In a typical radio communications network, communication devices, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a Base Station (BS), e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
A main striving force in the development of wireless/cellular communication networks and systems is to provide, apart from many other aspects, increased coverage or support of higher data rate, or a combination of both. At the same time, the cost aspect of building and maintaining the system is of great importance and is expected to become even more so in the future. As data rates and/or communication distances are increased, the problem of increased battery consumption is another area of concern. Until recently the main topology of wireless communication systems has been fairly unchanged, including the three existing generations of cellular networks. The topology of existing wireless communication systems is characterized by the cellular architecture with the fixed radio base stations and the communication devices as the only transmitting and receiving entities in the networks typically involved in a communication session.
One way to introduce diversity in a received signal is to exploit the spatial diversity offered when multiple antennas are used at the transmitter with the possibility of using one or more antennas at the receiver. The use of multiple antennas offers significant diversity and multiplexing gains relative to single antenna systems, Multiple-Input Multiple-Output (MIMO) wireless systems can thus improve the link reliability and the spectral efficiency relative to Single-Input Single-Output (SISO) systems. Another method that offers macro-diversity is relaying systems also referred to as distributed systems, such as Distributed Antenna Systems (DAS), or cooperative relaying systems.
A relaying system is a conventional radio network that is complemented with relay nodes. The relay nodes communicate wirelessly with other network elements, e.g. Base Stations (BS), another relay node or a user equipment (UE).
A cooperative relaying system is a relaying system where the information sent to an intended destination is conveyed through various routes and combined at the destination. Each route may comprise one or more hops utilizing the relay nodes. In addition, the destination node may receive the direct signal from the source. Cooperative relaying systems may be divided into numerous categories based on desired parameters. For instance the way the signal is forwarded and encoded at the relay station may be classified into two categories: amplify-and-forward and decode-and-forward. As its name indicates, in amplify-and-forward systems the relay nodes simply amplify and forward the received signal. In the decode-and-forward case, the relay nodes demodulate and decode the signal prior to re-encoding and retransmission. The present-day communication networks, mentioned above, share the same fundamental principle of operation: the information or packet sent from one source, O1, to a destination, D, is transported independently from other information sent from another source, O2, to the same destination, D. Routers, repeaters or relays simply forward the data to the destination D. In contrast to those communication networks, Network Coding (NC) is a new area of networking, in which data is manipulated inside the network, e.g. at an intermediate node, N, to improve throughput, delay, and robustness. In particular, NC allows the nodes to instead recombine several input packets into one or several output packets. At the intermediate node N, may also be referred to a network coding node, some linear coding could be performed on the packets present at the network coding node, and the resulting encoded packet may be broadcasted for different recipients simultaneously instead of transmitting each packet separately.
The area of network coding was first introduced by Ahlswede et al. in [ACLY00]. In Ahlswede, network coding was presented as a method that allows intermediate nodes to perform some processing, e.g. coding, on the packets they receive, in exchange for throughput gain. In [LYC03], the authors showed that linear network coding is sufficient to achieve the maximum low bounds between the source-destination pairs in wired networks. Ho et al. extended this result and showed in [HKM+03] that a random linear combination of packets is enough to achieve the capacity for multicast traffic. Moreover, these works mainly targeted wired networks. Nevertheless, researchers soon came to realize that the broadcast property of the wireless medium makes it a natural environment for the application of network coding.
Up to the 2006, only few works [WCK05, CKL06] have hitherto considered a network coding solution for wireless communication networks. Since then, a plethora of work of applying NC to wireless communications has flourished, in particular in the scenarios of Multiple Access Relay Channel (MARC), bidirectional relaying, also called two-way relaying, and multi-cast transmission. When NC is used, it can be deployed either separately or jointly with channel coding. Although the Separate Network-Channel Coding (SNCC) approach has been the most commonly assumed [AB10], the Joint Network-Channel Coding (JNCC) has been considered and studied. It was shown that the JNCC in [HSO05, HD06, Hau08] exploits more effectively the relay transmission as additional redundancy, and not only to obtain diversity gain, especially in the case of MARC scenario. Finally Du To et al. [TC10] have recently proposed to extend joint network-channel coding to the two-way relaying scenario.
Problems with Existing Solutions
The existent network coding scheme in e.g. the MARC scheme combines the signals of the cooperating communication devices at the relay node as illustrated in FIG. 1 [WK07]. In FIG. 1, two communication devices cooperate through a Relay Node (RN). To get the best of this kind of network coding is to employ joint detection using JNCC [HD06, Hau08] or Low Density Parity Check (LDPC) codes [CHZK09]. This requires more complex decoder structures that necessitates an iterative decoding and relies on exchanging soft information between the decoders at the receiver. Besides the exorbitant complexity of the decoding operation, it is vulnerable to error propagation due to the usage of soft information between the decoders. When the cooperating communication devices are employing convolutional codes, the optimum decoder is a Viterbi decoder based on the combined channel-network coding schemes.
In the MARC scheme, three orthogonal coded words are received at the BS, two directly from the communication devices and one from the relay node. By considering the three received coded words together, one can see the combined convolutional code and network coding as an augmented convolutional code with a total number states v1×v2 where vi is the number of states of the convolutional code used by communication device i [BSO12]. Hence, using these three received coded words, the decoder at the BS can apply a single Viterbi algorithm to decode the information of the two cooperating communication devices. In that, the decoder will consider as if the output of the encoder consisted of three encoded sequences, e.g. c(1), c(2) and c, instead of decoding each encoded sequence separately. This gives a channel-network decoding applied to a constituent or composite encoder. For instance if communication device i employs a convolutional code with rate Ri=ki/ni and a total number of states vi, where ki is the length of the information bits, i.e. input at the encoder, and ni the size of coded block length, i.e. output of the encoder. Then the equivalent code that takes into account the network operation at the relay node and the coding operations at the communication devices, seen at a base station receiver is a convolutional code with rate
  Re  =                    k        1            +              k        2                            n        1            +              n        2            +              max        ⁢                  {                                    n              1                        ,                          n              2                                }                    and a total number of states v1×v2. Hence, the complexity of the joint decoder of conventional MARC with network coding increases exponentially with the number of cooperating communication devices while its free Hamming distance is twice that of the individual user channel code which may not be optimum for a convolutional code of the same constraint length.
For instance, if the channel encoder at the communication device is a convolutional encoder of constraint length K=3 as depicted in FIG. 2a with input bit stream b(i), and output encoded sequences c1(i), c2(i), the equivalent encoder of the JNCC is rate 2/6 16 state convolutional code as illustrated in FIG. 2b where input streams b(1), b(2) that are jointly encoded into encoded sequences c1(1), c2(1), c1(2), c2(2), c1, c2. It is to verify that a free Hamming distance of this encoder is dfree=10.
Another limitation of conventional MARC with network coding is the difficulty in combining communication devices with different data rates and also the cooperating communication devices need to be close to the same relay node, since if one or the two communication devices are far away from the cooperating relay node, the relay node will not be able to hear the communication devices and no cooperation will be possible. In the conventional MARC, communication devices cooperate through the same relay node. This limits the cooperation within the cell. Furthermore, the relay node needs to decode the messages of the different cooperating communication devices before applying network coding and forwarding the obtained message to the final destination. This puts limitations on the possible communication devices that can cooperate. In other words, the cooperating communication devices need to be in proximity of the same relay node for the MARC scheme to operate properly and provide possible gain. This leads to an inflexible solution that only in certain situations shows an improved performance of wireless communication network using relay nodes.