Behavioral correlates of the distributed coding of spatial context
High-Rate Distributed Space-Time-Frequency Coding for Wireless Cooperative Networks
Transcript of High-Rate Distributed Space-Time-Frequency Coding for Wireless Cooperative Networks
614 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 10, NO. 2, FEBRUARY 2011
High-Rate Distributed Space-Time-FrequencyCoding for Wireless Cooperative Networks
Jinsong Wu, Member, IEEE, Honggang Hu, and Murat Uysal, Senior Member, IEEE
AbstractβIn this paper, we propose high-rate distributedspace-time-frequency codes (DSTFCs) to exploit maximumachievable diversity gains over frequency-selective fading chan-nels. The proposed designs achieve full-rate for any number ofcooperative nodes, and allow channel variations over multipleOFDM blocks within one DSTFC codeword. We analyze diversitygains of DSTFCs through both conditional and average pairwiseerror probability (PEP), and we proposes better design criteriabased on one-side channel conditional PEP. We show thatthe difference between the frequency-selective channel ordersof source-to-relay and relay-to-destination links may provideextra diversity advantages, thus additional performance gains.Through Monte-Carlo simulations, we demonstrate that pro-posed high-rate DSTFCs provide notable diversity advantagesover existing designs.
Index TermsβAmplify-and-forward relaying, high-rate, dis-tributed space-time-frequency coding, one-side conditional pair-wise error probability, diversity.
I. INTRODUCTION
THE performance of wireless communications is highlydegraded in the presence of fading, shadowing, and mul-
tiuser interference. To overcome these limitations, cooperativediversity has been proposed [1], [2] which extracts spatial di-versity advantages in a distributed manner by creating a virtualantenna array. Relay-based cooperative communications cannotably increase capacity, and this capacity gain can also betranslated into reduced power for the users [3].
To extract the spatial diversity in distributed scenarios, anumber of distributed space-time codes (DSTCs) have beendeveloped for cooperative communications over frequency-flatchannels [2], [4], [5]. On the other hand, broadband channelsexhibit frequency-selectivity and require the deployment oftransceiver techniques which will handle the resulting inter-symbol interference (ISI). Orthogonal frequency division mul-tiplexing (OFDM) is a kind of multicarrier communicationsystem where the high-rate data stream is demultiplexed andtransmitted over a number of frequency subcarriers. If thesubcarrier width is sufficiently small compared to the channelcoherence bandwidth, a frequency-flat channel model can beassumed for each subcarrier and channel distortion can beeasily compensated at the receiver.
Manuscript received April 15, 2010; revised August 28, 2010; acceptedOctober 27, 2010. The associate editor coordinating the review of this paperand approving it for publication was B. Sundar Rajan.
J. Wu and H. Hu are with the Department of Electrical and ComputerEngineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1(e-mail: [email protected], [email protected]).
M. Uysal is with the Faculty of Engineering, Ozyegin University, 34662,Istanbul, Turkey (e-mail: [email protected]).
H. Hu is supported by the National Natural Science Foundation of Chinaunder Grant No. 60802029.
Digital Object Identifier 10.1109/TWC.2011.120810.100615
There has been a growing attention on the design ofcooperative OFDM systems [6]β[8]. Seddik and Liu have pro-posed distributed space-frequency codes (DSFCs) assumingboth decode-and-forward (DF) and amplify-and-forward (AF)relaying [7]. The proposed code structure of AF DSFC isonly shown to achieve full diversity for some special cases[7]. In [6], Zhang et al. have introduced some AF DSFCsand provided diversity analysis for arbitrary number of relaynodes and channel lengths based on some approximations. Themaximal rate of AF-DSFCs proposed in both [7] and [6] is oneOFDM block per two-phase cooperative OFDM transmission,which is termed as βfull rateβ. Wang, et al. have proposed twolower-rate (i.e., less than full rate) DSFCs to combat multiplecarrier frequency offsets (CFO) [8].
Space-time-frequency codes (STFCs) [9] have been fur-ther proposed for use in cooperative OFDM systems withmultiple relays over multiple OFDM blocks and multiplesubcarriers [10]β[12]. Oguz et al. have proposed distributedSTFC (DSTFC) for a two-user multiple-access-channel withcooperating transmitters [10]. The DSTFC design proposedby Tran et al. [11] is to improve the system performanceof multi-band OFDM-based ultra-wideband communicationsusing the cooperation of two nodes. Yang et al. have studiedspace-time-frequency block coding for DF relaying version[12]. All the existing DSTFC designs [10]β[12] are basedon orthogonal space-time block-codes [13], which introducesome limitations to applications. The current designs [10]β[12] only consider two cooperative nodes. In practice, usuallymore than two cooperative nodes are available. Therefore,extending those designs for more than two cooperative nodesis of practical concern. However, orthogonal designs [14]based approaches cannot achieve full-rate if the number ofcooperative nodes is more than two. It should be also notedthat orthogonal designs ensure simpler maximum-likelihoodreceiver; however, this requires that the channels are constantover multiple OFDM blocks, that is to say, this prevents fromexploiting time diversity over multiple OFDM blocks.
To overcome the design limitations mentioned above, inthis paper, we propose high-rate DSTFC for relay-based AFcooperative communications. The proposed designs achievefull-rate for any number of cooperative nodes, and allowchannel variations over multiple OFDM blocks within oneDSTFC codeword. Unlike the designs of AF DSFC whichwere based on diversity analyses using average pairwise errorprobability (PEP) [6], [7], we design AF DSFC using one-side-channel conditioned PEP based diversity analysis, whichenables to shed light on code construction. In addition, wealso provide some discussions on average PEP based diversityanalysis for our design creteria, and show some insights on
1536-1276/11$25.00 cβ 2011 IEEE
WU et al.: HIGH-RATE DISTRIBUTED SPACE-TIME-FREQUENCY CODING FOR WIRELESS COOPERATIVE NETWORKS 615
differences between conditional and average PEP-based baseddesigns.
Notations: π =ββ1, (β )π― matrix transpose, (β )β conju-
gate, (β )β matrix conjugate transpose, β Hadamard productoperator, π denotes empty set, β Kronecker product, πΏ (β )Kronecker delta, [A]π,π the (π, π) entry (element) of matrixA, tr (β ) matrix trace operation, Re (β ) real part of the object(matrix or variable), Im (β ) imaginary part of the object (matrixor variable), [A]
.π element-wise power of π for matrix A, andππ = exp
(π 2ππ
), and EπΌ (β ) expectation over random variable
or random variable set πΌ.
II. SYSTEM MODEL AND PROPOSED APPROACH
We consider a wireless cooperative communication systemwhich consists of a source node, πΎ relay nodes, and adestination node. The source node sends ππΆ symbols overππΆ
OFDM subcarriers per transmission. Perfect synchronizationis assumed for all transmissions between different relay nodesand the destination node. One OFDM block transmission inthe half-duplex cooperative-communications consists of twophases. In the first phase, the source node adds cyclic-prefix(CP) and broadcasts the OFDM block to all πΎ relay nodes.Each relay node receives the channel symbols with additivenoise at that relay. During the second phase, the sourcenode stops the transmission. All relay nodes remove the CPand process the received block, i.e., energy normalization,precoding, power-amplifying, etc. Then, all relay nodes (π =1, ...,πΎ) simultaneously retransmit the processed signals tothe destination node .
