in the path except the detailed content of the signal modulating the carrier, and that is largely (but not entirely) of very little concern to the antenna. In contrast, the receiver signal path operates in a much more difficult, random-like scenario. It must somehow locate and capture a tiny amount of RF signal power, and act as an electromagnetic (EM) field transducer to convert that power into a usable voltage. It must do this despite in-band noise and interference of various types and sources, as well some transmitter drift, and even Doppler- induced frequency shifts in some applications. This received power is quite low, on the order of milliwatts (mW) in a few cases and microwatts (µW) in most, so the corresponding voltage created at the antenna is usually on the order of microvolts. The voltage is too small to be used directly for demodulation in most cases, so the answer is obvious: just amplify it. To get some perspective, the received signal power for GPS signals is typically between -127 and -25 decibels (dB) relative to one milliwatt (dBm), and viable Wi-Fi signals range between -50 dBm and -75 dBm. Low SNR is the complementary problem The amplification solution answer tells only part of the receiver story. It’s not hard to amplify
Figure 1: The standard plot of BER versus SNR reveals a great deal about system performance; note that more advanced modulation techniques such as 256-QAM can increase the effective data rate, but at a penalty in BER at a given SNR. Image source: Julia Computing, Inc.
even a microvolt signal by several orders of magnitude. However, the original signal also has noise, and what really affects the ability to demodulate and decode the received signal is its signal-to-noise ratio (SNR). Any amplification of the received signal will also amplify the embedded noise. Using a larger antenna with higher passive gain will increase the received signal power, but the received SNR will be unchanged. One of the key metrics of system performance is its bit error rate (BER) versus SNR (Figure 1). The specifics of those curves depend
on many factors including received signal strength, SNR, and what type of error-correction code (ECC) encoding of the raw data is used at the transmitter; for this reason, the more-detailed graphs show the BER vs SNR for the raw, uncorrected bits stream as well as the corrected bit pattern (QAM = quadrature amplitude modulation). What are some typical SNR values which result in successful demodulation with acceptably low BER? There is no universal answer, of course, but an acceptable Wi-Fi signal SNR is 20 to 40 dB, 40 to 50 dB for an old-fashioned all-analog
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