An Investigation of the Impact of Signal Strength on Wi-Fi Link ...

output power is one Watt (1w) and <strong>the</strong> effective radiated power is four Watts (4w) [18].
Three spread spectrum technologies: FHSS, DSSS and OFDM are adopted in <strong>the</strong> IEEE
802.11 standards. The original IEEE 802.11 standard supports three physical layers:
FHSS, DSSS and infrared (IR) [13]. IR PHY and FHSS PHY support 1 Mbps with an
optional 2 Mbps. DSSS PHY supports both 1 Mbps and 2 Mbps.
The IEEE 802.11a standard uses OFDM technique to support higher data rate. OFDM
technique divides a channel into 52 sub-carriers, including 4 pilot carriers and 48 data
carriers and <strong>the</strong>n distributes signals into 52 sub-carriers [14]. In order to support various
data rates, OFDM uses different modulated techniques (refer table 2.1)[19]. For
example: 64-QAM modulation is used for 54 and 48 Mbps data rate.
Data rate (Mbps) Modulation Technique
6 BPSK
9 BPSK
12 QPSK
18 QPSK
24 16-QAM
36 16-QAM
48 64-QAM
54 64-QAM
Table 2.1: Data rate and modulation techniques for <strong>the</strong> IEEE 802.11a.
The IEEE 802.11b <strong>of</strong>fers higher data rates (5.5 and 11 Mbps) in addition to 1 Mbps and
2 Mbps by using 8-chip CCK. The IEEE 802.11b standard has some optional features to
improve data throughput, for example, packet binary convolutional coding (PBCC) is an
alternative coding method. The PBCC provides higher data rate (22 Mbps) than CCK
modulation. <strong>An</strong>o<strong>the</strong>r optional feature is a short preamble mode, such as HR/DSSS/short
and HR/DSSS/PBCC/short. With enabled short preamble function can improve
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