The frequency-selective channels are modeled using widelyadopted discrete symbol-spaced tap-delay-line (SSTDL) [15].The channel between the source node and the π-th relaynode in the π‘-th OFDM block experiences frequency-selective,temporally flat Rayleigh fading with channel coefficients
h(π,π‘) =
[β(π,π‘)(0) , ..., β
(π,π‘)
πΏ(π)β
]π―, π = 1, ...,πΎ . Similarly, the
channel coefficient vector between the π-th relay node andthe destination node in the π‘-th OFDM block is represented
by g(π,π‘) =
[π(π,π‘)(0) , ..., π
(π,π‘)
πΏ(π)π
]π―, π = 1, ...,πΎ . Set πΏ =
max{πΏ(π)β , πΏ
(π)π , π = 1, ...,πΎ} where πΏ
(π)β and πΏ
(π)π are
the frequency-selective channel orders for source-to-relay andrelay-to-destination links, respectively. The entries of h(π,π‘)
and g(π,π‘) are Rayleigh fading channel gains, and are mod-eled as complex Gaussian with zero mean, whose variancesdepend on power delay profile. We assume that the channelcoefficients remain constant within one OFDM block and varyindependently among different OFDM blocks. We further as-sume channel power constraints as πΈ
((h(π,π‘)
)βh(π,π‘)
)= 1
and πΈ((
g(π,π‘))β
g(π,π‘))
= 1. Let π»(π,π‘)π denote the π-th
subcarrier channel gain from the source node to the π-th relaynode during the π‘-th OFDM block. It is given by
π»(π,π‘)π =
πΏ(π)ββ
π=π
β(π,π‘)(π) πβπ(2π/ππ)π(πβ1) = w
(πΏ(π)β )
π h(π,π‘), (1)
where w(πΏ)π =
[1, ππβ1, π2(πβ1), β β β , ππΏ(πβ1)
]π―. Let πΊ(π,π‘)
π
denote the π-th subcarrier channel gain from the π-th relay
node to the destination node during the π‘-th OFDM block. Itis given by
πΊ(π,π‘)π =
πΏ(π)πβ
π=π
π(π,π‘)(π) πβπ(2π/ππ)π(πβ1) = w
(πΏ(π)π )
π g(π,π‘). (2)
One DSTFC codeword is of size ππΉ ΓπΎ Γ π , i.e., acrossπΎ relays, π OFDM blocks, and ππΉ subcarriers. Thus, πOFDM blocks contain π = ππΆ
ππΉDSTFC codewords, and the
rest of subcarriers may be padded by zero. The constructionand transmission procedure for the π-th DSTFC codeword,π = 1, ..., π, is described as follows: At the source node, adata symbol vector of size ππΉπ Γ 1, xπ, is encoded in sourcenode by
xπ = πsπ, π = 1, ..., π, (3)
where sπ is the source data vector carved from the signal alpha-bet π β β€ [π]. The matrix π is designed such that, for any two
different non-zero vector s(1)π and s
(2)π ,[x(1)π β x
(2)π
]π,1
β= 0
always holds for all π = 1, ..., ππΉπ , where x(1)π = πs
(1)π
and x(2)π = πs
(2)π . The design of π can be carried out using
signal-space-diversity concepts [16], [17]. For example, π canbe constructed as π = [Fππ ]
βππππ
(1, πΌ, ..., πΌππ β1
), where
ππ should be chosen as a power of 2, ππ = ππΉπ , πΌ =
exp(π 2π4ππ
), where [Fππ ]π1,π2
= exp(π 2π(π1β1)(π2β1)
ππ
)[16]β[19].
Denote x(π,π‘) = [xπ](π‘β1)ππΉ+1:π‘ππΉ ,1. During the π‘-th
OFDM block, using subcarriers{π(π)1 , ..., π
(π)ππΉ
}, the source
node transmits the sequence x(π,π‘) of size ππΉ Γ 1 to the relaynode. The received signal vector r(π,π,π‘)π at the π-th relay nodein frequency domain is given by
r(π,π,π‘)π =
βπ1x
(π,π‘) β h(π,π,π‘)π + v
(π,π,π‘)π , (4)
where h(π,π,π‘)π =
[π»
(π,π‘)
π(π)1
, β β β , π»(π,π‘)
π(π)ππΉ
], v(π,π,π‘)
π is the relevant
complex Gaussian noise vector at the π-th relay node, and
πΈ
(v(π,π,π‘)π
(v(π,π,π‘)π
)β)= IππΉ .
At the destination node, the received signal vector infrequency domain is
y(π,π‘)π = u
(π,π‘)π
+βπ1
πΎβπ=1
(π½(π)
βπ
(π)2 a(π,π,π‘) β r(π,π,π‘)π β g(π,π,π‘)
π
),
(5)
where g(π,π,π‘)π =
[πΊ
(π,π‘)
π(π)1
, β β β , πΊ(π,π‘)
π(π)ππΉ
], π½(π) = 1β
π1+1, u(π,π,π‘)
π
is the relevant complex Gaussian noise vector at the destina-
tion node, and πΈ
(u(π,π‘)π
(u(π,π‘)π
)β)= IππΉ . The design of
a(π,π‘) will be later discussed in Section III and AppendicesA-B.
Combining (4) with (5), the received symbol vector duringthe π‘-th OFDM block can be rewritten as
y(π,π‘)π = u
(π,π‘)π
+βπ1diag
(x(π,π‘)
) πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π,π‘)
π
),
(6)
616 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 10, NO. 2, FEBRUARY 2011
where h(π,π,π‘)
π = a(π,π,π‘) β h(π,π,π‘)π β g
(π,π,π‘)π , and u
(π,π‘)π =
u(π,π‘)π +
πΎβπ=1
(βπ
(π)2 π½(π)a(π,π,π‘) β v(π,π,π‘)
π β g(π,π,π‘)π
). Thus, the
received symbol vector for the π-th DSTFC codeword is givenby
y(π)π =
βπ1diag
(x(π))h(π)
π + u(π)π , (7)
where
x(π) =
[[x(π,1)
]π―, ...,[x(π,π )
]π― ]π―,
u(π)π =
[[u(π,1)π
]π―, ...,[u(π,1)π
]π― ]π―,
y(π)π =
[[y(π,1)π
]π―, ...,[y(π,1)π
]π― ]π―,
h(π)
π =
[[h(π,1)]π―
, ...,[h(π,π )]π― ]π―
, and h(π,π‘)
=
πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π,π‘)
π
). The received vectors are fed
to maximum likelihood (ML) decoder given by
argmaxsπ
(y(π)π
β£β£β£ sπ) =
argminsπ
β₯β₯β₯y(π)π ββ
π1diag(x(π))h(π)
π
β₯β₯β₯2 . (8)
III. DIVERSITY ANALYSIS AND DESIGN CRITERIA
In this section, in order to analyze the diversity gains,we first derive conditional PEP of the π-th codeword of theDSTFC under consideration and discuss relevant code designparameters. Second, we present an average PEP based analysisand point out the differences between average and conditionalPEP based designs.
A. Conditional PEP based analysis and code design
Given the channel coefficients h(π,π,π‘)π , g
(π,π,π‘)π ,
π = 1, ...,πΎ, π‘ = 1, ..., π , the conditional PEP can bebounded by
Pr
(x(π) β x(π)
β£β£β£β£β£ h(π,π,π‘)π ,g
(π,π,π‘)π ,
π = 1, ...,πΎ, π‘ = 1, ..., π
)
β€ exp
ββββββ₯β₯β₯βπ1Ξ
(π)h(π)
π
β₯β₯β₯22πΈ{
v(π,π,π‘)π ,u
(π,π‘)π
}(β₯β₯β₯u(π)
π
β₯β₯β₯2)ββββ ,
(9)
where Ξ(π) = diag(x(π) β x(π)
). The expectation of u
(π,π‘)π
over{v(π,π,π‘)π ,u
(π,π‘)π
}can be performed as
πΈ{v(π,π,π‘)π ,u
(π,π‘)π
}(β₯β₯β₯u(π)
π
β₯β₯β₯2) =
πβπ‘=1
(πΈ{
v(π,π,π‘)π ,u
(π,π‘)π
}(β₯β₯β₯u(π,π‘)
π
β₯β₯β₯2)), (10)
where
πΈ{v(π,π,π‘)π ,u
(π,π‘)π
}(β₯β₯β₯u(π,π‘)
π
β₯β₯β₯2)= tr
(πΈ{
u(π,π‘)π
}(u(π,π‘)π
(u(π,π‘)π
)π»))+ 11 + π1
πΎβπ=1
tr
(π
(π)2 a
(π,π‘)g R
(π,π,π‘)vπ
(a(π,π‘)g
)π»)= ππΉ + 1
1 + π1
πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)π»g(π,π,π‘)π
),
(11)
Here
R(π,π,π‘)vπ
= πΈ{v(π,π,π‘)π
}(v(π,π,π‘)π
(v(π,π,π‘)π
)π»),
a(π,π,π‘)g = diag(a(π,π,π‘) β g(π,π,π‘)
π
),
and
diag
(a(π,π,π‘) β
(a(π,π,π‘)
)β)= IππΉ .
