Adaptive transmission in MIMO AF relay networks with orthogonal space-time block codes over Nakagami-m fading
© Phan et al.; licensee Springer 2012
Received: 13 August 2011
Accepted: 11 November 2011
Published: 12 January 2012
In this article, we apply different adaptive transmission techniques to dual-hop multiple-input multiple-output amplify-and-forward relay networks using orthogonal space-time block coding over independent Nakagami-m fading channels. The adaptive techniques investigated are optimal simultaneous power and rate (OSPR), optimal rate with constant power (ORCP), and truncated channel inversion with fixed rate (TCIFR). The expressions for the channel capacity of OSPR, ORCP, and TCIFR, and the outage probability of OSPR, and TCIFR are derived based on the characteristic function of the reciprocal of the instantaneous signal-to-noise ratio (SNR) at the destination. For sufficiently high SNR, the channel capacity of ORCP asymptotically converges to OSPR while OSPR and ORCP achieve higher channel capacity compared to TCIFR. Although TCIFR suffers from an increase in the outage probability relative to OSPR, it provides the lowest implementation complexity among the considered schemes. Along with analytical results, we further adopt Monte Carlo simulations to validate the theoretical analysis.
During the last decade, multiple-input multiple-output (MIMO) techniques have attracted great attention as a way of improving spectral efficiency and reliability in wireless communications. MIMO systems with orthogonal space-time block coding (OSTBC) transmission are considered as a means of providing full diversity gain and linear decoding complexity [1–3]. In recent years, the combination of MIMO systems using OSTBC transmission with relay networks has been significantly considered (see, e.g., [4–9] and the references therein). In  and , the outage probability and symbol error rate (SER) performance of MIMO decode-and-forward (DF) relay networks with OSTBC transmission over Rayleigh fading channels were investigated, respectively. In , the SER of MIMO systems in which the source employs OSTBC transmission to transmit the signal to the destination through the help of semi-blind amplify-and-forward (AF) relays over Rayleigh fading channels was derived. In , the authors investigated the bit error rate (BER) performance of MIMO channel state information (CSI)-assisted AF relay networks with OSTBC transmission over Rayleigh fading channels. More recently, by taking the direct link between source and destination into account, the SER and outage probability of MIMO CSI-assisted AF relay cooperative networks with OSTBC transmission over Rayleigh fading channels were investigated . Furthermore, closed-form expressions for the outage probability and the SER of dual-hop MIMO CSI-assisted AF relay networks with OSTBC transmission have been derived for independent and correlated Nakagami-m fading channels in [9, 10], respectively.
Although MIMO relay networks with OSTBC transmission have received a lot of research efforts, all of the above mentioned contributions concentrate on cooperative communications with constant transmission rate and power. For adaptive transmission, depending on the link quality provided by the fading channels, the system will adapt its transmission power, transmission rate, coding rate/scheme, modulation scheme, or the arbitrary combination of these techniques to the fluctuations induced by the fading channels in order to enhance the spectral efficiency [11–17]. In particular, a cooperative relay network where the source employs constant transmission power and adapts transmission rate through M-ary quadrature amplitude modulation (M-QAM) has been studied in terms of outage probability, SER, and spectral efficiency for Rayleigh fading channels . The combination of opportunistic incremental relaying with adaptive modulation deployed in cooperative relay networks was analyzed in . This scheme has been shown to guarantee a specific BER performance level and improve both spectral efficiency and outage probability. In , approximations for the channel capacity of opportunistic cooperative multiple relay networks over Rayleigh fading channels under optimal simultaneous power and rate (OSPR), optimal rate with constant power (ORCP), and truncated channel inversion with fixed rate (TCIFR) were investigated. The upper bounds of channel capacity for AF cooperative systems over Rayleigh fading channels under adaptive transmission were derived in . Furthermore, the use of different adaptive schemes in AF multi-hop relaying networks over Nakagami-m fading environments was studied in  wherein the achievable channel capacity was evaluated by using the characteristic function (CHF) of the reciprocal of the instantaneous SNR at the destination. In , the Shannon channel capacity of the maximum ratio combining (MRC) receiver over η-μ fading channels and adaptive transmission has been investigated. Upper bounds for the capacity of different adaptive transmission techniques for an AF system with best relay selection over Rayleigh fading channels have been reported in . The study of  has presented a framework for practical application of adaptive transmission to MIMO systems to enhance the transmission rate of broadband wireless systems.
