No Downlink Pilots are Needed in Massive MIMO

We consider the Massive Multiple-Input Multiple-Output (M I O) downlink with maximum-ratio and zero-forcing processing and time-division duplex (TDD ) operation. To decode, the terminals must know their instantaneous effective channel gain. Conventi onally, it is assumed that by virtue of channel hardening, this instantaneous gain is close to its average a nd hence that terminals can rely on knowledge of that average (also known as statistical channel info rmation). However, in some propagation environments, such as keyhole channels, channel hardening does not hold. We propose a blind algorithm to estimate the effective chann el gain at each user, that does not require any downlink pilots. We derive a capacity lower boun d of each user for our proposed scheme, applicable to any propagation channel. Compared to the case of no downlink pilots (relying on channel hardening), and compared to training-based estimation usi g downlink pilots, our blind algorithm performs significantly better. The difference is especiall y pronounced in environments that do not offer channel hardening. H. Q. Ngo and E. G. Larsson are with the Department of Electric al Engineering (ISY), Linköping University, 581 83 Linkö ping, Sweden (Email: nqhien@isy.liu.se; egl@isy.liu.se). This work was supported in part by the Swedish Research Counc il (VR) and ELLIIT. Part of this work was presented at the 2015 IEEE Internationa l Conference on Acoustics, Speech and Signal Processing (ICASSP) [1].