The equality in (10) is based on the assumption that theequivalent noise vectors, u(π,π‘)
π , π‘ = 1, .., π, are independent.Now we return our attention to the expressionβ₯β₯β₯βπ1Ξ
(π)h(π)
π
β₯β₯β₯2 in (9). It can be easily shown thatβ₯β₯β₯βπ1Ξ(π)h
(π)
π
β₯β₯β₯2 = π1tr(Ξ(π)A
(π)β
(Ξ(π)
)β) (12)
where
A(π)β = A(π)
(h(π)π β g(π)
π
)(h(π)π β g(π)
π
)β (A(π)
)β,
h(π)π =
[[h(π,1)π
]π―, ...,[h(π,π )π
]π― ]π―,
g(π)π =
[[g(π,1)π
]π―, ...,[g(π,π )π
]π― ]π―,
A(π) = diag(A(π,1), ...,A(π,π )
),
h(π,π‘)π =
[[h(π,1,π‘)π
]π―, ...,[h(π,πΎ,π‘)π
]π― ]π―,
g(π,π‘)π =
[[g(π,1,π‘)π
]π―, ...,[g(π,πΎ,π‘)π
]π― ]π―,
A(π,π‘) =
[π½(1)
βπ
(1)2 A(π,1,1), ..., π½(πΎ)
βπ
(πΎ)2 A(π,πΎ,π‘)
],
and A(π,π,π‘) = diag(a(π,π,π‘)
).
Using the results in Appendix A, (12) can be rewritten asβ₯β₯β₯βπ1Ξ(π)h
(π)
π
β₯β₯β₯2 =
π1tr(Ξ(π)M(π)πgh (h)π» (πg)
π» (M(π))π» (
Ξ(π))π»)
= π1 (h)π»M
(π)Ξ h,
(13)
where M(π)Ξ = (πg)
β (M(π)
)β (Ξ(π)
)βΞ(π)M(π)πg. Through
averaging over{h(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}, the PEP
conditioned on channel{g(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}can be bounded by (14), where the probability density functionπ« (h) of h is defined in (15). Denote R
(π)Ξ in (16). Note that
WU et al.: HIGH-RATE DISTRIBUTED SPACE-TIME-FREQUENCY CODING FOR WIRELESS COOPERATIVE NETWORKS 617
Pr(x(π) β x(π)
β£β£β£ {g(π,π,π‘)π
}, π = 1, ...,πΎ, π‘ = 1, ..., π
)
β©½ πΈ{h} exp
ββββββ π1 (h)β M
(π)Ξ h
2
(πβπ‘=1
(ππΉ + 1
1 + π1
πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)βg(π,π,π‘)π
)))βββββ
=
β« β§β¨β©exp
ββββββ π1 (h)βM
(π)Ξ h
2
(πβπ‘=1
(ππΉ + 1
1 + π1
πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)βg(π,π,π‘)π
)))βββββ π« (h)
β«β¬βπh
ππ
πΎβπ=1
(πΏ
(π)β +1
)
det (h)
.
(14)
π« (h) =1
ππ
πΎβπ=1
(πΏ
(π)β +1
)
det (Rh)
exp{β (h)β (Rh)
β1 h}. (15)
R(π)Ξ =
βββββ π1M(π)Ξ
2
(πβπ‘=1
(ππΉ + 1
1 + π1
πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)βg(π,π,π‘)π
))) + (Rh)β1
βββββ β1
. (16)
1
ππ
πΎβπ=1
(πΏ
(π)β +1
)
det(R
(π)Ξ
)β« {
exp
(β (h)
β (R
(π)Ξ
)β1
h
)}πh = 1. (17)
Pr(x(π) β x(π)
β£β£ {g(π,π,π‘)π
}, π = 1, ...,πΎ, π‘ = 1, ..., π
)β€
det(R
(π)Ξ
)det (Rh)
= 1
det
((R
(π)Ξ
)β1
Rh
)= 1
det
βββββ π1M(π)Ξ Rh
2
(πβπ‘=1
(ππΉ + 1
1+π1
πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)βg(π,π,π‘)π
))) + Iπ
πΎβπ=1
(πΏ
(π)β +1
)
βββββ .
(18)
the relation (17) always holds, and thus (14) can be simplifiedas (18), where the second equality in (18) is obtained throughinserting (16) within.
The corresponding rank and product criteria based onconditional PEP bound in (18) can be expressed as
1) Rank criterion: The minimum rank of M(π)Ξ Rh over
all pairs of different x(π) and x(π) should be as large aspossible.
2) Product criterion: The minimum value of the productof all non-zero eigenvalues of M(π)
Ξ Rh over all pairs ofdifferent x(π) and x(π) should be maximized.
To investigate the maximal achievable conditional diversityorder, i.e. the rank of M(π)
Ξ Rh, we further impose the following
assumptions:Assumption 1: Channels for DSTFC are assumed uncorre-
lated over different relays and OFDM blocks, although themulti-path channels to or from each relay may be corre-lated, and Rh is assumed to be full rank, i.e, rank (Rh) =
ππΎβπ=1
(πΏ(π)β + 1
).
Assumption 2: Assume that, for all 1 β€ π β€ πΎ and 1 β€π‘ β€ π , there exists at least one entry in g(π,π‘),
[g(π,π‘)
]π(π,π‘),1
,
such that[g(π,π‘)
]π(π,π‘),1
β= 0, 0 β€ π(π,π‘) β€ πΏ(π)π , for each (π, π‘)
pair.The achievable conditional diversity order for conditional
PEP in (18) is analyzed in Appendix B along with the design
618 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 10, NO. 2, FEBRUARY 2011
of a(π,π,π‘), where π = 1, ...,πΎ and π‘ = 1, ..., π . Those resultsare summarized in the following theorem.
Theorem 1: Based on discrete frequency-selective chan-nel model for the OFDM-based relay network as de-scribed in Section II and under Assumptions 1 and 2,the achievable conditional diversity order of the π-thcodeword of the proposed DSTFC for PEP conditionedon channel
{g(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}in (18) is
ππΎβπ=1
(πΏ(π)β + 1
). This can be achieved through using
a(π,π,π‘) =[f (π)].((πβ1)πΏπ)
(19)
and
ππΉ β₯πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
), (20)
where πΏπ > max
{ (πΏ(π)π + πΏ
(π)β + 1
),
1 β€ π β€ πΎ
}, π = 1, ...,πΎ ,
and π‘ = 1, ..., π . These results hold for all πΏ(π)π < πΏ
(π)β ,
πΏ(π)β = πΏ
(π)π , and πΏ
(π)π > πΏ
(π)β cases.
Example: Based on Theorem 1, one DSTFC design exam-ple is provided here. Suppose ππΆ = 32, πΏ(π)
β = πΏ(π)π = 1,
π = 1, 2, and πΎ = π = 2, thus ππΉ β₯ 6. As mentionedearlier in Section II, ππΉ has to be a power of 2, thusππΉ = 8, πΌ = exp(π π
32 ). We therefore have a(π,π,π‘) =[π128(πβ1)(πβ1), ..., π16(8πβ1)(πβ1)
]π―, where π = 1, 2, π =
1, 2, and π‘ = 1, 2.Now we return our attention to find PEP conditioned on
channel{h(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}. Through aver-
aging (9) over{g(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}, the PEP
conditioned on channel{h(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}can be bounded by (21). Using the approximations (22)and (23), (21) can be approximately written as (24).Based on (24), similar to our previous discussions, itcan be shown that the achievable conditional diversityorder of the π-th codeword of the proposed DSTFC
is ππΎβπ=1
(πΏ(π)π + 1
). However, unlike the result condi-
tioned on channel{h(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}, this
conditional diversity order bound conditioned on channel{g(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}for (24) may be notably
larger than the actual achievable conditional diversity orderbound due to the use of the aforementioned approximations.
