5. Exercise: Asynchronous Channel Hopping
In this exercise we will develop a simple frequency hopping system. In a frequency hopping system the transmitter chooses a frequency out of a set of available channels and send data (commonly a packet). After sending the packet, the sender is using another channel. The receiver knows the frequency the sender will select and jumps to that channel. Now, the receiver is able to receive the packet from the sender. After reception, the receiver jumps to the next channel, that will be used by the sender.
Regarding the description of that basic behavior, two major problems have to be solved:
- Synchronisation: The receiver needs to know, at which time the sender has selected a new frequency. If the reiceiver changes the frequency too late, the packet will be lost.
- Hopping Sequence: The receiver has to know the frequency the sender will select transmitting the next packet.
In practice, it is not easy to build such systems. Especially the synchronization problems are hard to takle and introduces a large amount of overhead. To alleviate the synchronization problem, asynchronous frequency hopping systems are introduced. A simple system has been proposed by Texas Instruments (look at).
In that system (see Fig. 1), the sender selects an arbitrary frequency from a set of available channels and transmits a packet as usual. But the packet has a huge and very large preamble (green part in Fig. 1). The receiver will sense a channel for some time (blue boxes in Fig. 1). If he does not recognize a signal, he will change the channel and sense this new channel again and if there is also no signal, he will change again and so on. But, if there is signal on a channel, he will wait for a sync word (orange boxes in Fig. 1) and after a sync word has been found, he will start receiving the packet.
The idea behind this system is, that the receiver will sense all channels during the time the sender is transmitting the preamble of the packet. In that case, the receiver is able to always find the frequency the sender is using. In this system, receiver need not to always synchronized with the sender and always finds the channel on which the sender will send its packet.
Fig. 1: TI Asynchronous Channel Hopping System
This system has also some drawbacks. E.g. if the receiver detects a signal on a channel, but this signal was not generated by sender (e.g. from an microwave owen). In that case, the receiver is blocked. He will wait receiving a packet, that will never come. But for our purposes this system is best fit for a small project. To implement such system, we will divide it in multiple parts:
Each part consists of some exercises. For some exercises you can use previous already done exercises.
