- Research Article
- Open Access
Novel Precoded Relay-Assisted Algorithm for Cellular Systems
© Sara Teodoro et al. 2010
Received: 14 July 2010
Accepted: 11 November 2010
Published: 30 November 2010
Cooperative schemes are promising solutions for cellular wireless systems to improve system fairness, extend coverage and increase capacity. The use of relays is of significant interest to allow radio access in situations where a direct path is not available or has poor quality. A data precoded relay-assisted scheme is proposed for a system cooperating with 2 relays, each equipped with either a single antenna or 2-antenna array. However, because of the half-duplex constraint at the relays, relaying-assisted transmission would require the use of a higher order constellation than in the case when a continuous link is available from the BS to the UT. This would imply a penalty in the power efficiency. The simple precoding scheme proposed exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, achieving full diversity, while significantly reducing power penalty. Analysis of the pairwise error probability of the proposed algorithm with a single antenna in each relay is derived and confirmed with numerical results. We show the performance improvements of the precoded scheme, relatively to equivalent distributed SFBC scheme employing 16-QAM, for several channel quality scenarios.
Multiple-input, multiple-output (MIMO) wireless communications are effective in mitigating channel fading, thus improving the cellular system capacity . However, there is significant correlation between channels in some environments, and using an antenna array at the user terminal (UT) may not be feasible due to size, cost, and hardware limitations. Cooperative systems are promising solutions for wireless systems to overcome such limitations, when the direct link does not have good transmission conditions . It can be achieved through cooperation of terminals (either dedicated or user terminals acting as relays), which share their antennas and thereby create a virtual antenna array (VAA) or a virtual MIMO (VMIMO) system . These allow single antenna devices to benefit from spatial diversity without the need for colocated additional physical antenna arrays.
Several cooperative diversity protocols have been proposed and analysed to demonstrate the potential benefits of cooperation [4, 5]. In , a theoretical diversity-multiplexing trade-off study is presented regarding a cooperative system with 1 and 2 antennas in a single-relay scheme. Furthermore, in  the Rayleigh performance of a single-relay cooperative scenario with multiple-antenna nodes is investigated, deriving pairwise error probability (PEP) expressions. In order to get higher diversity orders, one can also consider the use of multiple-relay nodes . However, increasing the number of relays reduces the bandwidth efficiency of the system, as the source uses only a fraction of the total available degrees of freedom to transmit the information.
In , the idea of space-time coding devised for point-to-point colocated multiple antenna systems is applied for a wireless relay network with single antenna nodes and PEP of such a scheme is derived. The authors show that in a relay network with a single source, a single destination with single antenna relays, distributed space-time coding (DSTC) asymptotically approaches the diversity of a colocated multiple antenna system with transmit antennas and one receiving antenna. More recently, in , distributed orthogonal space-time block codes (DOSTBCs) achieving single-symbol decodability have been introduced for cooperative networks. The authors considered a special class of these codes (row monomial DOSTBCs), deriving upper-bounds on the maximum symbol-rate. In , single-symbol decodable (SSD) DSTBCs have been studied when the relay nodes are assumed to know the corresponding channel phase information. An upper bound on the symbol rate for such a set up is shown to be one half of the rate of the direct link transmission when this link is available, which is independent of the number of relays. In , a semi-orthogonal precoded DSTBC was proposed, wherein the source performs precoding on the information symbols before transmitting it to all the relays. Several relays are considered all equipped with a single antenna. A systematic construction of these codes is presented for more than 3 relays and the constructed codes are shown to have higher rates than those of DOSTBCs. Although achieving full diversity, these distributed orthogonal algorithms cannot achieve full spectral efficiency, since they use 2 phases for transmission. For this reason, these cooperative systems achieve half of the bandwidth efficiency of the equivalent non-cooperative systems.