B. Discussions on differences between average and condi-tional PEP -based designs
In Theorem 1 of [6], it is claimed that, in the case of πΏ(1)β =
... = πΏ(πΎ)β = πΏβ and πΏ
(1)π = ... = πΏ
(πΎ)π = πΏπ, the achievable
diversity for DSFCs is πΎmin {πΏβ, πΏπ}, which implies that,when πΏβ β= πΏπ, πΎ (max {πΏβ, πΏπ} βmin {πΏβ, πΏπ}) extrafading paths cannot provide diversity benefits for systemperformance. However, according to our present diversityanalysis based on conditional PEP, even in the case of πΏ =
πΏ(1)β = ... = πΏ
(πΎ)β = πΏβ > πΏπ β₯
{πΏ(1)π , ..., πΏ
(πΎ)π
}, the
achievable diversity order of DSFC based on conditional PEPcan be πΎ(πΏβ + 1). Unlike our design approach, the DSFCswere designed under average PEP based diversity analysis, theresulting ππΉ was set as πΎmin (πΏβ + 1, πΏπ + 1) to achievetheir claimed diversity πΎmin {πΏβ, πΏπ}, which actually maynot fully exploit available diversity in the relay frequency-selective channels.
To explain the reasons for these somehow conflicting con-clusions, we provide the following remarks:
1) The full frequency diversity properties of DSTFC cannotbe determined only by the numbers of paths
2πΎβπ=1
(min
(πΏ(π)β + 1, πΏ(π)
π + 1))
,
where the number 2 is used for counting both sides ofmultipath channels. The extra number of paths
πΎβπ=1
ββ max(πΏ(π)β + 1, πΏ
(π)π + 1
)β
min(πΏ(π)β + 1, πΏ
(π)π + 1
) ββ may contribute to the system diversity of DSTFC.
2) The diversity analyses using one-side-channel condi-tional PEP leads to different DSTFC design parametersfrom those using average PEP. This difference intro-duces performance gains, which will be verified throughsimulations in Section IV.
3) There are two kinds of different one-side-channel con-ditional PEP ( source-to-relay and relay destination ).However, both one-side-channel conditional PEP baseddiversity analyses will lead to the same design parame-ters of DSTFC.
4) Note that, in [6], it was claimed that the diver-sity order be limited by πΎmin (πΏβ + 1, πΏπ + 1).However, we have found that the diversity limi-tation of πΎmin (πΏβ + 1, πΏπ + 1) is only valid infrequency flat fading relay channels. In the caseof{πΏ(π)π = πΏ
(π)β = 0, π = 1, ...π
}, i.e. in flat (non-
frequency-selective) fading channels, the numbers ofpaths
2
πΎβπ=1
(min
(πΏ(π)β + 1, πΏ(π)
π + 1))
does fully determine conditional diversity properties ofDSTFC for each OFDM block, which is coincidentalwith average PEP based diversity analysis of DSTC in[20].
To provide some further insights on differences betweenaverage and conditional PEP based designs, it is beneficialto also investigate diversity gains directly through derivingaverage PEP. Here, as an example, we only consider the caseof πΏ(π)
β > πΏ(π)π > 0, π = 1, ..,πΎ .
Denote g =[[g(1)]π―
, ...,[g(π )
]π― ]π―with g(π‘) =[[
g(1,π‘)]π―
, ...,[g(πΎ,π‘)
]π― ]π―. We use set partitioning to cat-
egorize whether or not there exists at least one non-zero entryin g(π,π‘). Adopting similar proof steps of Appendices A andB and using
{a(π,π,π‘)
}specified in Theorem 1, the following
relations can be proved:
WU et al.: HIGH-RATE DISTRIBUTED SPACE-TIME-FREQUENCY CODING FOR WIRELESS COOPERATIVE NETWORKS 619
Pr(x(π) β x(π)
β£β£ {h(π,π,π‘)π
}, π = 1, ...,πΎ, π‘ = 1, ..., π
)
β€ πΈ{g(π,π,π‘)π
} exp
ββββββββ
β₯β₯β₯β₯β₯βπ1diag(Ξ(π)
) πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π)
π
)β₯β₯β₯β₯β₯2
2πΈ{v(π,π,π‘)π ,u
(π,π‘)π
}(β₯β₯β₯u(π)
π
β₯β₯β₯2)βββββββ .
(21)
πΈ{g(π,π,π‘)π
} exp
ββββββββ
β₯β₯β₯β₯β₯βπ1diag(Ξ(π)
) πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π)
π
)β₯β₯β₯β₯β₯2
2πΈ{v(π,π,π‘)π ,u
(π,π‘)π
}(β₯β₯β₯u(π)
π
β₯β₯β₯2)βββββββ
β πΈ{g(π,π,π‘)π
} exp
ββββββββ
β₯β₯β₯β₯β₯βπ1diag(Ξ(π)
) πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π)
π
)β₯β₯β₯β₯β₯2
2πΈ{v(π,π,π‘)π ,u
(π,π‘)π ,g
(π,π,π‘)π
}(β₯β₯β₯u(π)
π
β₯β₯β₯2)βββββββ
(22)
πΈ{v(π,π,π‘)π ,u
(π,π‘)π
}(
πβπ‘=1
(πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)βg(π,π,π‘)π
)))β
πβπ‘=1
(πΎβπ=1
(π
(π)2 ππΉ
)),
(23)
Pr(x(π) β x(π)
β£β£ {h(π,π,π‘)π
}, π = 1, ...,πΎ, π‘ = 1, ..., π
)
<βΌπΈ{g(π,π,π‘)π
} exp
ββββββββ
β₯β₯β₯β₯β₯βπ1diag(Ξ(π)
) πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π)
π
)β₯β₯β₯β₯β₯2
2
πβπ‘=1
(πΎβπ=1
(π
(π)2 ππΉ
))βββββββ .
(24)
1) For ππΉ β₯πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
), we have
π (π,g,1) = rank(M
(π)Ξ Rβ
)β£β£β£g
= rank(M
(π)Ξ B)β£β£β£
g
=πβπ‘=1
(πΎβπ=1
((πΏ(π)β + 1
)πΏ(g(π,π‘) β= 0
))),
(25)
2) For ππΉ =πΎβπ=1
(min
(πΏ(π)β + 1, πΏ
(π)π + 1
)), we have
(26)
Denote the PEP conditioned on channel{g(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}as πΌ(πβ£g), i.e.,
πΌ(πβ£g) = Pr
βββx(π) β x(π)
β£β£β£β£β£β£β£{g(π,π,π‘)π
},
π = 1, ...,πΎ,π‘ = 1, ..., π
βββ . (27)
To remove the conditions of channel
{g(π,π,π‘)π , π = 1, ...,πΎ, π‘ = 1, ..., π
}, we need to calculate
πΌ(π,π) =
β«β«β«βπΌ(πβ£g)πgπg, (28)
where πg =πΎβπ=1
πβπ‘=1
πΏ(π)πβ
ππ=0
ππ(π,π‘)ππ
, and πg is the probability
density function of g. (28) can be rewritten as the form ofdiscrete summations
πΌ(π,π) =β
...β
πΌ(πβ£g)πgΞg (29)
where Ξg is the discrete multiple integration unit area.It is useful to derive the inequality (30), where {ππ}
are non-zero eigenvalues of (Rh)1/2 M
(π)Ξ (Rh)
1/2, π (π) =
rank((Rh)
1/2M
(π)Ξ (Rh)
1/2)
= rank(M
(π)Ξ Rh
), π(π,Ξ) is
defined in (31), πg is symbol signal-to-noise ratio (SNR)conditioned on g, and π is some positive constant dependingon the system structure.
Denote πΌ(πβ£g) in (27) for the case of ππΉ β₯
620 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 10, NO. 2, FEBRUARY 2011
π (π,g,2)= rank(M
(π)Ξ Rβ
)β£β£β£g= rank
(M
(π)Ξ Rβ
)=
πβπ‘=1
(πΎβπ=1
(min
(πΏ(π)β + 1, πΏ(π)
π + 1)πΏ(g(π,π‘) β= 0
))). (26)
(det
((R
(π)Ξ
)β1
Rh
))β1
=
(det
((R
(π)Ξ
)β1
Rh
))β1
=
ββdet
ββπ(π,Ξ)π1 (Rh)1/2
M(π)Ξ (Rh)
1/2+ I
ππΎβ
π=1
(πΏ
(π)β +1
)
ββ ββ β1
=π(π)βπ=1
(1 + ππgππ)β1 β€ (πmin {ππ} πg)βπ(π)
.
