Frequency hopping is one of the most popular techniques. This is a proactive technique built into the functionality of the radio where the transmitter is changing its output frequency very quickly over time as it transmits the data. The receiver has the same frequency hopping algorithm, so it is able to tune to the same frequencies and receive the data correctly. This technique is jamming resistant due to the fact that not all channels can be jammed effectively. If one channel is jammed, there is still information transmitted on the other channels. Error detection and correction is used to fix the information lost in the jammed channel.
Direct Sequence Spread Spectrum techniques are also used to combat jamming. In this technique, a pseudo-random code is used to augment the data. This code spreads the signal out over a wider frequency range while lowering its power. The fact that this signal can be hidden within the noise floor makes it harder to detect. If the signal cannot be detected then it is less likely that someone will attempt to jam it. Even if it is jammed, the fact that the signal is spread over a wider range makes it less susceptible to jamming.
Additionally, since the jammer is essentially degrading the signal to noise ratio, one way to combat jamming is to increase the power level of the transmitter. This has an obvious tradeoff where more power is consumed as well as the fact that it is easier to track the location of the transmitter, but it will provide a more reliable link in the presence of a jammer.
Another way to combat jamming is to establish more focused antenna patterns between the receiver and transmitter. Since the jammer is typically coming from a direction other than the transmitter, it could be possible to steer the antenna or modify the beam pattern (such as with a phased array system). If the direction of the jammer is known, a null of the antenna can be placed at the jammers location while at the same time focusing the beam toward the desired transmitter.
In cases where a mesh network is used, if one node on the network is being jammed, preventing data transmission through that node, the network can recognize this and re-route the data through a different path in the network.
