Relay selection (RS) has widely been studied for both decode-and-forward (DF) and amplify-and-forward (AF) protocols, and relay ordering (RO) was recently proposed for the AF protocol only . The two strategies, RS and RO, have been individually shown to be very effective to enhance the performance of cooperative relay networks. In this letter, we first discuss the fundamental difference of the two schemes and investigate an important tradeoff in terms of spectral efficiency and energy efficiency between RS and RO. Specifically, RS is more spectrally efficient, whereas RO can be more efficient in energy consumption. Then we demonstrate that indeed RS and RO generally have different outage performance and may complement each other depending on channel conditions. To optimize the outage performance, therefore, we combine the two strategies, RS and RO, and we propose joint RS-RO, followed by exact outage probability derivation of the joint RS-RO in closed-form. Finally, we obtain some interesting insights into the issue of selecting efficient transmission schemes between RS and RO.

We study the analog network coding (ANC), which is a well-known amplify-and-forward (AF)-based bidirectional protocol, for a bidirectional network consisting of two different sources and a relay. In this protocol, the two sources exchange information with the help of the relay during two time slots in a half-duplex mode. For this system, we first derive a tight lower bound of outage probability, which is very close to the exact outage probability in the whole signal-to-noise ratio (SNR) range irrespective of the values of channel variances. Using the tight lower bound, we obtain finite-SNR diversity-multiplexing tradeoff of the ANC protocol. Furthermore, we propose an optimum power allocation scheme, which simultaneously minimizes the outage probability and maximizes the total mutual information of the ANC protocol.

Time division broadcasting (TDBC) is a well-known bidirectional protocol. In this protocol, two sources exchange information with the help of a relay terminal. For amplify-and-forward (AF)-based TDBC, we first derive a tight lower bound of the outage probability in closed-form, and it is very close to the exact outage probability in the whole signal-to-noise ratio (SNR) range irrespective of the values of channel variances. Using the tight lower bound, diversity-multiplexing tradeoff of the TDBC protocol is obtained for finite-SNR. Furthermore, we investigate how to optimize the TDBC protocol; specifically, an optimum method to combine the received signals at the relay terminal is developed. This method minimizes the outage probability and maximizes the total mutual information of the TDBC protocol at the same time.

This paper focuses on the average bit error rate (BER) and diversity order analysis of the distributed Alamouti's code in dissimilar cooperative networks with channel state information (CSI)-assisted relays. We first assume that the relays adopt the amplifying coefficient proposed in . Lower and upper bounds of the average BER of the distributed Alamouti's code are derived. Those two bounds tightly bound the exact average BER. Then we show that, surprisingly, the distributed Alamouti's code achieves only diversity order one when the relays use the amplifying coefficient proposed in . To resolve this problem, we propose a new threshold-based amplifying coefficient for the distributed Alamouti's code based on the work in . This new amplifying coefficient makes the distributed Alamouti's code achieve the full diversity order two. Moreover, based on three different CSI assumptions, four amplifying schemes are developed in order to determine the value of the threshold used in the new amplifying coefficient.

We study relay selection with the physical-layer network coding (PNC), which is a major decode-and-forward (DF)-based bidirectional protocol. We consider a network consists of two different end-sources and multiple relays, where each relay adopts XOR-encoding to combine two received symbols from the two end-sources. For this network, we propose a relay selection scheme by modifying the well-known selection cooperation (SC) to use in the PNC protocol, and it is referred to as SC-PNC. The SC-PNC consists of two phases: a multiple access channel (MAC) phase and a broadcast channel (BC) phase. In the MAC phase, a set of relays that correctly decode two received symbols from the two end-sources is determined; in the BC phase, among the relays in the determined set, a single best relay is selected such that the minimum mutual information of the two links from each relay to the two end-sources is maximized. Finally, we derive the exact outage probability of the SC-PNC in closed-form.

An amplify-and-forward, multi-branch, multi-hop relay system with K relays, in which the relays broadcast to other relays as well as the destination, is analyzed. An approximate symbol-error-rate (SER) expression, which is valid for any number of relays and for several modulation schemes, is found for the multi-hop system. Also, the cumulative density function (CDF) and probability density function (PDF) are found for the random variable, Z = XY/(X + Y + c), where X and Y are sums of independent, Erlang random variables, and c is a constant. The moment generating function (MGF) of Z is found for the special case in which c = 0. It is shown that these results are generalizations of previously published results for special cases of Z. The MGF of Z is used to develop the approximate SER expression. Results for the analytic SER expression are included and compared with simulation results for various values of K, for various modulation schemes, and for two choices of system parameters (channel variances). Results for the multi-hop system are also compared to results for the two-hop system (in which relays transmit only to the destination).