Other works were developed with the objective of increasing capacity or diversity order of cooperative systems, using non-orthogonal protocols for cooperative systems with 2 or more relays, as in [13, 14]. In  a generalised non-orthogonal amplify-and-forward protocol is proposed with a low decoder complexity, achieving better error performances than in , depending on the coding. In  coding strategies are studied for non-orthogonal cooperative channels, using one or more designed space-time precoders, in a protocol where inter-relay communication is allowed, but again having no full spectral efficiency. In these non-orthogonal algorithms, transmission via an existing direct path is required. Thus, in situations with poor direct link conditions, performance is significantly degraded and in case of outage of one relay some information can be lost.
Further along the development of cooperative systems, some relay precoder designs were also proposed, however with different goals [15, 16]. In , the precoder maximizes the capacity between the source and destination nodes in a non-regenerative relay system, with a single relay node, considering all the nodes with multiple antennas. In , MIMO relay provides robustness against imperfect channel state information (CSI), for a multipoint-to-multipoint communication through the use of a relay precoder design.
Our previous work included a distributed space-frequency block coding (SFBC) scheme, designed for orthogonal frequency-division multiplexing-based cellular systems, requiring a direct path . We have thereby considered the use of an antenna array at the base station (BS) and a single antenna at both the UT and relay node (RN). We observed better performances with the cooperative systems against the non-cooperative, when one has better link quality in the first hop to the relay than the direct link. However, as the other cooperative systems recently proposed, this scheme has also half of the spectral efficiency of the respective non-cooperative system. Because of the half-duplex constraint at the relays, transmission of a data rate equivalent to that of a modulation technique with bits per symbol in the case the direct link would be available from the BS to the UT would require the use of a constellation with bits per symbol. This would imply a penalty in the power efficiency.
The aim of this paper is to propose a novel data-precoded relay-assisted (RA) algorithm, which can achieve full spatial diversity, obtaining a coding gain in comparison with the equivalent distributed SFBCs scheme, maintaining the same spectral efficiency of the non-cooperative system. Two RNs are used for cooperation, equipped with either 1 or 2 antennas. The simple precoding scheme proposed exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, achieving full diversity, while significantly reducing power penalty. Also there is no need to transmit through the direct link, in alternative to the non-orthogonal algorithms proposed previously. This is beneficial for most scenarios, since the direct link is usually strongly affected by path loss or shadowing. In this algorithm, we perform precoding of the data symbols prior to transmission and posterior decoding at the UT by using Viterbi algorithm . Analysis of the pairwise error probability of the proposed algorithm with a single antenna in each relay is derived and confirmed with numerical results.
The remainder of the paper is organized as follows: in Section 2, a general description of the system model considered is presented; we then describe the proposed algorithm and derive the main link equations as well as compare the proposed scheme with the equivalent distributed SFBC system, for both cases of 1 and 2 antennas at relays, in Section 3; also, pairwise error probability derivation and diversity analysis is made for the proposed algorithm with 1 antenna at each relay, in the same section; then, in Section 4, we present the comparison between theoretical and simulation results, and the performance of the precoded algorithm is assessed and compared with the reference cooperative and non-cooperative systems; finally, main conclusions are pointed out in Section 5.
2. System Model
The fact that we have two independent paths from the relays to the destination allows us to achieve diversity, assuming error-free links from source to relays. We consider the case that the relays are half-duplex; that is, they cannot transmit and receive at the same time. Considering the half-duplex nature, we may have several options.
The relay nodes are allowed to simultaneously transmit over the same channel by emulating a space-frequency code. Concerning this scheme with single antenna relays, RA system, Alamouti coding can be implemented in a distributed fashion by the 2 relay nodes . Assuming now a system with 2 antennas in each relay, the quasi-orthogonal SFBC (QO-SFBC) proposed in  can also be implemented in a distributed manner.
Another case occurs when the relays receive and transmit alternately and the source is transmitting continuously, firstly sending the information to the RN1 and then repeating it to the RN2. In this approach, diversity is achieved without need for any extra processing at the relays and using maximum ratio combining at the UT.
