Page 81 - Kaleidoscope Academic Conference Proceedings 2020

Industry-driven digital transformation

OFDM as shown in Figure 1(b), while the low PAPR 4.1 RRC Idle/Inactive
advantage is kept.
With the key enablers in section 3, connection-free
3.3 Enhanced Pilot Design transmission becomes possible which enables high-
efficiency and instant MTC without any connection
To support mMTC and massive MIMO, novel pilot designs establishment. That is to say, the transmission can be realized
are proposed to support more users and reduce allocation in a radio resource control (RRC) idle or inactive state. These
overheads as shown in Figure 1(c). One mainstream research enablers can also revolve the random access protocols with
area is non-orthogonal pilots based on compressed sensing a much higher successful rate and lower latency especially in
(CS) [6]. The sparsity of user activity, multiple paths and dense environment, i.e., they are beneficial for RRC
angles of arrival can be used to increase the performance of connection establishment. A related standardization work is
user detection and channel estimation. However, a CS-based 2-step random access channel (RACH) [19]. However, it
non-orthogonal pilot scheme leads to large computational only acts as a substitution of 4-step RACH, and the collision
complexity when the pilot pool is large or the receiving problem has not been studied as orthogonal multiple access
antenna array is large. Also, the inter-cell interference and and orthogonal pilots are still in use. In regard to the
time/frequency offset problems are hard to solve. collisions, the protocols relating to multiple access and pilot
sequences are expected to evolve.
The other research direction is special non-orthogonal pilot
design with partial orthogonality. To be specific, multi-pilot 4.2 Uplink and Sidelink NOMA
[7] is proposed which consists of multiple pilots. Multiple
orthogonal pilots are usually employed, although multiple As mentioned before, the non-orthogonal domain has not
non-orthogonal pilots can also be used. The detection of been well utilized in 5G. Actually, the discussion of uplink
every orthogonal pilot is very simple, and the detection NOMA didn’t reach a consensus in 5G standards [20]. One
complexity is reduced a lot compared with general non- reason is that the performance and complexity comparison is
orthogonal pilot design. Moreover, an orthogonal pilot has not enough to decide a winner among different uplink
been used and verified during a long period, and many NOMA schemes. The demands of mMTC should be further
existing engineering methods can be used to ensure the analyzed, and some crucial key performance indicators
performance of multi-pilot use. should be especially emphasized. The contention-based
grant-free feature [21] is a potential one in the future
3.4 SIC of diversity discussion as it is exceedingly friendly to low-cost and low-
power MTC devices. Moreover, contention-based NOMA is
The joint use of diversity and SIC is first proposed in [17], also a potential standardization direction for sidelink (SL)
and some novel schemes improve the performance without central control, e.g., LTE-V SL mode 4 [22].
optimizing a bipartite graph of Figure 1(d). It allows the
users to transmit multiple replica packets at any time slot, 4.3 Comprehensive Synchronization
and SIC of packets is used after demodulating any packet in
each round. This strategy was designed for satellite The connection-free transmissi brings some
communications where the round-trip time is very long. The synchronization (SYNC) challenges, and a comprehensive
achievable loading of these strategies is approaching 1 with SYNC process is required. In the current protocols,
the cost of replicas. This method greatly reduces the synchronization is realized by the SYNC signal and
transmission delay and increases transmission efficiency. It measured time advance (TA) from BS [23]. TA is hard to be
is also named as modern random access, which is seen as one obtained in connection-free transmissions, and a
potential next-generation random access protocol [10]. comprehensive SYNC is required. Some possible
modifications can be based on the information of the UE
Unlike satellite communications, the channel gain of status (e.g., position, speed), BSs’ positions, SYNC signals
different users varies a lot in widely-used terrestrial of multiple BSs, etc. Moreover, an overall framework is
communications. Therefore, the near-far effect becomes a required to jointly use all the prior knowledge relating to
practical factor that requires consideration. NOMA is able to SYNC which can be obtained by the end device.
utilize the near-far effect to separate different users. The
combination of NOMA and SIC of diversity requires a joint 5. CASE STUDIES
design of the medium access control (MAC) and PHY layer.
An example is shown in [18] which jointly uses the code Two representative cases of both massive and critical MTC
domain NOMA and SIC of diversity. are shown in this section. The advantage of novel access
technologies are shown, which verifies the necessity of them
4. IMPACTS ON PROTOCOLS in future standards.

With the novel access technologies, the transmission 5.1 Case Study of mMTC
procedure can be simplified a lot, which deeply affects the
future protocol. This case study is for mMTC, and a Poisson arrival model is
used with an average arrival rate of λ. NOMA provides a

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