26 research outputs found
A Random Access Protocol for Pilot Allocation in Crowded Massive MIMO Systems
The Massive MIMO (multiple-input multiple-output) technology has great
potential to manage the rapid growth of wireless data traffic. Massive MIMO
achieves tremendous spectral efficiency by spatial multiplexing of many tens of
user equipments (UEs). These gains are only achieved in practice if many more
UEs can connect efficiently to the network than today. As the number of UEs
increases, while each UE intermittently accesses the network, the random access
functionality becomes essential to share the limited number of pilots among the
UEs. In this paper, we revisit the random access problem in the Massive MIMO
context and develop a reengineered protocol, termed strongest-user collision
resolution (SUCRe). An accessing UE asks for a dedicated pilot by sending an
uncoordinated random access pilot, with a risk that other UEs send the same
pilot. The favorable propagation of Massive MIMO channels is utilized to enable
distributed collision detection at each UE, thereby determining the strength of
the contenders' signals and deciding to repeat the pilot if the UE judges that
its signal at the receiver is the strongest. The SUCRe protocol resolves the
vast majority of all pilot collisions in crowded urban scenarios and continues
to admit UEs efficiently in overloaded networks.Comment: To appear in IEEE Transactions on Wireless Communications, 16 pages,
10 figures. This is reproducible research with simulation code available at
https://github.com/emilbjornson/sucre-protoco
Random Access Protocols for Massive MIMO
5G wireless networks are expected to support new services with stringent
requirements on data rates, latency and reliability. One novel feature is the
ability to serve a dense crowd of devices, calling for radically new ways of
accessing the network. This is the case in machine-type communications, but
also in urban environments and hotspots. In those use cases, the high number of
devices and the relatively short channel coherence interval do not allow
per-device allocation of orthogonal pilot sequences. This article motivates the
need for random access by the devices to pilot sequences used for channel
estimation, and shows that Massive MIMO is a main enabler to achieve fast
access with high data rates, and delay-tolerant access with different data rate
levels. Three pilot access protocols along with data transmission protocols are
described, fulfilling different requirements of 5G services
Achieving Fair Random Access Performance in Massive MIMO Crowded Machine-Type Networks
The use of massive multiple-input multiple-output (MIMO) to serve a crowd of
user equipments (UEs) is challenged by the deficit of pilots. Assuming that the
UEs are intermittently active, this problem can be addressed by a shared access
to the pilots and a suitable random access (RA) protocol. The strongest-user
collision resolution (SUCRe) is a previously proposed RA protocol that often
privileges the UEs closer to the base station (BS). In contrast, we propose a
novel RA protocol using a decentralized pilot power allocation method that aims
at a fairer performance. The proposed access class barring with power control
(ACBPC) protocol allows each UE to estimate, without additional overhead, how
many UEs collided for the chosen pilot and calculate an ACB factor, which is
then used to determine the pilot retransmission probability in the next
protocol step. The results show that the proposed ACBPC protocol is superior to
SUCRe in terms of providing a fair connectivity for very crowded networks,
although still being distributed and uncoordinated as the original SUCRe
protocol.Comment: 12 pages, 4 figure