Optimal Packet Length for Free-Space Optical Communications with Average SNR Feedback Channel

. In this article, a method to enhance data rates of free-space optical (FSO) systems using packet length optimization is proposed. The average signal-to-noise ratio (ASNR) is measured at the receiver and sent back to the transmitter to optimize packet length. In addition, the length of packet is optimized to enhance the average throughput. We concluded that packet length can be reduced at low ASNR. However, packet length should be increased at higher values of received ASNR. For each ASNR, we also choose the optimal modulation and coding scheme (MCS) and optimal packet length to maximize the throughput. Diﬀerent MCSs are investigated such as 4-pulse amplitude modulation (PAM) with and without channel coding, 8-PAM, 16-PAM, and 32-PAM. The proposed method gives 0.8–1.9dB gain with respect to conventional FSO with adaptive modulation and coding (AMC) and ﬁxed packet length. This is the ﬁrst paper to deal with packet length optimization for FSO systems.


Introduction
FSO communications allow high data rates as compared to radio frequency (RF) communications [1][2][3][4][5].It is easy to install and does not require uncovering of walkways to introduce fiber joins.Moreover, FSO spectrum is license free.erefore, FSO communications become viable due to its low implementation cost from one side and to overcome RF spectrum scarcity on the other side.However, the performance of FSO systems can be degraded due to rain, mist, tide, warm, or pointing errors [1][2][3][4][5][6][7][8][9][10].To circumvent this drawback, we can use cooperative or spatial diversity combined with error control coding [11][12][13][14][15][16][17].In cooperative FSO, relays amplify or decode the source message before transmitting it to the destination.Cooperative protocols allow us to benefit from cooperative diversity as the same information is transmitted over independent relayed channels [11][12][13][14][15][16][17].Cooperative communications and channel coding techniques allow us to combat the effects of atmospheric turbulence and improve the performance of FSO systems.
Adaptive modulation and coding (AMC) allows us to increase data rates in free-space optical (FSO) communications.In fact, the best modulation and coding scheme (MCS) is selected for each instantaneous or average SNR.At high SNRs, we can use 32-pulse amplitude modulation (32-PAM) to increase data rates since its spectral efficiency is 5 bit/s/Hz.At low SNR, robust on-off keying (OOK) modulation with channel coding can be used.It is well known that channel coding is required to overcome channel impairments and noise.Different MCSs can be deployed for each range of SNR: i.e., MCS i is used if D i ≤ SNR < D i+1 .In this paper, we will provide a methodology to derive the values of thresholds D i for fixed and optimal packet length.
e thresholds for optimal packet length are denoted by D i , and thresholds for fixed packet length are denoted by S i .
In all previous studies, packet length is fixed and only the MCS is varied where different-PAM can be used with different channel encoders [18][19][20][21][22][23][24][25][26].e main contribution of this paper is to optimize packet length in order to maximize the system throughput.For each average signal-to-noise ratio (ASNR), we choose the optimal packet length and MCS to have the largest throughput.e proposed solution allows 0.8-1.9dB gain with respect to conventional FSO systems with AMC and constant packet length.is is the first paper to deal with packet length optimization for FSO systems.
e system model is presented in Section 2. e packet error probability is analyzed in Section 3 in the absence and presence of channel coding.e optimal packet length is derived in Section 4. Adaptive packet length and MCS using ASNR is described in Section 5. Numerical results are given in Section 6. Section 7 concludes the paper.

Signal Model
In FSO communications, the received signal is written as follows: where x l is the transmitted signal, l is the symbol index, g is the atmospheric fading coefficient, I p is the effect of pointing errors, n l is an AWGN with normalized variance, and Γ is the average SNR.e average SNR can be expressed as follows: where E b is the transmitted energy per bit and N 0 is the power spectral density of the noise.e instantaneous photo SNR is expressed as follows: We denote as the atmospheric turbulence.Using a Gamma-Gamma distribution for atmospheric turbulence and in the presence of pointing errors, the probability density function (PDF) is equal to [27] where G is Meijer's G function and b is the ratio between the equivalent beam radius at the receiver and pointing error displacement standard deviation.For small pointing errors, i.e., b ⟶+ ∞, the above distribution converges to the nonpointing error case, where ζ � 2.064 and ε � 1.342 correspond to strong turbulence, ζ � 2.296 and ε � 1.822 correspond to moderate turbulence, and ζ � 2.902 and ε � 2.51 correspond to weak turbulence [28,29].e cumulative distribution (CDF) of SNR is given by the following equation [27]:

