data stream to be decoded at an acceptable BER. For Ir, the link budget calculation is far simpler than for RF, as terms like antenna gain, free space loss and multipath fading no longer apply. As a result, propagation behaviour can be more easily predicted for Ir than for RF. The unit of infrared power intensity, or radiant intensity, is mW/sr, with sr being the abbreviation for steradian. The steradian is the unit of solid angle measure, and this is the key concept in understanding the link budget for Ir communication. As shown in Figure 5-1, the solid angle (S) subtended by an area A on the surface of a sphere of radius R is given as: S=AIR2 steradians (5.1) A = 2 R2 (1-cos(a)) S = 2 (1-cos(a)) (5.2) Note from Eq. 5.1 that at a distance of 1 metre a solid angle of 1 steradian subtends an area of 1 m². For small solid angles, the area A on Infrared Communication Basics Radius of sphere = R Area = A Solid angle = A/R2 Figure 5-1: Solid Angle Subtended by an Area the sphere can be approximated by the area of the flat circle of radius r, giving: S =r2/R2steradians As an example, the IrDA physical layer standard specifies a half angle (a) of between 15° and 30°. For 15°, , S =2n (1-cos(15°)) = 0.214 steradians. For an LED with a given emitter power density or radiant intensity, I, in mW/sr, the equivalent power density in mW/m² will be approximately given as: P=1/R2mm/m2 (5.3) Emitter Beam Pattern Similar to an RF antenna, an LED has a beam pattern in which radiated power drops off with increasing angle off-axis. In the example shown in Figure 5-2, the power density drops to about 85% of the on-axis value at an off-axis angle of 15°. Inverse Square Loss Equation 5.3 shows that the on-axis power density is inversely proportional to the square of the distance from the source. If R is doubled, the power density P drops by a factor of 4, as shown in Figure 5-3. This is the equivalent of the free space loss term in the RF link budget. Chapter Five Offset angle Figure 5-2: Typical LED Emitted Power Polar Diagram Ir Detec