b receiver with a 2 MHz de-spread bandwidth, operating at 20°C (290°K) and with a noise figure of 10 dB: N = 1.38 X 10-23 J/K X 290°K X 2 106 Hz = 8.8 X 10-12 mW = - -110.6 dBm RNF = -110.6 dBm + 10 dB =-100.6dBm Receiver Sensitivity The receiver sensitivity, PRX, is the sum of the receiver noise floor (RNF) and the signal-to-noise ratio (SNR) required to achieve the desired bit error rate: PRx=RNF + SNR (4.6) For the example; PRX = 100.6 + 10.4 dBm = -90.2 dBm From this discussion it can be seen that as the data rate in the example increases from 2 Mbps towards the 802.11b maximum of 11 Mbps, different modulation methods will be needed to achieve the higher bandwidth efficiency (more bits per Hz of bandwidth, or fb/W in Eq. 4.1). 1 Again, the de-spread bandwidth is used here. Chapter Four Table 4-17: PRX Versus Data Rate for a Typically 802.11b Receiver Data Rate (Mbps) Modulation technique PRX (dBm) 256 CCK + DQPSK 16 CCK + DQPSK Barker + DQBSK Barker + DBPSK This will result in a higher signal-to-noise ratio requirements for the same bit error rate, SO that the receiver sensitivity will decrease at higher data rates. This is shown in Table 4-17 for a typical 802.11b receiver. This dependence of PRX on data rate underlies the gradual deterioration in wireless network throughput as signal strength decreases. There is no abrupt cut-off in performance, but rather a gradual reduction in throughput as the transmitter and receiver switch to a lower data rate at which a low BER can be maintained. RF Signal Propagation and Losses Between the transmitting and receiving antennas, the RF signal is subject to a number of factors that affect signal strength. These are considered in the following sections. Free Space Loss Once the signal is radiating outwards from the antenna, the signal power falls off with distance due to the spreading out of the radio waves. This is known as free space loss, and overall is the most significant factor affecting received signal strength. Free space loss is measured in dB, and depends on the