Preliminary Results
e average packet error probability (PEP) can be tightly upper bounded by the following equation [30]: where f Γ (c) is the PDF of SNR Γ and w 0 is a waterfall threshold.Equation (7) shows that the PEP for a given instantaneous SNR c ≤ w 0 is approximated by 1.However, the PEP for a given instantaneous SNR c > w 0 is approximated by 0.
e waterfall threshold can be written as follows [30]: where g(c) is the PEP for a given instantaneous SNR c.

Without Channel Coding.
For uncoded M-PAM, we have where N is the number of data bits per packet, n d is the number of parity bits per packet, and P es is the symbol error probability (SEP) of M-PAM [31].

With Channel Coding.
For convolutionally coded communications, we have where in which d f and a d are, respectively, the free distance and distance spectrum.
2 Journal of Computer Networks and Communications where R c is the rate of convolutional encoding.
Using equation ( 11), we have where We have where i � 1 for coded communications and i � 2 for uncoded communications.

Waterfall reshold.
Appendix shows that waterfall threshold is written as where where E ≃ 0.577 is the Euler constant.

Optimal Packet Length Using ASNR
e number of trials of HARQ is [32][33][34] calculated as follows: e throughput in bit/s/Hz is expressed as follows: where B � 1/T s is the used bandwidth and T s is the symbol period.In equation (23), we used the expression of PEP provided in equation ( 7). e optimal packet length maximizing the throughput can be obtained using the gradient algorithm: Usually, the gradient search is used to minimize a function f, and we write N i+1 � N i − μ((zf(N � N i ))/zN).Here, we aim to maximize the throughput which is equivalent to minimize f � −Thr.
e derivative of throughput with respect to packet length is equal to is expression is valid for Gamma-Gamma atmospheric turbulence with pointing errors.
e used packet length is the closed solution to that obtained by gradient search such that N + n d is a multiple of log 2 (M) so that bits can be converted to (N + n d )/log 2 (M) M-PAM symbols.

Optimal Packet Length with AMC
Figure 1 shows the average throughput of FSO communications with respect to ASNR using optimal N obtained with gradient search equation (24).From Figure 1, we can deduce the following strategy: As shown in Figure 2, a similar approach is used when packet length is fixed, N � 410 and n d � 10 with thresholds S 1 � 16.1 dB, S 2 � 23.3 dB, and S 3 � 30 dB.
Figure 3 shows the proposed adaptive packet length and adaptive MCS using ASNR Γ. e ASNR is measured at the receiver to select the optimal packet length and MCS. e ASNR is compared to thresholds D 1 , D 2 , D 3 , and D 4 to determine the optimal MCS.Packet length is computed using the gradient algorithm as explained in equations ( 24) and (25) .

Numerical Results
Numerical results were obtained in the presence of both atmospheric turbulence and pointing errors.Except in Figure 4, the ratio b between the equivalent beam radius at receiver and pointing error displacement standard deviation is 12. Weak, moderate, and strong turbulence were considered.Figures 5 and 6 show, respectively, the average throughput of FSO communications for 4-PAM, ASNR � 15 dB (respectively, ASNR � 20 dB) with respect to packet length N. In all results, n d � 10.We notice that throughput can be maximized by optimizing packet length.