(30)
π(π,Ξ) =
(2
(πβπ‘=1
(ππΉ +
1
1 + π1
πΎβπ=1
(π
(π)2
(g(π,π,π‘)π
)βg(π,π,π‘)π
))))β1
. (31)
πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
)as πΌ
(πβ£g)1 . Denote πΌ(πβ£g) for the case
of ππΉ =πΎβπ=1
(min
(πΏ(π)β + 1, πΏ
(π)π + 1
))as πΌ
(πβ£g)2 . Using
(25), (26), and (30), (27) can be bounded as
πΌ(πβ£g)1 β©½ π(π,g,1) (πg)
βπ(π,g,1)
= t(π,πg,1) (32)
for ππΉ β₯πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
)and
πΌ(πβ£g)2 β©½ π(π,g,2) (π)βπ(π,g,2)
= t(π,πg,2), (33)
for ππΉ =πΎβπ=1
(min
(πΏ(π)β + 1, πΏ
(π)π + 1
)), where π(π,g,1)
and π(π,g,2) are two positive values conditioned on g. In high
SNR, (πg)βπ(π,g,1)
and (πg)βπ(π,g,1)
dominate t(π,πg,1) andt(π,πg,2) instead of π(π,g,1) and π(π,g,2), respectively. Using(25), (26), (32), and (33), we have the following observations:If g β= 0, we have π (π,g,1) > π (π,g,2) and πΌ(πβ£g)
1 < πΌ(πβ£g)2 , and
the slope for log(t(π,πg ,1)
)versus SNR conditioned on g is
larger than that for log(t(π,πg,2)
)versus SNR conditioned on
g. If g = 0, π (π,g,1) = π (π,g,2), it means that communicationlinks from relay nodes to the destination node are completelydisconnected (which usually happens at very low probability),and DSTFC cannot provide any help in this case.
Using the above mentioned results, we conclude that thefreedom order for average PEP (28) or (29) under ππΉ β₯πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
)is larger than that under ππΉ =
πΎβπ=1
(min
(πΏ(π)β + 1, πΏ
(π)π + 1
)).
IV. SIMULATION RESULTS AND DISCUSSIONS
In this section, we present Monte-Carlo simulation resultsto demonstrate the error-rate performance of the proposedschemes. We assume that the underlying frequency-selectivechannels follow uniform power delay profile, and remainconstant within a fixed integer number of OFDM blocks,denoted as the channel change interval (πΆπΆπΌ), and change
independently from one block to another block. The followingare further assumed in all simulations:
β πΏ(1)β = ... = πΏ
(πΎ)β = πΏβ and πΏ
(1)π = ... = πΏ
(πΎ)π = πΏπ.
β π1 = π and π(1)2 = ... = π
(πΎ)2 = π
πΎ .β CCIs for source-to-relay and relay-to-destination chan-
nels are equal to 1.β Before DSTFC precoding, data symbols are modulated
using 4-QAM modulation.The horizontal axes of performance figures is 10 log10 π . Notethe average symbol SNR is proportional to π .
In Fig. 1, for π = 1 and πΎ = 2, we compare the biterror rate (BER) performance of our proposed DSFC withthat proposed in [6]1. From Figure 1, it is observed that,
β The performance of DSFC for the case of πΏβ = πΏπ = 2is worse than that for the cases of (πΏβ = 5) > (πΏπ = 2)and (πΏβ = 2) < (πΏπ = 5),
β The performance of DSFC for the case of (πΏβ = 5) >(πΏπ = 2) is better than that for the case of (πΏβ = 2) <(πΏπ = 5),
β The performance of DSFC for the case that ππΉ is set toat least πΎ(πΏβ + πΏπ + 1) is better than for the case thatππΉ is set to at least πΎmin {πΏβ + 1, πΏπ + 1}.
The second observation implies that the extra frequency-selective order for source-to-relay paths provided strongerdiversity advantages over that for relay-to-destination paths.This phenomenon incurs due to the existing relay noises, andthus we would provide the following conjecture:
Conjecture 1: Due to the deterioration incurred by relaynoises, the degradation of the conditional diversity orderof DSFC and DSTFC in frequency-selective fading relaychannels is much larger than that of DSTC in frequency-flatfading relay channels. This effect may be more significant inlow and medium SNR regions.
1We remark that the discussions in [6] are based on general discretefrequency-selective channel models (channel taps are not necessarily uni-formly spaced), while our discussions are based on SSTDL as described inSection II. In the comparisons of Figure 1, the BER performance resultsfor DSFC approaches of Zhang, et al. are also based on the same SSTDLfrequency-selective channel models.
WU et al.: HIGH-RATE DISTRIBUTED SPACE-TIME-FREQUENCY CODING FOR WIRELESS COOPERATIVE NETWORKS 621
10 15 20 25 30 3510
β8
10β7
10β6
10β5
10β4
10β3
10β2
10β1
10log10
P (dB)
BER
DSFC (Zhang et. al. settings),NF=8,L
h=2,L
g=2
DSFC (Zhang et. al. settings),NF=8,L
h=2,L
g=5
DSFC (Zhang et. al. settings),NF=8,L
h=5,L
g=2
DSFC (Our settings),NF=16,L
h=2,L
g=2
DSFC (Our settings),NF=16,L
h=2,L
g=5
DSFC (Our settings),NF=16,L
h=5,L
g=2
Fig. 1. Performance comparison of proposed DSFCs with those in [6] (ππΆ =64).
27 28 29 30 31 32 3310
β4
10β3
10log10
P (dB)
BE
R
DFC (Our settings),K=1,T=1,NF=16,L
h=6,L
g=6
DFC (Our settings),K=1,T=1,NF=16,L
h=6,L
g=9
DFC (Our settings),K=1,T=1,NF=16,L
h=9,L
g=6
Fig. 2. BER Performance of DFCs (ππΆ = 32).
As mentioned in Section III, the authors of [6] have statedthat the achievable diversity order based on average PEPis πΎmin {πΏβ + 1, πΏπ + 1}, and the corresponding minimumππΉ is set to be at least πΎmin {πΏβ + 1, πΏπ + 1}. How-ever, according to our results based on the conditional PEPbound in Section III, we have stated that the achievablediversity order based one-side-channel conditional PEP isπΎmax {πΏβ + 1, πΏπ + 1}, and the corresponding minimumππΉ is set to be at least πΎ(πΏβ + πΏπ + 1). Observations fromFig. 1 support our statements.
For πΎ = 1 and π = 1, DSTFC becomes frequency-onlycoding, which can be termed as distributed frequency code(DFC). In Fig. 2, we investigate the BER performance of DFCunder different frequency-selective channel conditions. It isobserved that,
β The performance of DFC for the case of πΏβ = πΏπ = 6
10 15 20 25 30 3510
β5
10β4
10β3
10β2
10β1
10log10
P (dB)
BER
DSFC (Zhang et. al. settings),K=2,T=1,NF=4,L
h=1,L
g=1
DSFC (Our settings),K=2,T=1,NF=8,L
h=1,L
g=1
DSTFC (Our settings),K=2,T=2,NF=8,L
h=1,L
g=1
Fig. 3. Performance comparison of DSTFC and DSFCs (ππΆ = 32).
12 14 16 18 20 22 24 2610
β6
10β5
10β4
10β3
10β2
10β1
10log10
P (dB)
BE
R
DSTFC (Our settings),K=2,T=2,N
F=8,L
h=1,L
g=1
DSTFC (Our settings),K=2,T=2,NF=8,L
h=2,L
g=1
Fig. 4. Performance of DSTFC under different channel conditions (ππΆ =32).
is worse than that for the cases of (πΏβ = 6) < (πΏπ = 9)and (πΏβ = 9) > (πΏπ = 6),
β The performance of DFC for the case of (πΏβ = 9) >(πΏπ = 6) is better than that for the case of (πΏβ = 6) <(πΏπ = 9),
The above results for single relay-case further confirm ourdiversity analysis in Section III.
