Interference Alignment for Line-of-Sight Channels
Leonard H. Grokop, David N. C. Tse, Roy D. Yates · IEEE Transactions on Information Theory · 2011
The fully connectedK-user interference channel is studied in a multipath environment with bandwidthW. We show that when each link consists ofDphysical paths, the total spectral efficiency can grow linearly withK. This result holds not merely in the limit of large transmit powerP, but for any fixedP, and is, therefore, a stronger characterization than degrees of freedom. It is achieved via a form of interference alignment in the time domain. A caveat of this result is thatWmust grow withK, a phenomenon we refer to as bandwidth scaling. Our insight comes from examining channels with single path links (D=1), which we refer to as line-of-sight (LOS) links. For such channels, we build a time-indexed interference graph and associate the communication problem with finding its maximum independent set. This graph has a stationarity property that we exploit to solve the problem efficiently via dynamic programming. Additionally, the interference graph enables us to demonstrate the necessity of bandwidth scaling for any scheme operating over LOS interference channels. Bandwidth scaling is then shown to also be a necessary ingredient for interference alignment in theK-user interference channel.