In the above two approaches spatial diversity can be achieved, but because of the half-duplex constraints of relays, the information has to be transmitted in half of the time that would be needed in the case of a continuous link available from the source to the destination. This means that, assuming that a modulation scheme carrying bits per symbol could be used in the case when continuous direct link was available, one would need to switch towards a modulation carrying bits per symbol (if the symbol duration was kept identical), for example, going from QPSK to 16-QAM when 2 RNs are available. As a major consequence, increasing modulation order leads to a decrease of power efficiency. However, as we will show in the next sections, the relation between 16-QAM and QPSK defines an inherent trellis structure that can be used to bring the performance closer to the one that would be obtained with the more power efficient modulation scheme.
We further assume that each relay node is capable of deciding whether or not it has decoded correctly. If an RN decodes correctly, it will forward the BS data in the second phase; otherwise it remains idle. This can be achieved through the use of cyclic redundancy check codes. This performance can also be approximated by setting a signal-to-noise ratio (SNR) threshold at both RNs; the RN will only forward the source data if the received SNR is larger than that threshold .
3. Data-Precoded Algorithm
where is the th QPSK symbol of the original sequence information, with unitary power; is a normalization factor, so that the average transmitted power is 1, in this case being . From (1), we can easily recognize that each symbol is a 16-QAM symbol. However, the receiver will interpret it as a sum of 2 QPSK symbols, allowing, because of the fact that each QPSK symbol is received through two paths, to bring the performance close to the one that would be achieved if the QPSK symbols were transmitted continuously.
In the proposed precoded algorithm, while BS transmits data continually to the RNs, relays transmit and receive alternately: RN1 transmits in even time slots, or symbol duration, while RN2 receives; RN2 transmits in odd time slots, during the reception period of RN1 (Figure 2). At the UT, we use the Viterbi decoding algorithm to separate the QPSK data symbols, since it is the optimal decoding method . In the following the signal expressions and methods are presented in detail, separately for each scheme.
3.1. RA Scheme with 1 Antenna at Relays
where represents the cooperative channel for links RN l UT; is the complex flat-fading Rayleigh channel realization for time slot , with unit average power and represents the long-term channel power; and are the zero mean complex additive white Gaussian noise samples with variance of .
Therefore, the proposed scheme asymptotically achieves the performance of QPSK, in the case of high SNR and when the channels have equal average power gain, that is, an improvement of 4 dB relatively to 16-QAM; taking to 0, the performance will be that of distributed Alamouti, as we have only one path, reducing to a 16-QAM demodulation situation.
3.2. RA Scheme with 2 Antennas at Relays
with in odd time slots and in even time slots, where represents the complex flat-fading cooperative Rayleigh channel of the th antenna of RN l and UT, on time slot and th subcarrier position. Channels are considered flat for adjacent subcarriers; that is, the subcarrier separation is significantly lower than the coherence bandwidth of the channel.
3.3. Error Probability and Diversity Gain Analysis
where is the remainder term of order 2 .
Diversity order, which is an important measure that we ultimately also need to keep track of, has been defined as the absolute values of the slopes of the error probability curve plotted on a log-log scale in high SNR regime . From (24) we can see that the error probability decays as , which means that our scheme achieves diversity order of 2.
4. Numerical Results
4.1. Assumptions and Conditions
Some assumptions were considered for this work, such as perfect CSI at the relays and at the UT, the transmitted power per time slot normalized to 1, and the distance between antenna elements of each BS and RNs large enough to assume uncorrelated antenna propagation channels. The block length used in the simulations, , is of 3600 symbols.
In order to characterize propagation aspects as a whole, including the effects of path loss, shadowing, scattering, and others, we considered different link quality combinations, quantifying it in terms of SNR, given by the ratio between the link's received power and noise power. We define different SNRs for the second-hop cooperative links: RN1 UT and RN2 UT, referred to as SNRc1 and SNRc2, respectively; and for the direct link (the alternative link between the BS and the UT of the non-cooperative systems) as SNRd. For simplicity and also assuming that the relays only forward BS data if decoding is correct, we do not refer to SNR differences in the first cooperative hop.
