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Figure 5.4 Constellation diagram FONG et al 2011
Based on the constellation diagram above, the cross section rep- resents the amplitudes and phases of signal. More dots in dia-
gram means that there is a higher modulation of above 64 and carries more data bits. Nearer dots mean that it is harder to iden-
tify individual dot. As resolving packed dots requires more complex recipients, there is a need to trade-off between the ef-
fectiveness if spectral usage and the framework of receiver. For example, with something as high as 1024; the amplitudes and
phases difference between neighboring signals points may make them cannot be differentiated.
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Compared to high order modulation, low order modulation such as QPSK is more receivable with the same transmittance ener-
gy. High order modulation always experience signal loss issue. Therefore, the 16-QAM is suitable to serve receivers that are
close to the transmitter and at the same time QPSK can be use to serve receivers that are further from the transmitter.
5.2.2 Cellular Configuration
A wireless channels can be configured as single and macro cell. They have the technique of reusing frequency but in different
methods. reusing frequencies is a technique to enhance spectral performance and expand channel performance by re-using the
channel and frequency of the same network by separating and RF coverage area into segments. In the diagram, there is an ex-
ample that shows the re-use different frequency. Although they belonged to identical bands, sufficient separation is required
from adjacent segment to lower the risk of interference. All the- se frequencies are re-used at least two segments away from each
others.
A wider coverage is normally achieved using high-gain anten- nas with clear line-of-sight to receive signal in single cell archi-
tecture. Similarly, different polarizations can be achieved in frequency re-use. Macro cell architecture usually uses structural
frequency re-use that able to provide acceptable signal reception without clear line-of-sight. Therefore, it is clear that we can im-
prove spectral efficiency by combining a high-order modula- tions and frequency re-use. However, it may results in the
amplification of co-channel interference, where two neighbor- ing channel shares the same frequency that interfere with each
other which mainly caused by network congestion and bad fre- quency planning.
