Modulation Network Scalability for Network Development

169 This causes the reduction of modulation order in the opposite way. The spectral efficiency, which is estimated in the unit of bits per second per hertz transferred. Co-channel interference mainly occurred through the scatter of re-use sectors in cells. As macro cell architecture adopts frequency re-use structurally sep- arated segments and the sharing of frequency among neighbor segments can cause co-channel interference, Shannons Theo- rem can be use to perform frequency re-use as shown in the formula below: L represents the frequencies of network usage, CI represents the channel interference ratio and represents the structurally re-use variable while m is the overhead assigned to guard bands which separates the frequency into different channel that shown in Figure 5.5. From Equation 5.7, the increasing of L or de- creasing of K in macro cell architecture can perform frequency re-use at optimum level. Besides that, CI in single cell and macro cell architecture can be approximated as Equation 5.8 and Equation 5.9 respectively Sheikh, 1999, love et al 2003, celebi et al 2007. In the equation, c represents the arbitrary constant that especial- ly for a network deployment. 5.8 5.9 170 Figure 5.5 Sub-channels separated by a guard band FONG et al 2011 Figure 5.6 Time division multiplexing FONG et al 2011 5.2.3 Multiple Accesses Multiple accesses or multiplexing is a process of breaking a high speed circuit into smaller logical circuit for sharing pur- pose. In order to perform multiplexing, two multiplexors are required; one for combination and the other for separation. There are two techniques that can be used to perform multiplex- ing namely TDM Time Division Multiplexing and FDM Fre- quency Division Multiplexing. Multiple accesses are common things in the early 90’s during the boom of digital cellular phones. The working principle of 171 multiple accesses can be easily understood by referring to Fig- ure 5.6 and 5.7. A network is divided into individual time slots in Figure 5.6. In this scenario, the time slot happens to be equal but this situation does not occur most of the time. Every trans- mitting device has its own allocated duration and the next user will be allocated another duration. For instance, A, B and C are the devices allocated with 10ms time slot each. During the transmission process, device A will be given the first access and spend its first 10ms. After that, B is given the same time slot and followed by C. This means that at the time slot between 0ms to 10ms, A will have the transmission followed by B in between 10ms to 20ms and C in between 20ms to 30ms. As C completed its time slot of 10ms, it goes back to A and the entire cycle continues. In every 10ms the device is changing its se- quence A to B to C and again with A. The theory of the multi- ple accesses is well explained in the example of three devices above. However in real time, when switching between the de- vices, there is a lag time between them. There were no instanta- neous transmission goes on when each device completes its time slot. Hence the lag time should be considered when switch- ing the transmission cycle. Figure 5.8 describes the transmis- sion process discussed earlier where a logical switch enable the devices to gain entry to the channel. Figure 5.7 Multiplexing of frequency division FONG et al 2011