Figure 3 illustrates performance comparisons betweenDSTFC and DSFC approaches. In time-varying channels (i.e,channel changes from one OFDM block to another), theproposed DSTFC demonstrates significant performance gainsover DSFC at the price of longer decoding delay and highercomputational complexity. In Fig. 4, the BER performancesof DSTFC under different channel conditions are compared,which further confirms that, the channel order differences be-tween source-to-relay and relay-to-destination may contribute
622 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 10, NO. 2, FEBRUARY 2011
to extra gains in diversity performance as discussed in SectionIII.
V. CONCLUSION
We have proposed high-rate distributed space-time-frequency coding for AF OFDM-based cooperative relay net-works over frequency-selective fading channels. Using bothconditional and average pairwise error probability, we haveanalyzed DSTFC diversity performance in block time-varyingfrequency-selective fading channels and pointed out the signif-icant differences between diversity performance in frequency-selective channels and that in frequency-flat fading channels.Such observations help obtain better design criteria for DST-FCs. We have shown that high-rate DSTFC may significantlyoutperform high-rate DSFC due to efficient exploitation oftime diversity over multiple OFDM blocks.
APPENDIX ADERIVATION OF A
(π)β
Denote f (π) =
[ππ
(π)1 β1, ..., π
π(π)ππΉ
β1
]π―οΈΈ οΈ·οΈ· οΈΈ
ππΉ
.
The term π»(π,π‘)π πΊ
(π,π‘)π is derived in (34),
where[z(π)π
]π2+
(πΏ
(π)π +1
)π1+1,1
= πβ(π1+π2)(πβ1),[b(π,π‘)
]π2+
(πΏ
(π)π +1
)π1+1,1
= β(π,π‘)(π1)
π(π,π‘)(π2)
, π1 =
0, ..., πΏ(π)β , π2 = 0, ..., πΏ
(π)π , b(π,π‘) = π(π,π‘)h(π,π‘), and
π(π,π‘) = diag(IπΏ
(π)β
β g(π,π‘)). Therefore, we have (35),
where[Ξ©(π,π)
]:,π2+
(πΏ
(π)π +1
)π1+1
=[f (π)].(π1+π2).
Note that
h(π)
π =
πΎβπ=1
(π½(π)
βπ
(π)2 h
(π,π)
π
)=[Ξ(π,1), ...,Ξ(π,π )
]= M(π)b.
(36)
In (36), each element can be written as
Ξ(π,π‘) =
πΎβπ=1
(π½(π)
βπ
(π)2
[a(π,π,π‘) β h(π,π,π‘)
π β g(π,π,π‘)π
]π― )= M(π,π‘)b(π‘),
(37)
where
h(π‘) =
[[h(1,π‘)
]π―, ...,[h(πΎ,π‘)
]π― ]π―,
h =
[[h(1)]π―
, ...,[h(π )
]π― ]π―,
M(π) = diag(M(π,1), ...,M(π,π )
),
M(π,π‘) =[M(π,1,π‘), ...,M(π,πΎ,π‘)
],
M(π,π,π‘) = π½(π)
βπ
(π)2 diag
(a(π,π,π‘)
)Ξ©(π,π),
b(π‘) =
[[b(1,π‘)
]π―, ...,[b(πΎ,π‘)
]π― ]π―,
b =
[[b(1)]π―
, ...,[b(π )
]π― ]π―,
b(π‘) = π(π‘)g h(π‘), π(π‘)
g = diag(π(1,π‘)g , ..., π
(πΎ,π‘)g
), b = πgh,
and πg = diag(π(1)g , ..., π
(π )g
). Therefore, A(π)
β is calculatedas
A(π)β = M(π)b (b)
β (M(π)
)β= M(π)πgh (h)β (πg)
β (M(π))β
.(38)
APPENDIX BACHIEVABLE RANK OF M
(π)Ξ Rh
In this section, we investigate the rank of M(π)Ξ Rh. Under
Assumption 1, we have Rh = diag (Rh(1) , ...,Rh(π)), where
Rh(π‘) = πΈh(π‘)
(h(π‘)
(h(π‘))β)
= diag (Rh(1,π‘) , ...,Rh(πΎ,π‘)),
Rh(π,π‘) = πΈh(π,π‘)
(h(π,π‘)
(h(π,π‘)
)β). Further, denote Ξ¦h =
(Rh)12 = diag (Ξ¦h(1) , ...,Ξ¦h(π)), where Ξ¦h(π‘) = (Rh(π‘))
12 =
diag (Ξ¦h(1,π‘) , ...,Ξ¦h(πΎ,π‘)), Ξ¦h(π,π‘) = (Rh(π,π‘))12 , π = 1, ...,πΎ ,
π‘ = 1, ..., π .Recall that M
(π)Ξ = (πg)
β (M(π)
)β (Ξ(π)
)βΞ(π)M(π)πg,
whereπg = diag
(π(1)g , ..., π(π )
g
),
π(π‘)g = diag
(π(1,π‘)g , ..., π(πΎ,π‘)
g
),
π(π,π‘)g = diag
(IπΏ
(π)β
β g(π,π‘)).
Provided that Ξ(π) is to be full rank, i.e., rank(Ξ(π)
)= ππΉπ ,
the following holds
rank(M
(π)Ξ Rh
)= rank
(Ξ(π)M(π)πgΞ¦h
)= rank
(M(π)B
) (39)
where B = πgΞ¦h.
A. Investigation of B
Note that matrix B can be calculated as B =diag
(B(1), ...,B(π )
), where B(π‘) = π
(π‘)g Ξ¦h(π‘) , B(π‘) =
diag(B(1,π‘), ...,B(πΎ,π‘)
), B(π,π‘) = π
(π,π‘)g Ξ¦h(π,π‘) . We know that
Rh(π,π‘) , π = 1, ...,πΎ , π‘ = 1, ..., π , are positive definiteHermitian matrices, thus all diagonal entries of Rh(π,π‘) arenon-zero, so are Ξ¦h(1,π‘) , π = 1, ...,πΎ , π‘ = 1, ..., π .
B. Investigation of M(π)
Recall that M(π) = diag(M(π,1), ...,M(π,π )
),
M(π,π‘) =[M(π,1,π‘), ...,M(π,πΎ,π‘)
], M(π,π,π‘) =
π½(π)
βπ
(π)2 diag
(a(π,π,π‘)
)Ξ©(π,π). We thus know that
rank(M(π)
)=
πβπ‘=1
rank(M(π,π‘)
). To maximize the rank
of M(π), it is necessary to maximize the rank of eachM(π,π‘), respectively. This requires to properly design a(π,π‘).Recall that
[Ξ©(π,π)
]:,π2+
(πΏ
(π)π +1
)π1+1
=[f (π)].(π1+π2), where
WU et al.: HIGH-RATE DISTRIBUTED SPACE-TIME-FREQUENCY CODING FOR WIRELESS COOPERATIVE NETWORKS 623
π»(π,π‘)π πΊ(π,π‘)
π =
ββπΏ(π)ββ
π=π
(β(π,π‘)(π) πβπ(2π/ππ)π(πβ1)
)ββ ββπΏ(π)πβ
π=π
(π(π,π‘)(π) πβπ(2π/ππ)π(πβ1)
)ββ =
πΏ(π)ββ
π1=π
ββπΏ(π)πβ
π2=π
(β(π,π‘)(π1)
π(π,π‘)(π2)
πβπ(2π/ππ)(πβ1)[π1+π2])ββ =
[z(π)π
]π―b(π,π‘).
(34)
h(π,π,π‘)π β g(π,π,π‘)
π =
ββπΏ(π)ββ
π1=π
(β(π,π‘)(π1)
[f (π)].(π1))ββ ββπΏ(π)
πβπ2=π
(π(π,π‘)(π2)
[f (π)].(π2))ββ = Ξ©(π,π)b(π,π‘). (35)
f (π) =
[πβ(π(π)1 β1
), ..., π
β(π(π)ππΉ
β1)]π―
οΈΈ οΈ·οΈ· οΈΈππΉ
, π1 = 0, ..., πΏ(π)β , and
π2 = 0, ..., πΏ(π)π .