We analyze the error performance of the physical-layer network coding (PNC) protocol without channel coding in bidirectional relay networks for binary phase shift keying (BPSK) over Rayleigh fading channels. It is assumed that a bidirectional relay network consists of two sources and a relay, where each node has a single antenna and operates in a half-duplex mode, and the PNC over finite GF(2) is employed. In this system, since the maximum-likelihood (ML) detection metric of the multiple access channel (MAC) at the relay is given by the sum of two exponential functions, it is not possible to utilize the classical Euclidean distance rule. To make the performance analysis tractable, we approximate the ML detection metric by adopting the max-log approximation. Then we derive tight upper and lower bounds in closed form for the average symbol error probability of the MAC at the relay. Finally, we obtain tight upper and lower bounds in closed form for the end-to-end average bit-error rate (BER).

We study relay selection (RS) with the analog network coding (ANC) and time division broadcast (TDBC), which are two major amplify-and-forward (AF)-based protocols in bidirectional relay networks. We consider a bidirectional network consisting of two different end-sources and multiple relays, where each terminal has a single antenna and operates in a half-duplex mode. In this network, a single best relay is selected depending on channel conditions to help bidirectional communication between the two end-sources. Specifically, we first consider RS schemes for the ANC and TDBC protocols based on a max-min criterion to minimize the outage probabilities. Then, for the RS in the ANC protocol, we derive a closed-form expression of the outage probability; for the RS in the TDBC protocol, we derive a one-integral form of the outage probability and its lower bound in closed-form. Numerical results confirm that the closed-form expression of the ANC protocol and the one-integral form of the TDBC protocol are very accurate, and that the closed-form lower bound of the TDBC protocol is also tight.

For the decode-and-forward protocol in relay networks, opportunistic relaying (OR) and selection cooperation (SC) are two major relay selection schemes, which have been studied only for single antenna terminals. We study OR and SC in a multi-antenna relay network where each terminal has multiple antennas. To fully exploit multiple antennas without incurring high feedback overhead, we adopt transmit antenna selection (TAS). Specifically, we first propose two joint relay-and-antenna selection schemes which combine OR and SC, respectively, with TAS: joint OR-TAS and joint SC-TAS. For each joint selection scheme, a single best transmit antenna at the source, a single best relay, and a single best transmit antenna at this selected relay are jointly determined in an optimum sense. In this network, at the first time slot, the selected antenna at the source transmits a symbol to the selected relay; at the second time slot, the selected antenna at the selected relay retransmits the detected symbol to the destination. We derive the outage probability of joint OR-TAS. Also, we obtain the outage probability of joint SC-TAS by proving that the outage probability of joint SC-TAS is identical to that of joint OR-TAS.

In this paper, we combine opportunistic transmission and source/destination cooperation in a decode-and-forward (DF)-based two-hop cooperative diversity network consisting of multiple source-destination pairs and a single relay. Firstly, we consider a special scenario with two source-destination pairs. For this network, there are two possible strategies in each hop: no-cooperation or cooperation. Considering the combination of the two strategies in two cascaded hops, we investigate four different end-to-end transmission strategies, and we find that the four strategies complement one another depending on channel conditions in terms of outage performance. To maximize the mutual information, we propose an optimum joint selection of source-destination pair and end-to-end transmission strategy. Then we show that the optimum joint selection scheme can be simplified without loss of outage performance, and derive its exact outage probability in closed-form. Secondly, we generalize the proposed transmission strategy into a scenario with multiple source-destination pairs. For this network, we first propose an optimum end-to-end transmission strategy to maximize the mutual information, then a suboptimum end-to-end transmission strategy to reduce the signaling overhead and computational complexity. For the suboptimum strategy, we derive the outage probability and diversity order.

We analyze the error performance of two-hop relay networks adopting frequency shift keying (FSK) over frequency flat Rayleigh fading channels. It is assumed that relay networks consist of a source, a relay, and a destination without a direct path signal from the source to the destination and the relay adopts the amplify-and-forward protocol with a fixed gain. Firstly, considering imperfect frequency and phase synchronization, we obtain the exact error probability expressions for noncoherent and coherent binary FSK (BFSK). Secondly, assuming perfect frequency and phase synchronization, we derive a closed-form error probability approximation for coherent M-ary FSK (MFSK). The proposed methods can also be used for the error performance analysis of classical one-hop FSK systems with perfect/imperfect frequency and phase synchronization. The obtained error probability expressions will help the design of two-hop relay networks adopting FSK in determining the system parameters such as the transmission power at the source, the amplifying coefficient at the relay, and the maximum affordable frequency and phase offsets to satisfy the required error performance.

