xN /N x1xN with 222/334 OSTBC relay channel with and without
direct path forγth = 5dB
We use (42) for calculation of loose approximation of OP for N x1xN (m = N and b = c) and
N xN xN (m = N2
and b = 1) relay channels with direct path to the destination.
On Fig. 14 we show the OP of AF MIMO relay channel with and without direct path to the
destination for threshold γth = 5dB. Namely, on this figures we compare the OP obtained by
(38) and (42) which are based on the loose approximation of the CDF of a AF cascade MIMO
relay channel given with (39). From the figures we may conclude that the AF MIMO relay
channel with direct path has significantly better outage performance compared to cascade AF
MIMO relay channel, as it could have been assumed from the information-theoretic analysis in
chapter 1. For example, for 2x2x2 system for ρ = 10dB the OP of the cascade relay channel
is 4 · 10−2
, and the OP of the same system with direct path is 4 · 10−6
which is four orders of
magnitude improvement. Moreover, we may observe that the AF MIMO relay channel with
direct path has greater diversity gain [49, eq.(5.2)] compared to the cascade AF MIMO relay
channel. The difference in diversity gain is decreasing with increase of the number of antennas.
3 MIMO relay channel from information-
theoretic aspect
In chapter 1 we have analyzed OP and EP performance of MIMO relay channels with and
without direct path to the destination, and in this part of the thesis we analyze the ergodic
capacity (EC) and outage capacity (OC) of the relay MIMO channels which use OSTBC. The
EC is:
C = E [log2 (1 + γ)] bit/Hz/s . (43)
where γ is the instantaneous SNR, and the operator E [...] designates average per γ. OC is
probability that the instantaneous capacity falls bellow the certain threshold - Cth:
Poc = Pr [C ≤ Cth] =
ˆ Cth
0
fc (C) dC . (44)
15

3.1 Ergodic capacity of cascade AF MIMO relay channel
Аccording (24) the capacity of the AF relay channel without direct path is:
C =
1
2
log (1 + γ) =
1
2
log 1 +
γ32 · γ21
γ32 + γ21 + 1
. (45)
Starting from [32, eq.(8)] it can be shown that the fraction in the logarithm of (45) also
represents the end-to-end instantaneous SNR of cascade AF MIMO relay channel.
From other side if we use ampliﬁcation factor as given in (28) with b = 1 the end-to-end
instantaneous SNR is:
γ =
γ31 · γ32
γ31 + γ32
=
L · ρ
K · NT
·
H 2
F · G 2
F
H 2
F + G 2
F
. (46)
Hence the AF MIMO relay channel with OSTBC might be presented with equivalent point-
to-point system with ergodic capacity ([50], [51]):
CAF = EH
K
L
· log2 (1 + γ) = EH
K
L
· log2 1 +
γ32 · γ21
γ32 + γ21 + 1
≈ (47)
≈ EH
K
L
· log2 1 +
γ32 · γ21
γ32 + γ21
= EH
K
L
· log2 1 +
L · ρ
K · NT
·
H 2
F · G 2
F
H 2
F + G 2
F
. (48)
From other side for the AF MIMO relay channel we may use the loose approximation (40)
Figure 15: EC of N x1xN and N xN xN vs. N x1 and N xN vs. Shannon capacity for 222 and
334 OSTBC
according which the instantaneous end-to-end SNR of the AF MIMO relay channel is following
the Gamma PDF with scale parameter θ = γ
b+1
where γ = L
NT ·K
· ρ and parameter of shape
m. If we introduce (40) in (47) we obtain the ergodic capacity of the cascade AF MIMO relay
channel:
CAF ≈
K
L · Γ (m) · ln (2)
· G1,3
3,2
L · ρ
K · NT · (b + 1)
1−m,1,1
1,0
, (49)
where G1,3
3,2(...) is Meijer G function. The equation (49) may be used for N x1xN if use substi-
tution: m = NT and b = c and for N xN xN if we change: m = NT · NR = N2
и b = 1.
16

On Fig. 15 we compare the approximation of the capacity of AF MIMO relay channel (49)
with Shannon capacity [52] and upper bound given by the ergodic capacity of corresponding
point-to-point MIMO systems for the two system configurations (N x1xN and N xN xN ) in
case of 222 and 334 OSTBC. From the figures it is obvious that the approximation (49) is
not surpassing the upper bound given by the ergodic capacity of corresponding point-to-point
MIMO system and it is approaching the bound by increase of the number of the antennas.
Furthermore, the ergodic capacity for N x1xN system is negligibly lower than the ergodic
capacity of the corresponding N x1 point-to-point system and that the difference is increasing
by increasing the average SNR and reducing the number of antennas.
Figure 16: Ergodic capacity for 2x2x2 and 3x3x3 AF MIMO with and without direct path
3.2 Ergodic capacity of AF MIMO relay channel with
direct path
If (41) is introduced in (47) the ergodic capacity for the AF MIMO relay channel with direct
path to the destination is:
CAF =
1
(b + 1)m ·
∞
k=0
(m)k
k!
·
b
b + 1
k
·
ˆ ∞
0
γ2m+k−1
(b + 1)2m+k
Γ (2m + k) · γ2m+k
· e−
(b+1)·γ
γ ·
K
L
· log2 (1 + γ) dγ .
(50)
Without lost of generality in the numerical analysis for AF MIMO relay channel with direct
path we focus only on N xN xN system configuration (m = NT · NR = N2
и b = 1). However
(50) might be used for computation of ergodic capacity for N x1xN system configuration if we
use substitutions: m = NT and b = c = L/ (N · K). Оn Fig. 16 we observe that the system with
direct path outperforms the cascade system. Moreover the system with direct path has better
performance than point-to-point MIMO system with corresponding number of antennas. The
system with direct path do better than Shannon capacity for practical values of the average
SNR. The two-antenna system do better then Shannon capacity for whole range of the average
SNR, and the other systems has lower capacity than Shannon limit only for the very large
values of the average SNR. With increase of the number of antennas the intersection point is
shifting to right.
17

