Q: The FCC allows a transmit power of just under 1mW for operation in the 902 MHZ band. I feel this is insufficient. Should I:
use spread-spectrum to achieve processing gain?
use the 27 MHZ band where I can have an output of four watts?
Spread-spectrum "processing gain" is not an actual gain when the entire transmitter-receiver system is considered. It is the recovery of the loss, in the receiver, due to spreading the signal at the transmitter. See Advantages of spread-spectrum, below.
27 MHZ channels (US)
A: There are several 27 MHZ channels set aside for model control. A few other control services are also allowed. These channels are plagued by two problems:
Unpoliced citizens band hobbyist users, and
Worldwide propagation during the solar maxima, plus or minus a few years. This mode of propagation is supported by surprisingly small amounts of power. One or two watts is sufficient for effective communication from the California to the East Coast.
Q: Spread spectrum Processing gain looks like something for nothing. Is it?
A: No, something for nothing violates the second law of thermodynamics.
Let's look at processing gain. When we transmit a signal, we require a certain amount of bandwidth to pass our data stream. This is sometimes known a the information bandwidth. The amount of bandwidth is related to the bit speed and method of modulation. Normally it is only several kHz.
A spread spectrum transmission takes the data stream and combines it with a very wide band signal (direct sequence) or combines it with a signal that is sequencing over many frequencies spaced over a wide bandwidth (frequency hopping).
The term for this effect is spreading loss ...
The receiver must have a wide bandwidth to pass the signal to the detector. After the detector, the wideband signal is processed into an equivalent of the original narrow band data stream. Processing gain refers to the gain of this processor. This gain should be equal to the loss encountered during the spreading process.
Q: What is the advantage for Spread Spectrum?
A: 1. Spread-spectrum is useful because it provides some robustness against multipath and discrete signal interference, such as exists from microwave ovens in the 2450 MHZ band.
Both frequency hopping and direct-sequence systems operate over many frequencies. Multipath signal cancellation is highly dependant on propagation path geometry. A small frequency change alters this geometry, reducing or eliminating the cancellation. These spread-spectrum systems offer a frequency change that is faster than the bit rate. The result is there is some reduction in the severity of multipath cancellation.
The de-spreading process in a direct sequence receiver actually spreads discrete frequency signals. This has the affect of lowering the intensity of discrete frequency interference. Most of the potential interference in the ISM bands is from low power transmitters. The exception to this is leakage from microwave ovens. A direct sequence spread-spectrum processor will reduce this interference by the same amount as its processing gain.
The US rules also allow a much higher power level when frequency hopping or direct sequence spread-spectrum techniques are employed in the microwave ISM bands. The maximum difference amounts to about 36 dB. There are some applications that require this power increase.
Q: What are the disadvantages of Spread Spectrum?
A: On the downside, spread-spectrum increases system cost, reduces battery life, and, without using a higher power level, offers a reduced range. Other disadvantages are:
1. There is no processing gain during signal acquisition.
2. It is not a useful option outside the U.S. and Canada.
3. Signal Acquisition requires time.
4. Acquisition time, complex circuitry, and transmit power reduces battery life.
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