Page 35 - ITU Journal, Future and evolving technologies - Volume 1 (2020), Issue 1, Inaugural issue

ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1

nel and noise. Moreover, they proposed the idea of multi‑
plexing the data of each user equipment (UE) in the con‑
stellation domain based on a joint‑symbol, which is a su‑
perposition of the symbolssent by severalUEs. In thecase
of DL, the combination of NCDS with MIMO has been un‑
til now based on block codes [27] ‑ [31]. However, their
application requires that the channel response remains
quasi‑static during the transmission of a block code, and
they also need a high SNR in order to provide an accept‑
able performance. Moreover, they have the problem that
they are not scalable and when the number of antennas
at the BS is very large, the design of these block codes be‑
comes unaffordable. Typically, only two and four trans‑
mit antennas are taken into account [27] ‑ [31]. More Fig. 1 – Time domain scheme in the OFDM resource grid when = 12
recently, the combination of beamforming and NCDS has and = 14. The yellow box represents a reference symbol required by
the differential modulation.
been proposed in order to exploit the high number of an‑
tennas through compensating the path loss and enhanc‑
ing the quality of the link, and spatially multiplexing the
different UEs [18], [19]. In these cases a certain channel
knowledge is needed to point the beam towards the UE
through the beam‑management procedure, and the sig‑
nal is processed non‑coherently in each beam afterwards.
Even though the overhead is not completely eliminated,
the savings are considerable.

3. NCDS WITH MASSIVE MIMO FOR THE UP‑
LINK

We describe in this section how to integrate the NCDS
based on [12] ‑ [17] in a realistic communication sys‑ Fig. 2 – Frequency domain scheme in the OFDM resource grid when
tem for the particular scenario of UL. We consider one BS = 12, = 14 and ℐ = {1, 8}. The yellow and blue boxes de‑
equipped with antennas, which is simultaneously serv‑ note the reference symbols required by the differential modulation and
phase difference estimation, respectively.
ing UEs. These UEs are constrained to have a reduced
number of antennas, typically single‑antenna devices. Let tion is performed, in this case between resources that be‑
us assume that the UEs are simultaneously transmitting long to the same frequency and contiguous symbols in the
OFDM symbols. The OFDM signal has subcarriers, time domain. The differential encoding can be described
and the length of the cyclic pre ix (CP) is long enough to as
absorb the effects of the multi‑path channel. At the re‑ = { , = 1 ,
,
ceiver side, after removing the CP and performing a fast‑ , , 2 ≤ ≤ (1)
, −1 , −1
Fourier transform (FFT) to each block at each antenna of
the BS, we can process each subcarrier as one of a set of 1 ≤ ≤ , 1 ≤ ≤ ,
independent subchannels.
where is the reference symbol transmitted at the ‑
,1
3.1 Integration of differential encoding in th subcarrier of the irst OFDM symbol by the ‑th UE,
OFDM for high mobility scenarios and , are the complex data and differential sym‑
,
bol, respectively, transmitted at the ‑th subcarrier and
Similar to CDS, the NCDS can be also implemented in an ‑th OFDM symbol by the ‑th UE. The data symbol ,
OFDM system [17], suitable for dealing with a doubly‑ belongs to a PSK constellation due to the fact that the
dispersive channel. The stream of differential symbols differential encoding can only transmit information in
produced by the differential encoding can be mapped in the phase component. However, this time‑domain im‑
the two‑dimensional resource grid provided by the OFDM plementation has the drawback of an increased latency
(time and frequency). According to [17], the way this and memory consumption, since this mapping scheme re‑
mapping is performed will signi icantly impact on the quires waiting for the reception of two complete OFDM
overall system, especially for high mobility scenarios. symbols in order to obtain , , due to the fact that it per‑
Typically, NCDS based on differential modulation is per‑ forms a differential decoding of two contiguous symbols
formed using the time domain scheme. This scheme is in the time domain. Also, it cannot be exploited when
represented in Fig. 1, where the red arrows indicate the the Doppler shift is very high, since any two consecutive
direction in which differential modulation and demodula‑ OFDM symbols will not face a similar channel response.

© International Telecommunication Union, 2020 15