Design and Realization of a Prototype Hardware Platform for ...

The MegaWatch Wireless PlatformMaster's Thesis - Emanuel Corthay7 The ProtocolsDataSignalClockSignalTimeReceivedData StreamSamplingInstant1 1 0 1 0DetectedFrequency‘1’ Level‘0’ LevelDetectedFrequencyChip outputUnstableSignalStableSignalFigure 22 Binary Data & Recovered Clock SignalBetween two sampling times, the signal may change; going from one to zero, zero to one, orstaying unchanged indicating a repetition of the bit. The further away from the sampling instant weare, the more unstable the signal is.Unlike typical digital circuits, in which there is a data signal and a clock signal transmittedseparately, in this radio link the clock signal is not transmitted over-the-air. It must be reconstituted(i.e. recovered) by the receiver circuitry using the incoming analog signal as a base. This processis called clock recovery[] and is done in the Xemics radio circuit.In order for this synchronization process to work, the same transmission speed must be used bythe receiver and the transmitter, and must be programmed in the radio chip upon initialization.Also, in order to have a valid recovered clock at the receiver when the actual transmission bitsequence arrives, some initialization bit sequence is sent prior to the real data transmission. Thissequence is composed of 0 and 1 sent alternatively and is referred to as the preamble. It does notcontain any information, but is used merely to synchronize the receiver clock with its transmitter’sequivalent.The transmission structure is explained in details in section 7.1.2.Bit synchronizer:For each incoming bit, the chip makes the best possible decision on whether the incoming signalwas a 0 or a 1. It then outputs and maintains the corresponding voltage level to rest of the systemduring an entire clock cycle. This is to ensure that the signal is stable at the sampling time, even ifthe incoming signal varies in time.Pattern Recognition Function:When no signal is present at the receiver, the ambient noise still causes the receiver to outputones and zeros with a recovered clock, but this bit stream is totally random. In order to detect thebeginning of a transmission from the noise, the pattern recognition function is used.The RF circuit comes with a pattern recognition module. This is a very useful feature when two ormore RF circuits are trying to establish communication. When the pattern bit sequence that weprogrammed at initialization time is detected by one of the receiver RF circuits, it sends a signal tothe rest of the system indicating that the incoming bit stream is a transmission (rather than noise)from another node.This special pattern identifying the beginning of a data frame is referred to as the Starting FrameDelimiter or SFD and is the same for all of the units.Transmission speed:40 / 83