Assume that our design ensures[f (π)].(π1) and
[f (π)].(π2)
are linear-independent for all π1 β= π2, which can be achievedby choosing subcarrier indices
{π(π)π
}such that π(π)π + π =
π(π)π+1, where π = 1, ..., ππΉ β 1. If π = 1, adjacent subcarrier
indices are chosen. Observing that[Ξ©(π,π)
]:,π2+
(πΏ
(π)π +1
)π1+1
=[f (π)].(π1+π2), where π2 = 0, 1, ..., πΏ
(π)π , π1 = 0, 1, ..., πΏ
(π)β , al-
though Ξ©(π,π) have(πΏ(π)β + 1
)(πΏ(π)π + 1
)columns, there are
only(πΏ(π)π + πΏ
(π)β + 1
)different columns regardless of differ-
ent coefficients, since there are totally only(πΏ(π)π + πΏ
(π)β + 1
)different element-wise power π for
[f (π)].(π)
. This leads
to rank(Ξ©(π,π)
)=
(πΏ(π)π + πΏ
(π)β + 1
). Note that, in
the case of diag(a(π,π‘)
)= IππΉ , the maximal rank
of M(π,π‘) =
[π½(1)
βπ
(1)2 Ξ©(π,1), ..., π½(πΎ)
βπ
(πΎ)2 Ξ©(π,πΎ)
]is
max{πΏ(π)π + πΏ
(π)β + 1, π = 1, ...,πΎ
}. Now we set
a(π,π,π‘) =[f (π)].((πβ1)πΏπ )
, (40)
where πΏπ > max{πΏ(π)π + πΏ
(π)β + 1, π = 1, ...,πΎ
}. Using
a(π,π,π‘) as specified in (40), M(π,π‘) hasπΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
)linear independent columns, in other words, rank
(M(π,π‘)
)=
πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
), for all π‘ = 1, ..., π .
C. Rank of M(π)B
Now we are ready to investigate the rank of M(π)B. We canwrite M(π)B as
M(π)B = diag(M
(π,1)B , ...,M
(π,π )B
), (41)
where M(π,π‘)B = M(π,π‘)B(π‘) =
[M
(π,1,π‘)B , ...,M
(π,πΎ,π‘)B
], and
M(π,π,π‘)B = M(π,π,π‘)B(π,π‘)
= π½(π)
βπ
(π)2 diag
(a(π,π,π‘)
)Ξ©(π,π)B(π,π‘)
= π½(π)
βπ
(π)2 Ξ©
(π,π,π‘)a B(π,π‘)
= π½(π)
βπ
(π)2 Ξ©
(π,π,π‘)B ,
(42)
where[Ξ©
(π,π,π‘)a
]:,π2+
(πΏ
(π)π +1
)π1+1
=[f (π)].(π1+π2+(πβ1)πΏπ)
,
Ξ©(π,π,π‘)B = Ξ©
(π,π,π‘)a B(π,π‘).
Denote Ξ©(π,π,π‘)a(π1)
=[Ξ©
(π,π,π‘)a
]:,(πΏ
(π)π +1
)π1+1:πΏ
(π)π +
(πΏ
(π)π +1
)π1+1
,
and thus Ξ©(π,π,π‘)a =
[Ξ©
(π,π,π‘)a(0) , ...,Ξ©
(π,π,π‘)
a(πΏ(π)β )
]. Recall
B(π,π‘) = π(π,π‘)g Ξ¦h(1,π‘) and π
(π,π‘)g = diag
(IπΏ
(π)β
β g(π,π‘)).
We then obtain
Ξ©(π,π,π‘)B =
[g(π,π,π‘)(0) , ...,g
(π,π,π‘)
(πΏ(π)β )
]Ξ¦h(π,π‘) , (43)
where g(π,π,π‘)(π1)
= Ξ©(π,π,π‘)a(π1)
g(π,π‘), π1 = 0, ..., πΏ(π)β .
g(π,π,π‘)(π1)
= Ξ©(π,π,π‘)a(π1)
g(π,π‘)
=πΏ(π)
πβπ2=0
(π(π,π‘)π2
[f (π)].(π1+π2+(πβ1)πΏπ)
)=
πΏπ(π,π‘)πβπ2=1
(π(π,π‘)π(π2)
[f (π)].(π1+π(π2)+(πβ1)πΏπ)
),
(44)
where πΏπ(π,π‘)π is the total number of non-zero entries in
vector g(π,π‘), the non-zero entries of g(π,π‘) are denotedas π
(π,π‘)π(1) , ..., π
(π,π‘)
π(πΏπ(π,π‘)π )
, 0 β€ π (1) < π (2) < ... <
π(πΏπ(π,π‘)π
)β€ πΏ
(π)π . Based on Assumption 2, πΏπ(π,π‘)
π β= 0
hold for all 1 β€ π β€ πΎ and 1 β€ π‘ β€ π . Since g(π,π,π‘)(π1)
is
the linear combination of πΏπ(π,π‘)π linear independent columns{[
f (π)].(π1+π(π2)+(πβ1)πΏπ)
β£β£β£ 1 β€ π2 β€ πΏπ(π,π‘)π
}, g
(π,π,π‘)(π1)
β=0 holds for all 0 β€ π1 β€ πΏ
(π)β , 1 β€ π β€ πΎ , 1 β€ π‘ β€ π , and
0 β€ π1 β€ πΏ(π)β . Note that
[f (π)].(π1+π(πΏπ(π,π‘)
π )+(πβ1)πΏπ)does
not exist in the linear combination within g(π,π,π‘)(0) , ...,g
(π,π,π‘)(π1β1),
respectively, where 0 β€ π1 β€ πΏ(π)β . Thus, g
(π,π,π‘)(π1)
is linear
independent to all g(π,π,π‘)((π2)
for πΏ(π)β β₯ π1 > π2 β₯ 0. Using this
624 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 10, NO. 2, FEBRUARY 2011
recursive way, it can be proven that any two vectors within
the set
{g(π,π,π‘)(0) , ...,g
(π,π,π‘)
(πΏ(π)β )
}are pairwise independent.
Now we can calculate the rank of Ξ©(π,π,π‘)B . Using
rank (Ξ¦h(π,π‘)) = πΏ(π)β , the following holds
rank(Ξ©
(π,π,π‘)B
)= rank
([g(π,π,π‘)(0) , ...,g
(π,π,π‘)
(πΏ(π)β )
])= πΏ
(π)β + 1.
(45)
Further, note that the set{ [f (π)].(π1+π(π2)+(π1β1)πΏπ )
,
1 β€ π2 β€ πΏπ(π1,π‘)π , 0 β€ π1 β€ πΏ
(π1)β
}and
the set
{ [f (π)].(π1+π(π2)+(π2β1)πΏπ )
,
1 β€ π2 β€ πΏπ(π2,π‘)π , 0 β€ π1 β€ πΏ
(π2)β
}do not have
any common element vector, where 1 β€ π1 β= π2 β€ πΎ ,
πΏπ > max
{ (πΏ(π)π + πΏ
(π)β + 1
),
1 β€ π β€ π
}, thus any element
vector in
{g(π,π1,π‘)(0) , ...,g
(π,π1,π‘)
(πΏ(π)β )
}is linear-independent to any
element vector in
{g(π,π2,π‘)(0) , ...,g
(π,π2,π‘)
(πΏ(π)β )
}. Therefore,
rank(M
(π,π‘)B
)=
πΎβπ=1
rank(Ξ©
(π,π,π‘)B
)(46)
and
rank(M
(π)Ξ Rh
)= rank
(M(π)B
)=
πβπ‘=1
rank(M
(π,π‘)B
)= π
πΎβπ=1
(πΏ(π)β + 1
) (47)
To achieve the achievable rank provided in (47), there
areπΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
)available element vectors in set{[
f (π)].(π1+π2+(πβ1)πΏπ )
}, and note that those element vectors
have to be linear-independent. Thus we have to choose
ππΉ β₯πΎβπ=1
(πΏ(π)π + πΏ
(π)β + 1
). (48)
ACKNOWLEDGEMENTS
The authors would like to thank the editor and anonymousreviewers for invaluable comments and suggestions.
