08-08-2012, 03:21 PM
Selection Diversity for Wireless Optical Communications with Non-coherent Detection without CSI
[attachment=30612]
INTRODUCTION
Space diversity has been widely investigated in the
literature and includes several analytical reports assuming
coherent [1] and non-coherent (direct) detections [2]. In
wireless optical links, the channel changes slowly relative to
symbol rate. In this case, pilot tones provide information about
the channel. The chief advantages of optimal non-coherent
detection when perfect CSI is available at the receiver side are
the simplicity of the detector and lower bound to BER.
However, diversity combining schemes based on perfect CSI
availability is sensitive to channel estimation error,
particularly for low SNR signals. Accordingly, Equal Gain
Combining (EGC) can provide a solution when CSI is not
available.
CHANNEL MODEL
The received signal by On-OFF Keying modulation
can be expressed by
= ℎ +
(1)
where ∈ {0,1} is the transmitted signal, ℎ > 0 is the
normalized time-varying and ergodic channel fading due to
atmospheric turbulence, and is total additive Gaussian
noise. Assuming a discrete-time sampling, authors simply
neglect term time in the analysis presented herein.
CONCLUSIONS
When obtaining instantaneous CSI, the estimation error
causes performance loss in both combining and detection
processes. Such error appears even if the channel has a
relatively long coherence time. This paper has considered a
multi-receiving design without perfect instantaneous CSI
knowledge where apertures are either balanced or unbalanced.
An SNR estimation was performed to facilitate branch
selection with the highest short term SNR obtained through
SC combining. Unlike OC-based systems and because the
value of estimated SNR is not directly involved in the
combining process or detection purposes, estimation error
does not remarkably degrade performance. Simulation
demonstrates that regular EGC technique provides a superior
BER performance when branches are balanced. It was
observed that SC provides superior performance in the event
of unbalance, which is more common in practice.
[attachment=30612]
INTRODUCTION
Space diversity has been widely investigated in the
literature and includes several analytical reports assuming
coherent [1] and non-coherent (direct) detections [2]. In
wireless optical links, the channel changes slowly relative to
symbol rate. In this case, pilot tones provide information about
the channel. The chief advantages of optimal non-coherent
detection when perfect CSI is available at the receiver side are
the simplicity of the detector and lower bound to BER.
However, diversity combining schemes based on perfect CSI
availability is sensitive to channel estimation error,
particularly for low SNR signals. Accordingly, Equal Gain
Combining (EGC) can provide a solution when CSI is not
available.
CHANNEL MODEL
The received signal by On-OFF Keying modulation
can be expressed by
= ℎ +
(1)
where ∈ {0,1} is the transmitted signal, ℎ > 0 is the
normalized time-varying and ergodic channel fading due to
atmospheric turbulence, and is total additive Gaussian
noise. Assuming a discrete-time sampling, authors simply
neglect term time in the analysis presented herein.
CONCLUSIONS
When obtaining instantaneous CSI, the estimation error
causes performance loss in both combining and detection
processes. Such error appears even if the channel has a
relatively long coherence time. This paper has considered a
multi-receiving design without perfect instantaneous CSI
knowledge where apertures are either balanced or unbalanced.
An SNR estimation was performed to facilitate branch
selection with the highest short term SNR obtained through
SC combining. Unlike OC-based systems and because the
value of estimated SNR is not directly involved in the
combining process or detection purposes, estimation error
does not remarkably degrade performance. Simulation
demonstrates that regular EGC technique provides a superior
BER performance when branches are balanced. It was
observed that SC provides superior performance in the event
of unbalance, which is more common in practice.