Most efforts that have been made for the utilization of adaptive transmission focus on traditional cooperative relay networks over Rayleigh or Nakagami-m fading channels. To the best of the authors’ knowledge, there is no previous study considering adaptive transmission for MIMO AF relay networks with OSTBC transmission. Although OSTBC transmission provides diversity gain and decoding simplicity, it decreases transmission rate as compared to other kinds of space-time block codes (STBC). Therefore, it is beneficial to employ adaptive transmission schemes for MIMO AF relay networks with OSTBC transmission in order to enhance its spectral efficiency. In this article, we therefore analyze the performance of these systems over independent, identically distributed (i.i.d.) and independent, non-identically distributed (i.n.i.d.) Nakagami-m fading channels. Our key contributions can be summarized as follows. We investigate the performance of MIMO CSI-assisted AF relay networks with OSTBC transmission for the adaptive schemes of OSPR, ORCP, and TCIFR over Nakagami-m fading channels. Specifically, we derive analytical expressions for the channel capacity of the three adaptive MIMO AF relay networks with OSTBC based on the CHF of the reciprocal of the instantaneous SNR at the destination. Furthermore, we present expressions of the outage probability for the considered cooperative relay networks with OSPR and TCIFR wherein transmission will be suspended as long as the instantaneous SNR falls below an optimal value. For the ORCP scheme, however, there is no need to evaluate the outage probability as the source constantly keeps transmission regardless of the value of the instantaneous SNR.
The remaining parts of this article are organized as follows. In Section 2, we present the system and channel model, describing fundamental concepts of MIMO CSI-assisted AF relay networks with OSTBC transmission. In Section 3, we derive the CHF of the reciprocal of the instantaneous SNR for the considered cooperative relay networks with OSTBC and adpative transmission. The derivation of the channel capacity and the outage probability of these cooperative relay networks are presented in Section 4. We then derive the channel capacity of the investigated systems with ORCP in Section 5. In Section 6, the channel capacity and the outage probability of the cooperative relay networks using TCIFR are derived. Analytical results and Monte Carlo simulations along with further discussions are presented in Section 7. Finally, in Section 8, conclusions of this study are given.
Notation: Throughout this article, we will use the following notations. The Frobenius norm of a vector or matrix is denoted as . The probability density function (PDF) and the cumulative distribution function (CDF) of a random variable (RV) X are denoted as and , respectively. Then, and stand for the moment generating function (MGF) and the CHF of an RV X, respectively. In addition, represents an additive white Gaussian noise (AWGN) RV with zero mean and variance . We denote as the gamma function , eq. (8.310.1)] and as the incomplete gamma function , eq. (8.350.2)]. Furthermore, indicates the exponential integral function , eq. (184.108.40.206)], and is the n th order modified Bessel function of the second kind , eq. (8.432.1)]. Finally, is the real part of a complex expression.
2 System and channel model
where , denotes the fading severity parameter of the Nakagami-m channel, and is the average channel power, , .
It is noted that the tractable form of the instantaneous SNR in (6), which is expressed as the harmonic mean of and , is utilized for deriving the performance of dual-hop MIMO AF relay networks with OSTBC and adaptive transmission in the sequel.
3 Derivation of the CHF of the reciprocal of the instantaneous SNR
Depending on whether the fading channels are identically distributed, the obtained CHF of has different forms, resulting in different performance expressions for each case. We now continue to derive the MGF and CHF of Y based on its respective PDF for i.i.d. and i.n.i.d. Nakagami-m fading channels.
3.1 MGF and CHF for i.i.d. Nakagami-m fading channels
3.2 MGF and CHF for i.n.i.d. Nakagami-m fading channels
In the following sections, we utilize the CHF of the reciprocal of the instantaneous SNR to analyze the performance of dual-hop MIMO AF relay networks using OSTBC with different adaptive transmission schemes. Specifically, the source will adapt its transmission rate and/or power to the variations of the channel coefficients. Moreover, for implementing adaptive transmission, it is required that the instantaneous SNR is perfectly measured at the destination and is then sent back to the source through a feedback channel. This feedback channel is assumed to be error free with negligible delay and therefore enables the source to timely perform the transmission rate and/or power adaptation.
Recall that the Shannon capacity of a fading channel determines the theoretical upper bound on the maximum transmission rate with an arbitrarily small error probability. Adaptive transmission has been known as a means of achieving this bound . For dual-hop MIMO AF relay networks using OSTBC where adaptation is only implemented at the source, the channel capacity of different adaptive schemes is analyzed in the sequel.
4 Optimal simultaneous power and rate adaptation
In this section, we investigate the channel capacity and the outage probability of the considered MIMO AF relay network with OSTBC for the case of adaptive transmission with OSPR. Accordingly, in response to the instantaneous SNR fed back from the destination, the source will adapt its transmission power and transmission rate in an optimal way, i.e., maximizing the channel capacity subject to the average transmit power constraint. In order to conserve transmission power during deep fades, the transmission will be suspended under such channel conditions. This mode of operation remains as long as the instantaneous SNR, γ, that is fed back from the destination to the source, is below the optimal cutoff SNR, , for OSPR.
where denotes the secant function, i.e., . To the best of the authors’ knowledge, no closed-form solution is available for this integration. However, we now can readily evaluate the performance of the considered relay network by simply using the numerical integration method as in .
