In this paper, we introduce the idea of using adaptive hybrid modulation techniques to overcome channel fading effects on visible light communication (VLC) systems. A hybrid
M
-ary quadrature-amplitude modulation (
M
Q
A
M
) and multipulse pulse-position modulation (MPPM) technique is considered due to its ability to make gradual changes in spectral efficiency to cope with channel effects. First, the Zemax optics studio simulator is used to simulate dynamic VLC channels. The results of Zemax show that Nakagami and log-normal distributions give the best fitting for simulation results. The performance of
M
Q
A
M
–MPPM is analytically investigated for both Nakagami and log-normal channels, where we obtain closed-form expressions for the average bit-error rate (BER). The optimization problem of evaluating the hybrid modulation technique settings that lead to the highest spectral efficiency under a specific channel status and constraint of outage probability is formulated and solved using an exhaustive search. Our results reveal that the adaptive hybrid scheme improves system spectral efficiency compared to ordinary QAM and MPPM schemes. Our results reveal that the adaptive hybrid scheme improves system spectral efficiency compared to ordinary QAM and MPPM schemes. Specifically, at low average transmitted power,
−
32
d
B
m
, the adaptive hybrid scheme shows 280% improvement in spectral efficiency compared to adaptive versions of ordinary schemes. At higher power,
−
20
d
B
m
, 6.5% and 725% improvement are obtained compared to ordinary QAM and ordinary MPPM, respectively. Also, the adaptive hybrid scheme shows great improvement in average BER and outage probability compared to ordinary schemes. The hybrid scheme shows 28%, 34%, and 38% improvement, respectively, for
m
=
1
,
2
,
3
for Nakagami channels at
B
E
R
=
10
−
3
. Also, the outage probability of hybrid schemes of
B
E
R
=
10
−
3
shows 30% and 14% better performance than ordinary
M
Q
A
M
and MPPM schemes, respectively.
The performance of free-space optical (FSO) communication systems adopting multipulse PPM (MPPM) techniques is investigated taking into account the effects of both the atmospheric turbulence and receiver noise. The atmospheric turbulence is modeled by a gamma-gamma distribution, which is suitable for both weak and strong turbulence. As for the receiver noise, both shotand thermal-noise limited scenarios are considered. For the shotnoise limited system, both exact and approximate expressions of the average symbol-error rate (SER) of the system are obtained. For the thermal-noise limited system, a closed form for the upper bound of the average system SER, based on the Meijer G function, is obtained. Then, we validate it using Monte Carlo simulation results. Furthermore, we study the effects of changing the atmospheric conditions, operational wavelengths, and number of time slots on the average system performance. In addition, we compare the performance of the aforementioned system with that of the traditional PPM technique, in a gamma-gamma channel, under same constraints on the average energy per bit, transmission data rate, and bandwidth.
The communications in the millimeter wave (mmWave) band, e.g., WiGig (60 GHz), is considered as one of the main components of 5G and beyond 5G (B5G) networks. However, it is characterized by short range transmissions along with high susceptibility to path blockage, e.g., human shadowing. Thus, mmWave multi-hop relaying using device to device (D2D) connections turns to be an efficient solution to extend its communication range and to route around blockages. In this context, relay probing is essential to discover/explore the candidate multi-hop routes from source-to-destination and then select the best route among them. However, a trade-off exists between relay probing and the required overhead coming from mmWave beamforming training. In this paper, taking advantage of the multi-band µW /mmWave relay nodes, an efficient multi-hop relay probing scheme is proposed for mmWave D2D routing. In this scheme, the collected µW received signal strengths (RSSs) among the distributed relay nodes are used to estimate the probability of their mmWave signal-to-noise power ratios (SNRs) while considering line-of-sight (LOS) and non-LOS (NLOS) path availabilities. Then, based on a proposed probabilistic metric, a hierarchical search algorithm is proposed to jointly discover the relay nodes along the candidate multi-hop routes and enumerate the number of routes expected to maximize the spectral efficiency of the whole path from sourceto-destination. This is done in an offline phase, and only the relay nodes located within the pre-selected multi-hop routes are requested to do online relay probing. Numerical analysis confirms the superiority of the proposed mmWave multi-hop relay probing scheme over the candidate techniques. INDEX TERMS Millimeter wave D2D networks, multi-hop relaying, unlicensed µW band, multiband management protocol, relay probing.
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