# Spectral Domain-Based Data-Embedding Mechanisms for Display-to-Camera Communication
## 2 Display to Camera Communication System
### The Wireless Optical Channel  
D2C systems operating in the visible light spectrum (380 to 740 nm) use light-emitting sources to transmit information. These systems operate using a wireless optical channel, where transmission occurs over an unguided medium via visible optical carriers. The wireless optical channel is relatively secure and immune to electromagnetic interference. In addition, the optical channel does not emit harmful radiation, and it operates on inexpensive and readily available transceivers.  
The communication performance of a D2C system significantly depends on the orientation and position of the transceiver components, i.e., the \(AOC\) and \(D\). The \(AOC\) is the angle at which the display plane and camera are mutually aligned. An \(AOC\) of 90\({}^{\circ}\) affords excellent D2C communication, while a large \(D\) between the display and camera limits the image resolution at the receiver. Furthermore, the performance of a D2C system is seriously affected by interference from opaque objects in the path of the visual components.  
In a D2C system, the information signal is modulated by the light intensity. This is termed intensity-modulated direct detection (IM/DD), where the transmitted signal is proportional to the light intensity. Similarly, on the receiver side, the camera captures the pixel intensity of the display. The pixel is the primary unit for detection of received power, which corresponds to the photon arrival rate at the receiver. In OCC, the photon arrival rate is usually considered to be stochastic and is modeled using Poisson's distribution. Therefore, the signal received via an optical wireless channel is affected by several types of noise. The amount of noise captured by the receiver, along with the display content, is affected principally by the room configuration, reflective characteristics of the display, \(AOC\), transmission range, and position and orientation of the receiver. For an indoor D2C system, performance is limited by background illumination and external light sources, such as bulbs and incandescent lamps. Such sources induce a constant shot noise element at the receiver side, thus degrading the overall communication performance.  
Although both the display and camera are digital devices, the transmission medium is analog and completely wireless. This means that digital-to-analog (D/A) and analog-to-digital (A/D) conversion are performed during both transmission and reception, which tends to degrade the transmitted signals. In addition, depending on the capture position of the camera, complex phenomena such as blurring and geometrical distortion may be evident in a  
Figure 4: Data-embedded images obtained using different mechanisms. (**a**) Original input image, (**b**) M1: peak signal-to-noise ratio (PSNR) = 48 dB (\(\alpha\) = 10), (**c**) M2: PSNR = 26 dB (\(\alpha\) = 0.1), and (**d**) M3: PSNR = 25 dB (\(\alpha\) = 0.1).  
captured image. Since a D2C system should consider all these operating conditions of the optical wireless channel, it differs from traditional image watermarking systems.