Journal of Computer Networks and Communications
In fact, we notice that we can choose the value of packet length N to maximize the throughput.As shown in Figure 5, increasing packet length does not mean increasing the throughput all the time and there is a certain length that maximizes the throughput.e optimal packet length increases as the average SNR increases, as shown in Figures 5  and 6. e optimal packet length is 60 (respectively, 120) for E b /N 0 � 15 dB (respectively, 20 dB).By comparing the results of Figures 5 and 6, we concluded that packet length should be increased as the ASNR increases.
Figure 7 compares the average throughput of 4-PAM when packet length is optimal with respect to the fixed packet length (FPL) N of 200, 400, and 1000.We notice that packet length optimization allows 0.8 dB gains with respect to N � 200 and throughput equal to 0.5 bit/s/Hz.Optimal packet length allows 1.3 dB gains with respect to N � 400 and throughput equal to 0.5 bit/s/Hz.Optimal packet length allows 1.9 dB gains with respect to N � 1000 and throughput equal to 0.5 bit/s/Hz.At high ASNR, N � 1000 allows higher throughput than N � 400 and N � 200.At low ASNR, N � 200 allows higher throughput than N � 400 and N � 1000.Figure 2 shows the average throughput of FSO communications with N � 410 for different modulations and coding schemes (MCSs): 4-PAM with R c � 0.5 convolutional coding; 4-PAM, 8-PAM, 16-PAM, and 32-PAM without channel coding.If 4-PAM is used, the packet of length N + n d � 420 is converted to 420/2 � 210 symbols.If 8-PAM is used, the packet is converted to 420/3 � 140 symbols.If 16-PAM is used, the packet is converted to 420/ 4 � 105 symbols.If 32-PAM is used, the packet is converted to 420/5 � 84 symbols.erefore, packet length should be a multiple of 60.
e following AMC approach is suggested:   resholds S i correspond to intersection of different curves.
4-PAM transmits 2 bits per symbol, whereas 5 bits are coded in one 32-PAM symbol.As the number of bits per symbol increases, the spectral efficiency increases, but the PEP also increases since the symbols are closer to each other for a fixed transmit energy per bit.
Figure 8 shows the throughput of FSO systems with adaptive modulation and coding (AMC) for fixed and adaptive packet length.Optimal packet length offers 1 dB gain with respect to N � 410.e curve of optimal packet length with AMC is the upper bound of 5 curves of Figure 1.
e curve of fixed packet length N � 410 with AMC is the upper bound of 5 curves of Figure 2.
e effects of pointing errors are investigated in Figure 4 for b � 3 and b � 12. b is the ratio between the equivalent beam radius at the receiver and pointing error displacement standard deviation.We notice that the proposed optimal packet length allows us to increase the throughput even in the presence of pointing errors.When b is small, the pointing errors increase and the throughput decreases.
We compare in Figure 9 the throughput derived in our paper for N � 400 to that obtained from [27].We used the PDF of SNR [27] to compute the PEP and throughput of 4-PAM as an integral as follows: where g(x) is the PEP for SNR equal to x, as given in equation (12).Another contribution of the paper is to show that PEP can be deduced from CDF of SNR and yields close results to the PEP computed using the above integral.Figure 9 shows that our results are very close to those of [27].We have provided in equation ( 10) a tight upper bound of PEP.erefore, it is a tight lower bound on throughput.

Conclusion
In this paper, we have optimized the throughput for FSO communications.For each average SNR (ASNR), we choose the optimal packet length and MCS to have the largest throughput.Optimal packet length was obtained using the gradient search algorithm and yields 0.8-1.9dB gain with respect to conventional FSO.We have shown that the optimal packet length should be increased with average SNR to obtain higher throughput.We have also identified threshold to select the appropriate MCS for each average SNR and ensure higher data rates.e average SNR is compared to thresholds: when average SNR (ASNR) is less than 14.
per bit E b /N 0 (dB) Throughput bit (s/Hz) Optimal packet length

Figure 1 :
Figure 1: Average throughput of FSO systems for optimal packet length and different modulations and coding schemes with moderate turbulence.

Figure 2 :Figure 3 :Figure 4 :
Figure 2: Average throughput of FSO systems for fixed packet length and different modulations and coding schemes with moderate turbulence.

Figure 5 :Figure 6 :
Figure 5: Average throughput of FSO systems with respect to packet length for an average SNR of 15 dB.

Figure 7 :
Figure 7: Average throughput of FSO systems for optimal and fixed packet length: 4-PAM modulation and moderate turbulence.

+∞ 0 1 − 1 − 1 )
a i e −c i u  N+n d Let y � a i e −c i u , and we deduce per bit E b /N 0 (dB) Throughput bit (s/Hz) AMC with N = 410 AMC with optimal packet length

Figure 8 :
Figure8: roughput of FSO systems with AMC using ASNR: fixed versus optimal packet length and moderate turbulence.