4.1 Channel capacity and outage probability for i.i.d. Nakagami-m fading channels
4.2 Channel capacity and outage probability for i.n.i.d. Nakagami-m fading channels
5 Optimal rate adaptation with constant transmit power
In this section, we derive an analytical expression for the channel capacity of ORCP in the context of the considered relay networks. For the case of OSPR, the source only adapts its transmission rate in response to the channel states as . The instantaneous SNR at the destination is also provided to the source through a feedback channel. Compared to the fixed rate systems, wherein the transmission rate of the source is designed in advance to operate efficiently in specific level of channel quality, the ORCP scheme can take advantage of the variations of fading channels because it constantly adapts the transmission rate to the channel condition . In addition, as the source adapts only its transmission rate but not the power, the implementation of ORCP is less complex as compared to OSPR. Since the transmission remains at arbitrary value of the instantaneous SNR, no outage event occurs for ORCP scheme.
which basically represents the average channel capacity of a flat-fading channel with respect to the distribution of the instantaneous SNR at the destination.
where , and . In fact, the integral of the real part of a complex expression given in (36) can be numerically solved by mathematics software packages.
5.1 Channel capacity for i.i.d. Nakagami-m fading channels
5.2 Channel capacity for i.n.i.d. Nakagami-m fading channels
These above integrals given in (37) and (38) can be numerically solved with the help of standard mathematics software packages.
6 Truncated channel inversion with fixed rate
With TCIFR, the source only adapts its transmit power to provide a constant instantaneous SNR at the destination while keeping the transmission rate fixed. In other words, it inverts the channel response as long as channel conditions are better than a specific level. Hence, the fading channels appear as time-invariant and a fixed transmission rate can be maintained by changing the transmit power properly regardless of the channel conditions. Therefore, TCIFR incurs the least implementation complexity among OSPR and ORCP schemes. For TCIFR, transmission will be suspended during periods of deep fading. Specifically, as long as the instantaneous SNR is greater than the optimal cutoff SNR, , TCIFR is active.
6.1 Channel capacity and outage probability for i.i.d. Nakagami-m fading channels
Both integral expressions given in (45) and (46) can be numerically solved by mathematics software packages. The respective channel capacity, , is obtained by substituting (45) and (46) in (39).
6.2 Channel capacity and outage probability for i.n.i.d. Nakagami-m fading channels
The above expressions in (47) and (48) can be solved numerically. The respective channel capacity, , is obtained by substituting (47) and (48) in (39).
It is noted that the obtained expressions for the outage probability and channel capacity given in (28), (30), (31), (33), (37), (38), and (45)-(48) are represented in terms of one-dimensional integrals with definite limits. These expressions can readily be solved by standard mathematics software packages such as Mathematica. It is noted that for single antenna relay networks, the obtained expressions for the system performance are given in integral forms in .
7 Numerical results
In this section, we present numerical examples for the performance metrics derived above. Monte Carlo simulations are provided together with analytical results in order to verify our analysis. Importantly, in all tested scenarios, there is very close agreement between the analytical and simulated curves. This confirms the correctness of the analysis presented in this article.
Optimal cutoff SNR and for i.i.d. Nakagami- m fading channels and different number of antennas.
Optimal cutoff SNR and for i.n.i.d. Nakagami- m fading channels and different number of antennas.
We have analyzed the performance of the three adaptive transmission schemes of OSPR, ORCP, and TCIFR applied to MIMO CSI-assisted AF cooperative relay networks with OSTBC. Our analysis is based on both the i.i.d. and i.n.i.d. Nakagami-m fading channels, which generalizes a wide class of multi-path fading environments. Closed-form expressions for the MGFs of the reciprocal of the instantaneous SNR are derived and then utilized to evaluate the performance metrics. We present Monte Carlo simulations, which are in a very close agreement with analytical results, to validate our analysis. We also show that for sufficiently high SNR the channel capacity of OSPR and ORCP are almost the same and better than that of TCIFR for the considered scenarios. Regarding the practical implementation, ORCP is less complex to set up as compared to OSPR. It is also seen that for low SNR, the channel capacity of TCIFR outperforms that of the ORCP scheme. Moreover, it can be seen that the outage probability of OSPR is significantly lower than that of TCIFR; however, TCIFR offers the least complexity for practical implementation.
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