Page 79 - Kaleidoscope Academic Conference Proceedings 2021

A DESIGN OF NB-IOT RANDOM ACCESS PREAMBLE RECEIVER FOR LARGE
FREQUENCY OFFSET

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Chenchen Zhang ; Nan Zhang ; Wei Cao ; Kaibo Tian ; Zhen Yang 1
State Key Laboratory of Mobile Network and Mobile Multimedia Technology, ZTE Corporation, China 1
ZTE Corporation, China 2

ABSTRACT FO may be still considerable with respect to the Subcarrier
Space (SCS). Considering the pre-compensation, in this paper
In the fifth generation (5G) and beyond, the satellite the FO is assumed to be smaller than 0.5SCS.
communication system is expected to support Narrowband Generally, an IoT device can be assumed to be stationary
Internet of Things (NB-IoT). However, the residual frequency or only have very slow velocity. So the legacy NB-IoT
offset in a channel may be large because of the large velocity PRACH (NPRACH) preamble is designed mainly for low
of the satellites. In a legacy narrowband internet of things FO scenarios. Essentially, the legacy preamble has a small
system, the users are assumed to have low velocity, and SCS and long cyclic prefix (CP) [7]. Therefore, compared to
the random access preamble consists of single-subcarrier TO, the NPRACH preamble detection is more sensitive to FO.
orthogonal frequency-division multiplexing symbols. With a Because of the the single-subcarrier structure of the preamble,
large frequency offset, the power of the preamble leaks into when the residual FO is not small with respect to SCS,
adjacent subcarriers, which will cause severe performance the power of the received preamble will leak into adjacent
degradation. In this paper, the influence of the leaked subcarriers, which may result in severe miss detection and
power is theoretically investigated, based on which a interference to other preambles.
novel random access receiver is proposed to alleviate the There are already several receiver designs for NPRACH
performance degradation. The adjacent subcarriers are preambles in terrestrial network. The preamble detection
actually coherently combined to take advantages of the leaked is mainly performed based on the phase rotation of the
power. The weights in combination are designed to suppress received preamble. In the paper [8], the received preamble
the interference of other users when the frequency offset is with TO and FO is derived. Based on the derivation, a
large. Numerical results show that the proposed receiver has two-dimensional Fast Fourier Transform (FFT) method is
good performance in preamble detection. proposed to take advantages of all the subcarriers of the
NPRACH preamble to jointly estimate the TO and FO.
Keywords - Large frequency offset, NB-IoT, non-terrestrial Instead of jointly estimation, a two-stage receiver is proposed
network, random access, receiver design in [9] for multiple simultaneous UEs. In the first stage,

the phase difference of the Orthogonal Frequency-Division
1. INTRODUCTION Multiplexing (OFDM) symbols in one symbol group is used
NB-IoT is a key wireless technology to support enhanced to estimate the FO. Then the TO is estimated based on the FO.
coverage and long battery life for low-cost Internet of Things Another receiver [10] converts the jointly estimation into an
(IoT) devices [1]. To improve the global coverage and service iterative approximation problem, which has low computation
continuity of the IoT terrestrial networks, the 3rd Generation complexity. However, none of the above discussed the
Partnership Project (3GPP) has approved a study item [2] influence of a large FO.
to support NB-IoT over Non-Terrestrial Networks (NTN). In In this paper, a novel receiver is designed to alleviate
NTN, the Base Station (BS) or the Radio Frequency (RF) the performance degradation caused by a large FO. The
repeater is located in a satellite or High Altitude Platform performance of this receiver is compared with the receiver
System (HAPS) [3]. Because of the large velocity of the in [8]. The main contributions of this paper are summarized
satellites, a User Equipment (UE) may suffer from large as follows:
Doppler Frequency Offset (FO) and Timing Offset (TO), • The influence of FO and TO on NPRACH preamble
especially for Low Earth Orbit (LEO) case [4]. are theoretically analyzed. With FO and TO, the power
In Long Term Evolution (LTE) and 5G systems, the Physical of the single-subcarrier preamble will leak into other
Random Access Channel (PRACH) is used to initialize the subcarriers. In this paper, the received signals in all the
wireless communication [5, 6]. The initialization may fail other subcarriers are derived.
if the FO is too large. In 3GPP release 17, an NB-IoT UE
over NTN is assumed to have Global Navigation Satellite • Based on the derivation, a novel receiver is proposed by
Systems (GNSS) capability [2]. The FO is estimated coherently combining thesignals inadjacent subcarriers.
and pre-compensated based on the UE’s position and the The weights of different subcarriers in the combination
satellite’s ephemeris. Due to the limited cost of IoT devices, are designed according to the phase rotation and the
the estimation may be not accurate enough and the residual leaked power.

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