REFERENCES
[1] T. Cover and A. El-Gamal, βCapacity theorems for the relay channel,βIEEE Trans. Inf. Theory, vol. 25, no. 5, pp. 572β584, Sep. 1979.
[2] J. Laneman and G. Wornell, βDistributed space-time-coded protocolsfor exploiting cooperative diversity in wireless networks,β IEEE Trans.Inf. Theory, vol. 49, no. 10, pp. 2415β2425, Oct. 2003.
[3] A. Sendonaris, E. Erkip, and B. Aazhang, βUser cooperation diversityβpart I: system description,β IEEE Trans. Commun., vol. 51, no. 21, pp.1927β1938, Nov. 2003.
[4] P. A. Anghel and M. Kaveh, βOn the performance of distributed space-time coding systems with one and two non-regenerative relays,β IEEETrans. Wireless Commun., pp. 682β692, Mar. 2006.
[5] Y. Jing and B. Hassibi, βDiversity analysis of distributed space-time codes in relay networks with multiple transmit/receive antennas,βEURASIP J. Advances in Signal Process., Jan. 2008.
[6] W. Zhang, Y. Li, X.-G. Xia, P. C. Ching, and K. B. Letaief, βDistributedspace-frequency coding for cooperative diversity in broadband wirelessad hoc networks,β IEEE Trans. Wireless Commun., vol. 7, no. 3, pp.995β1003, Mar. 2008.
[7] K. G. Seddik and K. J. R. Liu, βDistributed space-frequency codingover broadband relay channels,β IEEE Trans. Wireless Commun., vol. 7,no. 11, pp. 4748β4759, Nov. 2008.
[8] H. Wang, X.-G. Xia, and Q. Yin, βDistributed space-frequency codesfor cooperative communication systems with multiple carrier frequencyoffsets,β IEEE Trans. Wireless Commun., vol. 8, no. 2, pp. 1045β1055,Feb. 2009.
[9] W. Su, Z. Safar, and K. J. R. Liu, βTowards maximum achievablediversity in space, time, and frequency: performance analysis and codedesign,β IEEE Trans. Wireless Commun., vol. 4, no. 4, pp. 1847β1857,July 2005.
[10] O. Oguz, A. Zaidi, J. Louveaux, and L. Vandendorpe, βDistributedspace-time-frequency block codes for multiple-access-channel with re-laying,β in Proc. IEEE GLOBECOM 2007, Nov. 2007, pp. 1729β1733.
[11] L. C. Tran, A. Mertins, and T. A. Wysocki, βCooperative communica-tion in space-time-frequency coded MB-OFDM UWB,β in Proc. IEEEVehicular Tech. Conf. 2008 Fall, Sep. 2008, pp. 1β5.
[12] W. Yang, Y. Cai, and L. Wang, βDistributed space-time-frequencycoding for cooperative OFDM systems,β Science in China Series F:Inf. Sciences, vol. 52, no. 12, pp. 2424β2432, Dec. 2009.
[13] S. Alamouti, βA simple transmitter diversity scheme for wireless com-munications,β IEEE J. Sel. Areas Commun., pp. 1451β1458, Oct. 1998.
[14] V. Tarokh, H. Jafarkhani, and A. R. Calderbank, βSpace-time block codefrom orthogonal designs,β IEEE Trans. Inf. Theory, vol. 45, pp. 1456β1467, July 1999.
[15] Y. S. Choi, P. J. Voltz, and F. A. Cassara, βOn channel estimation anddetection for multicarrier signals in fast and selective Rayleigh fadingchannels,β IEEE Trans. Commun., vol. 49, no. 8, pp. 1375β1387, Aug.2001.
[16] X. Giraud, E. Boutillon, and J. C. Belfiore, βAlgebraic tools to buildmodulation schemes for fading channels,β IEEE Trans. Inf. Theory,vol. 43, pp. 938β952, May 1997.
[17] J. Boutros and E. Viterbo, βSignal space diversity: a power- andbandwidth-efficient diversity technique for the Rayleigh fading channel,βIEEE Trans. Inf. Theory, vol. 44, pp. 1453β1467, July 1998.
[18] Y. Xin, Z. Wang, and G. B. Giannakis, βSpace-time diversity systemsbased on linear constellation precoding,β IEEE Trans. Wireless Com-mun., vol. 2, pp. 294β309, Mar. 2003.
[19] Z. Liu, Y. Xin, and G. B. Giannakis, βLinear constellation precodedOFDM with maximum multipath diversity and coding gains,β IEEETrans. Commun., vol. 51, no. 3, pp. 416β427, Mar. 2003.
[20] Y. Jing and B. Hassibi, βDistributed space-time coding in wireless relaynetworks,β IEEE Trans. Wireless Commun., vol. 5, no. 12, pp. 3524β3536, Dec. 2006.
Jinsong Wu received the B.Eng. degree fromChangsha Railway University (now part of CentralSouth University), Changsha, China, in 1990, theM.A.Sc. degree from Dalhousie University, Halifax,NS, Canada, in 2001, the Ph.D. degree from QueenβsUniversity, Kingston, ON, Canada, in 2006, all inelectrical engineering.
He experienced research and development posi-tions of information technology or telecommunica-tions industry with STONE in China, Great-WallComputer in China, Nortel Networks in Canada,
Philips Research North America in USA, and Sprint-Nextel in USA. He iscurrently working as a research fellow in University of Waterloo, Canada.His recent research interests lie in general communications theory and signalprocessing, space-time-frequency processing and coding, cooperative com-munications, iterative decoding or processing, communication optimization,and network protocols with applications. He has served as technical programcommittee members in IEEE Global Communications Conference 2008 and2010 and IEEE International Conference on Communications 2009 and 2011.
WU et al.: HIGH-RATE DISTRIBUTED SPACE-TIME-FREQUENCY CODING FOR WIRELESS COOPERATIVE NETWORKS 625
Honggang Hu received a B.S. degree in Mathemat-ics in June 2000, and a B.E. degree in ElectricalEngineering in June 2001 from the University ofScience and Technology of China (USTC), Hefei,China, and the Ph.D. degree in Electrical Engineer-ing from the Graduate School of Chinese Academyof Sciences, Beijing, China, in July 2005. From July2005 to April 2007, he was a Postdoctoral Fellowat the Institute of Software, Chinese Academy ofSciences, Beijing, China. From August 2007 toJuly 2009, he was a Postdoctoral Fellow at the
University of Waterloo, Canada. Since August 2009, he has been a ResearchAssociate at the University of Waterloo, Canada. His research interests includepseudorandom sequences, cryptography, and coding theory.
Murat Uysal was born in Istanbul, Turkey in 1973.He received the B.Sc. and the M.Sc. degree in elec-tronics and communication engineering from Istan-bul Technical University, Istanbul, Turkey, in 1995and 1998, respectively, and the Ph.D. degree in elec-trical engineering from Texas A&M University, Col-lege Station, Texas, in 2001. Since 2002, he has beenwith the Department of Electrical and ComputerEngineering, University of Waterloo, Canada, wherehe is now an Associate Professor. He is currently onleave at Ozyegin University, Istanbul, Turkey. His
general research interests lie in communications theory and signal processingfor communications with special emphasis on wireless applications. Specificresearch areas include MIMO communication techniques, space-time coding,diversity techniques and coding for fading channels, cooperative commu-nication, and free-space (wireless) optical communication. Dr. Uysal is anAssociate Editor for IEEE TRANSACTIONS ON WIRELESS COMMUNICA-TIONS, IEEE COMMUNICATIONS LETTERS, and Wireless Communicationsand Mobile Computing (WCMC) journal. In the past he served as a GuestCo-Editor for WCMC Special Issue on βMIMO Communicationsβ (October2004) and IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS
Special Issue on βOptical Wireless Communicationsβ (December 2009). Overthe years, he has served on the technical program committee of more than70 international conferences and workshops in the communications area. Herecently co-chaired IEEE ICC 2007 Communication Theory Symposium andCCECEβ08 Communications and Networking Symposium. He is currentlyserving as the Tutorial Chair of IEEE WCNC 2011. Dr. Uysal is a SeniorIEEE member.