Page 40 - 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. Time/Freq. Decision UE 1
bits Encoding ant. = 1 UE 1 Decoding Combiner bits
lation
DPSK
UE 2 Modu‑ Diff. Time/Freq. OFDM Tx Channel OFDM Rx Diff. Time/Freq. Decision UE 2
bits Encoding ant. = 2 UE 2 Decoding Combiner bits
lation Diversity
and
Precoding
⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮
DPSK OFDM
UE Modu‑ Diff. Tx ant. Channel OFDM Rx Diff. Time/Freq. Decision UE
bits Encoding UE Decoding Combiner bits
lation =
BS

Fig. 6 – Block diagram for DL, where the BS uses a beamforming technique and all UEs are single‑antenna devices.

symbols, but it will also add a common phase error [23]. fectively reduce its SER, while it is not enough for CDS to
2
2
The variance of the PN corresponds to = 10 −5 rad . work properly. When hardware impairments are consid‑

On other hand, the realistic transfer function of the HPA ered, the performance of NCDS is degraded by both HPA
is not a linear function for all possible input values. This and PN effects. In the same way as for the UL, we can see
implies that the output might be saturated for those in‑ that NCDS is very robust to the PN effects due to the dif‑
put values that are higher than the saturation point. This ferential modulation. However, the non‑linear HPA sig‑
non‑linear effect will not only degrade the quality of the ni icantly degrades its performance. In this case the BS
received signal, but it will also enhance the out‑of‑band is simultaneously transmitting the signals of the two UEs
emissions. According to [24], we consider a solid state and, consequently, the constant envelope characteristic
power ampli ier whose output back‑off (OBO) is OBO = 8 of each of the PSK signals is lost when they are combined.
dB. It turns out that now the OBO is not enough and some of
the signal peaks are clipped. This affects equally to both
5.2 Numerical results NCS and CDS. In Fig. 9, a comparison in terms of through‑
put is provided for the DL, whose expression is given by
In Fig. 7, we show the SER comparison between CDS
and NCDS for the UL. The constellations of the two UEs = log ( ) (1 − ) . (13)

2
are QPSK for CDS and EEP for NCDS, both using two bits We can see that even with the overhead due to a very
per symbol. The CDS performs a post‑equalizer at the BS high frequency averaging factor ( = 16), the NCDS
based on a ZF criterion. In the absence of PN and HPA, the still outperforms the traditional CDS. This difference is
NCDS outperforms the traditional CDS by almost two or‑
even higher when either PN or HPA effects are considered.
ders of magnitude of SER for moderate and high SNR sce‑
narios. When hardware impairments are considered, the Therefore, the throughput reduction due to the overhead
PN and HPA effects do not signi icantly degrade the per‑ produced by the frequency diversity is negligible as com‑
formance of NCDS. On the other hand, the performance of pared to the small throughput achieved by the CDS due to
CDS with and without the effect of the HPA is very poor, a poor performance obtained even with a large overhead.
and it is even worse with the PN. The PN does not affect
our proposed system due to the use of a differential mod‑ 6. CONCLUSION
ulation and the fact that the phase noise does not change We have provided a detailed description of the novel com‑
between two contiguous subcarriers [23]. The negative bination of NCDS and multi‑user MIMO‑OFDM based on a
effect of the HPA is negligible in both systems because the differential modulation scheme. Both DL and UL scenar‑
OBO is enough, in view of the robustness of the PSK sig‑ ios are considered and the performance is analyzed for
nals, which are ampli ied separately at the transmitter of realistic channel conditions including the effect of the PN
each UE. and HPA.
In Fig. 8, we plot the SER comparison between CDS and It is shown that for channels with high mobility, the NCDS
NCDS for the DL. The same beamforming is considered outperforms the traditional CDS obtaining a better per‑
for CDS and NCDS to spatially multiplex the two UEs. Ad‑ formance, even more noticeable when PN and non‑linear
ditionally, a frequency averaging of factor = 16 is HPA effects are taken into account. NCDS does not require
performed in both schemes to leverage diversity and im‑ any additional PN estimation and equalization since it is
prove the overall performance, which would be otherwise inherently robust to these effects. Moreover, it shows a
compromised. Again, in the absence of PN and HPA, the similar degradation with the non‑linear effects of the HPA
NCDS outperforms the CDS by several orders of magni‑ to that suffered by CDS, since they share the sensitivity of
tude, showing that the frequency averaging is able to ef‑ OFDM to these effects.

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