## 2 Unipolar Optical Digital Modulation Schemes and Spreading Codes  
### Unipolar Optical Digital Modulation Schemes  
Because the average optical power of LIDAR is constrained, it is useful to determine a modulation scheme that can provide the requisite bandwidth and use power efficiently. The unipolar optical digital modulation scheme converts a symbol to a digital data stream that is composed of pulses and empty slots. The symbol is represented as a bit sequence called a block and the size of the block is the number of bits. Many digital modulation schemes have been proposed for use in optical wireless communication systems. Given the requirements, the performance of a communication system depends on how the information is represented in the modulation scheme. The types of modulation are thus the critical determinants of the system design. In digital modulation schemes, information is embedded in both mark and space slots, which are generated in terms of a fixed time slot. Each discrete amplitude of a modulated signal appears by varying the characteristics of the pulse at a discrete time. Time characteristics such as pulse position, width, and spacing are modulated using the instantaneous modulation signal, but a constant sampling frequency is sustained. The OOK provides higher bandwidth efficiency, but poor optical power performance. Digital pulse time modulation (DPTM) techniques such as PPM, DPPM, MPPM, DPIM, and DH-PIM are recognized as block codes through the OOK, which provides a balance between bandwidth and optical power efficiency [31]. Digital modulation schemes can be divided into two main categories: Isochronous and anisochronous. In an isochronous mode, the length of the symbol is fixed. In the anisochronous mode, the length of the symbol is variable. The OOK, PPM, and MPPM are isochronous, whereas the DPPM, DPIM,and DH-PIM are anisochronous. An illustration of the overall conversion method and the time waveforms of modulation techniques with fixed pulse width (\(T_{s}\)) and fixed time slot rate (\(R_{s}\)) that are discussed--the OOK, PPM, DPPM, MPPM, DPIM, and DH-PIM--are shown in Figure 2. \(M\) is the size of the input block, \(E_{TX}\) is the pulse peak power, \(T_{f}=T_{s}L_{max}=\frac{L_{max}}{R_{s}}\) is the block duration, \(T_{s}\) is the time slot duration, \(R_{s}\) is the time slot rate, and \(L_{max}\) is the maximum number of time slots. OOK is the predominant pulse modulation format in optical wireless communication systems. It uses the simple method of amplitude-shift keying (ASK) modulation that represents digital data depending on the presence of an optical pulse [31; 40; 41; 42]. In its simplest form, the presence of a pulse for a particular bit duration is represented by '1', and its absence for the same bit duration is represented by '0'. OOK can either be return to zero (RZ) or non-return to zero (NRZ). In NRZ-OOK, the pulses fill the entire bit duration; and in RZ-OOK, they occupy a particular portion of the bit duration. Owing to the relatively wide pulse, NRZ-OOK has higher bandwidth efficiency, but lower power efficiency than RZ-OOK. In the OOK, symbols are displayed as amplitude pulse groups. A combination of an \(M\)-bit input block with symbols for on or off can represent \(L=2^{M}\) unique combinations. Three significant advantages of the OOK are that it provides a high SNR, low distortion performance, and superior system linearity. In PPM, each bit of an \(M\)-bit input block is mapped to one of \(L=2^{M}\) possible symbols [31; 40; 41; 42; 43; 44]. A frame consists of a pulse that occupies a slot, and the remaining slots have no pulse. Therefore, the information is displayed as a pulse position within the same symbol as the decimal value of the \(M\)-bit input block. Because PPM requires both slot and symbol synchronization at the receiver to demodulate the signal, it delivers impressive optical power performance, but at the cost of bandwidth and circuit simplicity.