2 research outputs found
Sparse Beamforming and User-Centric Clustering for Downlink Cloud Radio Access Network
This paper considers a downlink cloud radio access network (C-RAN) in which
all the base-stations (BSs) are connected to a central computing cloud via
digital backhaul links with finite capacities. Each user is associated with a
user-centric cluster of BSs; the central processor shares the user's data with
the BSs in the cluster, which then cooperatively serve the user through joint
beamforming. Under this setup, this paper investigates the user scheduling, BS
clustering and beamforming design problem from a network utility maximization
perspective. Differing from previous works, this paper explicitly considers the
per-BS backhaul capacity constraints. We formulate the network utility
maximization problem for the downlink C-RAN under two different models
depending on whether the BS clustering for each user is dynamic or static over
different user scheduling time slots. In the former case, the user-centric BS
cluster is dynamically optimized for each scheduled user along with the
beamforming vector in each time-frequency slot, while in the latter case the
user-centric BS cluster is fixed for each user and we jointly optimize the user
scheduling and the beamforming vector to account for the backhaul constraints.
In both cases, the nonconvex per-BS backhaul constraints are approximated using
the reweighted l1-norm technique. This approximation allows us to reformulate
the per-BS backhaul constraints into weighted per-BS power constraints and
solve the weighted sum rate maximization problem through a generalized weighted
minimum mean square error approach. This paper shows that the proposed dynamic
clustering algorithm can achieve significant performance gain over existing
naive clustering schemes. This paper also proposes two heuristic static
clustering schemes that can already achieve a substantial portion of the gain.Comment: 14 pages, 9 figures, to appear in IEEE Access, Special Issue on
Recent Advances in Cloud Radio Access Networks, 201
Energy Efficiency of Downlink Transmission Strategies for Cloud Radio Access Networks
This paper studies the energy efficiency of the cloud radio access network
(C-RAN), specifically focusing on two fundamental and different downlink
transmission strategies, namely the data-sharing strategy and the compression
strategy. In the data-sharing strategy, the backhaul links connecting the
central processor (CP) and the base-stations (BSs) are used to carry user
messages -- each user's messages are sent to multiple BSs; the BSs locally form
the beamforming vectors then cooperatively transmit the messages to the user.
In the compression strategy, the user messages are precoded centrally at the
CP, which forwards a compressed version of the analog beamformed signals to the
BSs for cooperative transmission. This paper compares the energy efficiencies
of the two strategies by formulating an optimization problem of minimizing the
total network power consumption subject to user target rate constraints, where
the total network power includes the BS transmission power, BS activation
power, and load-dependent backhaul power. To tackle the discrete and nonconvex
nature of the optimization problems, we utilize the techniques of reweighted
minimization and successive convex approximation to devise provably
convergent algorithms. Our main finding is that both the optimized data-sharing
and compression strategies in C-RAN achieve much higher energy efficiency as
compared to the non-optimized coordinated multi-point transmission, but their
comparative effectiveness in energy saving depends on the user target rate. At
low user target rate, data-sharing consumes less total power than compression,
however, as the user target rate increases, the backhaul power consumption for
data-sharing increases significantly leading to better energy efficiency of
compression at the high user rate regime.Comment: 14 pages, 8 figures, accepted by JSAC Energy-Efficient Techniques for
5G Wireless Communication Systems special issu