We analyze the bit-error rate (BER) performance of the distributed Alamouti's code for cooperative diversity networks consisting of a source, two relays and a destination node over Rayleigh fading channels. It is assumed that the relays adopt the amplify-and-forward protocol. Firstly, assuming the existence of the direct path component from the source to the destination, we derive the exact BER expression in a one-integral form for M-pulse amplitude modulation (PAM) and M-quadrature amplitude modulation (QAM) constellations. We also present a series expansion of a very accurate BER approximation, which does not require any numerical calculation, and we prove this series is convergent. Secondly, considering the system where the direct path component does not exist, we obtain the exact BER expression in a one-integral form and a series expansion of the exact BER expression. Numerical results confirm that the two exact BER expressions in a one-integral form perfectly match the simulation results and the two series expressions of BER are very accurate, even with a small truncation window.

We analyze the maximum-likelihood (ML) performance of the decode-and-forward protocol in a cooperative diversity network which consists of a source, a relay, and a destination with a direct path signal, but which is not equipped with cyclicredundancy- check (CRC) codes. In this system, due to a symbol error at the relay, the ML receiver at the destination needs to consider all the possible symbol detection scenarios at the relay as well as at the destination. Therefore, the ML detection metric is given by a linear combination of exponential functions, which prevents the use of the classical minimum Euclidean distance rule. Adopting the max-log approximation, we approximate the ML detection rule which makes the ML performance analysis tractable. In order to facilitate the derivation of decision regions, we simplify the ML detection rule in the two-dimensional real space such that two metric values of two adjacent constellation points are sequentially compared. Then we obtain decision regions in a form without union and intersection. Finally, based on the decision regions, we derive a very accurate closedform BER approximation for M-pulse amplitude modulation (PAM) and M-quadrature amplitude modulation (QAM). The obtained BER expression can serve as the error performance upper-bound of the decode-and-forward protocol in cooperative diversity networks.

We consider very general code-division-multiple- access (CDMA) systems, namely multiple-chip-rate CDMA systems, where signals can be transmitted at different chip rates, carrier frequencies, processing gains, and transmitted powers to satisfy the given quality of service (QoS) requirements. Also, nonzero and possibly different carrier frequency offsets are assumed for different users. For these systems, a closed-form bit error rate (BER) expression is derived based on the simplified improved Gaussian approximation. Numerical result demonstrates that the proposed method provides much more accurate BER values compared to the standard Gaussian approximation.

In this paper, a multi-coded variable spreading gain (MC-VSG) CDMA system employing a binary transmission of MC signals by introducing a level clipper, termed MC-VSG BNet system, is proposed for a possible candidate of wireless personal area network (WPAN) and 3G cellular applications. With an emphasis on the MC-VSG BNet physical layer and the system performance, we address the concise specification of the MC-VSG BNet system. From the receiver performance based on simulation results, in addition to simple receiver structure, an acceptable performance degradation of the MC-VSG BNet system over the existing DS/CDMA system is observed, while guaranteeing an high bit rate transmission.

A multi-coded CDMA system employing a variable spreading gain (VSG) approach is proposed for high-rate applications, which is based on two algorithms for selecting the multi-code set. From the simulation results, in addition to a simple receiver structure thanks to level clipping, the multi-coded VSG CDMA system provides a high bit rate transmission, while maintaining an acceptable performance degradation of the multi-coded VSG CDMA system over the existing DS-CDMA system.

This paper considers a channel estimation technique in a wireless personal area network (WPAN) system with complementary code keying (CCK) signaling over multipath fading channels. The performance of the proposed coherent detector is compared with that of a noncoherent detector, and a significant improvement of performance is observed in terms of the bit error probability.

We present a coarse frequency-offset synchronization technique for an OFDM-based transmission system with an emphasis on the Eureka-147 DAB receiver. The proposed frequency-offset synchronization algorithm using two defined correlation functions is shown to have high robustness against a large range of symbol timing offset with a moderate implementational complexity.

This paper describes an adaptive timing synchronization scheme and DC-offset compensation technique for a short-ranged Bluetooth system. The synchronization scheme estimates the variance of the partial-band interference, which is utilized for the trigger threshold value of the inquiry scan and page scan states, while DC-offset compensation scheme is designed using the access codes which are known to each Bluetooth device. Numerical results show the proposed synchronization algorithm is robust to the partial-band noise interference and of low complexity. Also, the new DC-offset compensation method shows more reliable estimation ability of DC-offset than the existing method without extra hardware burdens.