3.3 Outage capacity of cascade AF MIMO relay channel
For calculation of the OP of AF MIMO relay channel, the relay channel is treated as point-
to-point channel in which end-to-end instantaneous SNR is following (40).
Figure 17: Comparison of the OP for N xN and N xN xN MIMO with 222 and 334 OSTBC
for fixed Cth
By functional transformation of random variables we can obtain PDF of the instantaneous
capacity C = (K/L) · log2 (1 + γ). Then by usage of (44) we can obtain the OP for AF MIMO
OSTBC in Rayleigh fading:
Poc = Pr (C ≤ Cth) = 1 −
1
Γ (m)
· Γ

m,
2
L·Cth
K − 1
ρ · L
· NT · K · (b + 1)

 . (51)
Without lost in generality we have analyzed the N xN xN system configuration (m =
NT · NR = N2
и b = 1), although (51) might be used for computation of OP of N x1xN
system configuration. On Fig. 17 (left) we present the comparison of OP for 2x2 point-to-point
and 2x2x2 AF MIMO relay channel with 222 OSTBC for different values of the threshold -
Cth. The curves with solid and dashed lines are related to the relay channel1
, and the dotted
curves are related to the point-to-point system. It is obvious that the relay channel shows
worse performance compared to point-to-point system. The performance gap is increased with
lowering the threshold Cth and increasing the total average SNR - ρ . Note that for small values
of the average SNR the performance of the relay channel is approaching the performance of
the point-to-point system. By usage of [49, eq.(5.2)] we obtain that the 2x2x2 AF MIMO relay
channel has same diversity gain as 2x2 point-to-point MIMO system. On Fig. 17 (right) we
compare the OP for 3x3 and 3x3x3 MIMO with 334 OSTBC for different values of the thresh-
old - Cth. The difference in the performance is increasing by lowering the threshold Cth and
by increasing of the total average SNR - ρ. By usage of [49, eq.(5.2)] we can conclude that the
3x3x3 AF MIMO relay channel has same diversity gain as 3x3 point-to-point MIMO system.
If we compare 2x2x2 and 3x3x3 system, besides that 3x3x3 system is using 3/4 code rate it
has better performance than 2x2x2 system. For example for Cth = 1 b/s/Hz and ρ = 5dB the
OP for 2x2x2 is 0.039 and for 3x3x3 it is 4 · 10−4
that is almost two order of magnitude better
performance. Moreover by usage of [49, eq.(5.2)] we show that diversity gain of 3x3x3 system
is d = 9, more than twice larger than the diversity gain of 2x2x2 system (d = 4).
1
The designation of the threshold variable Cth contains additional index „r“, i.e. Cthr.
18

3.4 Outage capacity of AF MIMO relay channel with
direct path
For calculation of the OC of the AF MIMO relay channel with direct path to the destination
(Fig. 13) we assume that end-to-end instantaneous SNR is following the PDF given in (41). By
functional transformation of random variables we can obtain PDF of the instantaneous capacity
C = (K/L) · log2 (1 + γ). Then by usage of (44) we can obtain the OP for AF MIMO relay
channel with OSTBC in Rayleigh fading:
Poc = 1 −
1
(b + 1)m ·
∞
k=0
(m)k
k!
·
b
b + 1
k
·
1
Γ (2m + k)
Γ 2m + k,
2
LCth
K − 1
γ
· (b + 1) . (52)
Without lost of generality in numerical analysis of the AF MIMO relay channel with direct
Figure 18: OP for 2x2x2 and 3x3x3 AF MIMO relay channel with and without direct path
for Cth = 3bit/s/Hz
path we focus on N xN xN system configuration (m = NT · NR = N2
и b = 1), although
(52) might be used for N x1xN system configuration if we use substitutions: m = NT и
b = c = L/ (N · K). On Fig. 18 we compare OC for 2x2x2 and 3x3x3 AF MIMO relay channel
with (52) and without (51) direct path and corresponding point-to-point MIMO systems. We
observe that the systems with direct path have significantly better OC performance than the
cascade AF MIMO relay channel and point-to-point systems. Moreover the systems with direct
path show grater diversity gain.
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