6 research outputs found
A Graph-Based Collision Resolution Scheme for Asynchronous Unsourced Random Access
This paper investigates the multiple-input-multiple-output (MIMO) massive
unsourced random access in an asynchronous orthogonal frequency division
multiplexing (OFDM) system, with both timing and frequency offsets (TFO) and
non-negligible user collisions. The proposed coding framework splits the data
into two parts encoded by sparse regression code (SPARC) and low-density parity
check (LDPC) code. Multistage orthogonal pilots are transmitted in the first
part to reduce collision density. Unlike existing schemes requiring a
quantization codebook with a large size for estimating TFO, we establish a
\textit{graph-based channel reconstruction and collision resolution
(GB-CR)} algorithm to iteratively reconstruct channels, resolve collisions,
and compensate for TFO rotations on the formulated graph jointly among multiple
stages. We further propose to leverage the geometric characteristics of signal
constellations to correct TFO estimations. Exhaustive simulations demonstrate
remarkable performance superiority in channel estimation and data recovery with
substantial complexity reduction compared to state-of-the-art schemes.Comment: 6 pages, 6 figures, submitted to IEEE GLOBECOM 202
Unsourced Random Access Using Multiple Stages of Orthogonal Pilots: MIMO and Single-Antenna Structures
We study the problem of unsourced random access (URA) over Rayleigh
block-fading channels with a receiver equipped with multiple antennas. We
propose a slotted structure with multiple stages of orthogonal pilots, each of
which is randomly picked from a codebook. In the proposed signaling structure,
each user encodes its message using a polar code and appends it to the selected
pilot sequences to construct its transmitted signal. Accordingly, the
transmitted signal is composed of multiple orthogonal pilot parts and a
polar-coded part, which is sent through a randomly selected slot. The
performance of the proposed scheme is further improved by randomly dividing
users into different groups each having a unique interleaver-power pair. We
also apply the idea of multiple stages of orthogonal pilots to the case of a
single receive antenna. In all the set-ups, we use an iterative approach for
decoding the transmitted messages along with a suitable successive interference
cancellation technique. The use of orthogonal pilots and the slotted structure
lead to improved accuracy and reduced computational complexity in the proposed
set-ups, and make the implementation with short blocklengths more viable.
Performance of the proposed set-ups is illustrated via extensive simulation
results which show that the proposed set-ups with multiple antennas perform
better than the existing MIMO URA solutions for both short and large
blocklengths, and that the proposed single-antenna set-ups are superior to the
existing single-antenna URA schemes