Page 38 - 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

DPSK
UE 1 Modu‑ Diff. OFDM Tx Channel OFDM Rx Diff.
bits Encoding UE 1 ant. = 1 Decoding
lation UE 1 bits
Joint‑
DPSK Constellation UE 2 bits
UE 2 Modu‑ Diff. OFDM Tx Channel OFDM Rx Diff.
bits Encoding UE 2 ant. = 2 Decoding Decision UE bits
lation
Spatial
⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮
Combiner
DPSK OFDM
UE Modu‑ Diff. OFDM Tx Channel Rx ant. Diff.
bits Encoding UE Decoding
lation = BS
Fig. 5 – Block diagram for UL, where different UEs are multiplexed in the constellation domain.

The block diagram of a UL system addressing a more gen‑ are obtained through a beam management procedure to
eral multi‑user case is shown in Fig. 5, from [17]. In this point the beams towards each UE. Once this is achieved,
system, several UEs map their data bits into complex sym‑ the data information is sent over a non‑coherently pro‑
bols that belong to a speci ic DPSK constellation. Then, cessed link. Reference [18] proposed a similar idea as‑
the differentially encoded symbols are transmitted into suming, however, an ideal case where the MUI is com‑
an OFDM signal through a multi‑path channel. The re‑ pletely mitigated by the beamforming. Meanwhile, the
ceived signal at each antenna of the BS after the OFDM combination of NCDS with a practical beamforming tech‑
receiver corresponds to a superposition of different sig‑ nique is proposed in [19], taking into account the residual
nals coming from each UE. Later, these differential sym‑ MUI.
bols are non‑coherently combined by using a differential In [19] the beam‑management procedure de ined in 5G
decoder and averaged over the spatial domain in order to [1] is suggested to be performed as a irst step. This pro‑
obtain the desired joint‑symbol. Exploiting the spatial di‑ cedure is responsible for accurately determining the an‑
versity in order to reduce the noise and multi‑user inter‑ gle of the spatial clusters of the propagation channel con‑
ference (MUI) is crucial to obtaining a right decision from tributing to the signal of each UE, by transmitting some
the joint‑symbol [12]. Additionally, the constellation de‑ reference signals. These reference signals are speci i‑
sign can be combined with channel coding, where the soft cally the synchronization signals (SS) and channel state
information can either improve the performance or re‑ information‑reference signals (CSI‑RS). The former are
duce the number of required antennas at the BS [16] [32]. used when a UE would like to enter the system for the irst
time, while the latter are exploited for updating the angu‑
4. NCDS WITH MASSIVE MIMO FOR THE lar position of an existing UE in the system.
DOWNLINK Then, the BS transmits one or several differential data
streams to each UE by using beamforming [34] as
Now turning to the DL, the massive number of antennas , ,

at the BS are used for transmission and we can only count , = , , , 1 ≤ ≤ , (10)
on a few antennas at the UE receivers, just one per UE in 1 ≤ ≤ , 1 ≤ ≤ , 1 ≤ ≤ ,
many cases. The mapping schemes and multi‑user con‑
stellations proposed for the UL are still valid, while we where , is the precoding coef icient for the ‑th UE and
,
need a few more ingredients to make these schemes suit‑ ‑th antenna of the BS, placed at the ‑th subcarrier and
able for the DL. We explain in the following these key in‑ ‑th OFDM symbol. This precoding coef icient is obtained
gredients for a combination of NCDS with beamforming according to the estimated angular positions of each UE,
based on [19], where the BS is simultaneously transmit‑ and thus, it is in charge of focusing the energy in the ob‑
ting the data streams through its antennas, while the tained speci ic directions. In this way, the path loss is
UEs are receiving with their single‑antenna device. compensated and the MUI that results from spatially mul‑
tiplexing the UEs in different beams is avoided. Similarly,
4.1 Beamforming beamforming can be used in the UL for the BS to receive
the signal coming from these spatial directions.
The exploitation of the diversity from the transmitter Depending on the angular position of different UEs, it
without the knowledge of the CSI is still a challenge. Due may be dif icult to completely remove the MUI by exploit‑
tothefactthattechniquesbasedonblockcodes[27]failed ing the beamforming. Therefore, the overall performance
to exploit the large number of antennas at the transmitter, critically depends on the scheduler which is capable of
we propose the use of beamforming in order to take ad‑ properly selecting those UEs to be simultaneously served
vantage of the massive number of antennas of the BS at in the same time and frequency resources and minimizing
the expense of using some (reduced) channel knowledge. the negative impact of the mentioned MUI. Even though
Then, it is assumed that the angular positions of each UE we are making use of some reference signals to perform

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