162
Figure 5.3 an ad-hoc network FONG et al 2011
In centrally coordinated network, each access point is no longer required to perform most of the management task as it has been
taken over by the central controller. Hence, the observation of channel efficiency for the whole channel can be centrally con-
ducted. With such configuration, it is very easy to expand the coverage area. All we need to do is just plug in more entry
points to the central control and the user-traffic can be moni- tored among access points. Apart from that, we also can pro-
gram the central controller to re-configure the access point independently if the network condition changes such as pro-
gramming the central controller to disable failed access point and re-route traffic for load balancing. This provides the net-
work with the abilities of self-recovery. This configuration to- pology is shown in Figure 5.3. Instead of using a switch or
router, it uses controller. In conclusion, standalone ad-hoc net- work is more suitable for smaller network with small coverage
area and small number of access points, while centrally coordi- nated network is more suitable for network with dynamic
changes from time to time.
163
5.1.4 Assessment of Link Budget
It is a crucial procedure to identify the extent of coverage through a margin to prevent any unwanted problem. For out-
door wireless network, rain can be one of the factors that reduce the signal propagation. Other than that, regulations and polari-
zation too can significantly impact the signals. The gain and loss in the whole communicative network is described by the
link budget as shown in Figure 2.2. This is very simple and re- quires the calculation of
ܲ
ோ
after it passes the whole system in- clude air, obstacle and so on. Other than that, it also takes
account into the cables that used to connect device such as an- tenna and demodulator. The formula is shown in the following:
ࡼ
ࡾୀࡼ
ࢀశࡳషࡸ
In the formula above, ܲ
்
represents the transferred dBm signal, whereas G and L represent the total dB gain and loss. Equation
5.2 should be re-evaluated for link coverage expansion. Radio waves spread as it travels through air. If we double up the prop-
agation distance, it can cause the receive power to reduce to a quarter as shown in the formula below:
ࡸ ൌ െܔܗ ሺ
࣊ࢊ ڊ
ሻ
A link budget measurement is invented by NIST to estimate the ambient link budget. It is essential for us as both signal of link
budget as the transferring and acquiring characteristics of transmitter are different from each other. In the situation of tel-
emedicine network, a minimum of 10 dB of link margin is needed to overcome the reflection problem that can cause the
reduction of signal strength. Furthermore, another additional of 30 dB is needed in case polarity mismatch happens. Hence, link
margin becomes one of the elements that show the difference
5.2
5.3
164
between telemedicine from wireless network for a general pur- pose. Besides that, we also need to ensure that the wireless net-
work is able to perform well under consistent heavy rain. Extra link margin is estimated mainly based on the transmitter design
and the site environment. In addition, if we maximize the link margin, it will ensure the system reliability to be maintained at
the optimum level.
5.1.5 Antenna Placement
It is tiresome to decide the location to place an antenna. Some- times a location offers the best performance is not feasible to be
used. Impedance matching is also one of the major elements to be considered to effectively transmit signals between antennas,
radios and cables. Also, cable connected between antennas and radios must be the same to prevent losses caused by mismatch.
As many antennas are not identical with cables connected, im- pedance matching circuitry is crucial to transform the antenna
impedance to the cable. To measure the impedance match, Voltage Standing Wave Ratio VSWR is applied. The optimal
value must be below the ratio of 2:1 so that more energy can be sent with minimum reflection. O the other hand, a VSWR with
high ratio shows that the power is either lost or retransmitted and the percentage of bandwidth relates to carrier frequency
ࢌ
ࢉ
:
ࢃ ൌ
ࢌ
ࡴ
ିࢌ
ࡸ
ࢌ
ࢉ
࢞Ψ
ࢌ
ࢉ
ൌ
ࢌ
ࡴ
ାࢌ
ࡸ
ࢌ
ࡴ
And ࢌ
ࡸ
represent the maximum and minimum band fre- quencies. Since most of the antenna does not support 360 de-
gree omni-directional coverage, the angle of coverage needs to
5.4
5.6