In all considered systems, two information bits are transmitted per symbol interval, and thus all of them have the same spectral efficiency. We considered that the BS is equipped with 2 or 4 antennas for the non-cooperative systems. In systems where a space-frequency code is needed for 2 transmitting antennas, the well-known Alamouti coding is implemented . In systems with 4 antennas transmitting simultaneously, we implement the QO-SFBC code proposed by Tirkkonen, Boariu, and Hottinen (TBH) in .
The schemes considered in our evaluations are presented next, where the former bullet includes the proposed ones and the two last ones are used as references:
The results of the cooperative and non-cooperative schemes are presented in terms of bit error rate (BER) as a function of of the direct link used for reference, where is the received energy per bit at the UT and is the bilateral noise power spectral density.
4.2. PEP Analysis Validation
4.3. Scheme with 1 Antenna in Each Relay
When comparing the RA-Precoded scheme against RA Alamouti, we observe an improvement of 2.2 dB, for BER = 10−3. This, in turn, derives from the precoding used in the proposed scheme, which mitigates some of the penalty resultant from the half-duplex constraint at the relays, avoiding the use of a higher modulation order.
The proposed cooperative scheme has a penalty of about 1 dB from the best reference, that is, QPSK Alamouti coding with a continuous link available, for the same BER conditions. It is however worthwhile to point out that in our reference we assume independence between the channels. In practice using colocated antennas inevitably leads to some correlation between the channels, in fact reducing such 1 dB of penalty, or even outperforming it in the case of high correlation.
Comparing with the distributed RA Alamouti, we have an improvement of about 2.2 dB in using the proposed code, for BER = 10−3, which is the same difference than in Scenario 1.
4.4. Scheme with 2 Antennas in Each Relay
For the scheme with 2 antennas in RNs ( ), the same study was made, for the same scenarios. In this case, the chosen reference systems are the non-cooperative Alamouti and TBH systems and the RA scheme with the TBH code applied to the relay nodes.
We proposed a novel data-precoded relay-assisted scheme, which ensures spatial diversity for cooperative systems with 2 relays, while ensuring spectral efficiency. Numerical results for the precoded scheme were computed, in scenarios with different link quality conditions, for the cases of each relay node being equipped with either 1 or 2 antennas.
The implemented precoding mitigates some of the penalty resulting from the half-duplex constraint at the relays. It brings the performance very close to the one achieved when a direct continuous link is available and SFBC coding is used at the BS. Actually, for the case of 2 antennas in each relay, the precoded scheme outperforms the non-cooperative one for high SNR regime, due to the non-orthogonality of space-frequency codes for 4 transmitting antennas. Improvements are obtained for scenarios where cooperative links have higher quality than the direct link, being more pronounced as the quality of the cooperative links increases.
We observe that, independently of the propagation scenario, both precoded schemes outperform the equivalent distributed SFBC cooperative schemes, achieving better performance due to the coding gain obtained with precoding. Even for the most probable situation of asymmetric quality conditions between cooperative links, results show that the proposed scheme is better than the reference cooperative ones. In these cases, the difference between the 2 cooperative schemes is higher. For example, it is of about 7.5 dB for BER = 10−3, considering the case of .
We also observe that the extra antenna in each relay leads to a considerable improvement in the overall system performance, which is at least of 3 dB for BER = 10−3. Furthermore, the performance difference between the precoded schemes and the respective equivalent distributed SFBC schemes are higher for the case of having 2 antennas in each relay.
From the presented results, it is clear that the proposed cooperative schemes can be used to extend the coverage mainly in scenarios where the quality of the direct link is poor, as is the case of cluttered urban environments.
The work presented in this paper was supported by the European project CODIV, Portuguese projects CADWIN and AGILE, and by the Portuguese Foundation for Science and Technology (FCT) with a grant for the first author.
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