Study of Interference Alignment

• Main Author: Shokri Razaghi, Hazhir
• Format: Others
• Language: English
• Published: KTH, Kommunikationsteori 2013
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-120502

The concept of interference alignment has recently become one of the importanttools to analyze the capacity of many multiuser communication networks,e.g. K-user interference channel, wireless X networks, multi hop interferencenetworks, etc. The idea is to consolidate the interference into smallerdimensions of signal space at each receiver and use the remaining dimensionsto transmit the desired signals. Furthermore, most progress in understandingof the wireless networks capacity has been made on the single hop schemes andmulti-hop multi-cast networks. However, there has not been as much progressin multi-hop multi-flow networks where all messages are not required by alldestination nodes. One of the basic problems in this area, is the capacity of2 × 2 × 2 interference channel. It is proved that the upper bound value of 2degrees of freedom (DoF) for this channel can be achieved using the so called“aligned interference neutralization” method.In the proposed interference alignment schemes for network problems whichwe mentioned in the above, including 2 × 2 × 2 interference channel, there aresome theoretical assumptions which seem to be difficult to apply in practice,e.g. high transmit power, asymptotic symbol extension of the channel, globaland perfect channel state information (CSI), etc. Among these assumptionsthe availability of CSI specially at transmitter, is crucial for performing theinterference alignment technique. The CSI at transmitter (CSIT) is usuallyavailable through feedback from receiver and it is used to estimate the currentchannel state, given that the channel coherence-time is long enough. However,it has been shown recently that the delayed CSIT, which is assumed to be independentof current channel state, still can be used to increase DoF of somespecific network settings.In this work, we consider the 2 × 2 × 2 interference channel where twosource nodes communicate with corresponding destination nodes via two relaynodes. We investigated the degrees of freedom of 2×2×2 interference channelwith delayed CSIT and we derived the upper bound on the degrees of freedomof the channel under this condition. Furthermore, we showed that this upperbound can be achieved using interference alignment technique. We also showedthat this completely out-of-date information of the channel can still be usefulto achieve higher rate compared to the situation where no CSIT is availableat the source nodes. Moreover, we observed that using relay nodes in interferencechannel can improve DoF compared to one hop interference channelwhere transmitters and receivers directly communicate with each other.