JED - September 2011 - (Page 56)
Spectrum Warfare – Part 5
By Dave Adamy
The Journal of Electronic Defense | September 2011
receiver as a modulated RF signal with different states for “1” or “0” bits. The receiver demodulates this signal to recover the bits, and then must set a timing circuit (called a “bit synchronizer” that outputs a code clock signal aligned with the code clock in the transmitter, but delayed by the propagation time of the signal from the transmitter to the receiver (at the speed of light). THE TRANSMITTED BIT STREAM The bit synchronizer produces a clean digital bit stream with 1s As shown in Figure 1, a transmitted digital signal must and 0s determined from the demodulated received RF signal, but contain more than just the digitized data. A data frame is there is a series of bits that can be processed in digital circuitry. shown in the drawing. As shown in Figure 2, the bit synchronizer, in addition to generating the code Transmitted Digital Signal clock, also determines when the RF signal is sampled to decide whether a received bit is a 1 or a 0. When information is transmitted digSynchronization Address Information Bits Parity or EDC Bits Bits Bits itally, the transmitter sends a typically continuous series of bits (1s and 0s) that Less than 10% to is meaningless unless the receiver can more than 100% ~ 10% of Total of Info Bits determine the function of each bit. The information is organized into frames dd l l h dd f of many bits, and the receiver must be Figure 1: A transmitted digital signal contains synchronization, address, information and parity or EDC bits. able to determine the beginning of each frame. The position of each bit in the frame then identifies • There is typically a block of bits which provide frame its function. This process is called synchronization. In some synchronization. data transfer systems there is a separate modulation value for • In many systems, for example a command link to a UAV, the a synchronization pulse at the start of the data frame. Howinformation bits may need to be sent to one of several destiever, typically, there is a unique series of bits in the digital nations at the receiver location. In a UAV, this could be: the bit stream that the receiver can compare against a stored bit UAV navigation system, one of several payloads, etc. Thus, sequence to identify the beginning of the frame. there would need to be a block of address bits. Figure 3 shows the “thumb tack” correlation of a series of • The information bits carry the actual transmitted bits. A digital signal will have approximately the same numinformation. ber of 1s and 0s, and they will be close to randomly distrib• Because the transmitted data may be corrupted by noise, interference or jamming in the environment, special bits are added to allow the receiver to either detect and reject bad data blocks or to actually correct erroneous bits in the reDETECTED SIGNAL RECOVERED BITS ceived signal. The parity or error detection and correction WITH NOISE WITH ERRORS (EDC) block of bits support this function.
ast month, we ended by discussing bandwidth and data rate considerations in digital communication. Now we will consider some of the more detailed aspects of this important subject.
TRANSMITTED BIT RATE VS. INFORMATION BIT RATE
The transmitted bit rate must be fast enough to send the whole signal frame at the rate that the information in that frame is required at the receiver location. This means that the transmitted data rate could be significantly higher than the required information data rate. The link bandwidth must be wide enough to accommodate this higher bit rate.
SAMPLE TIMES FROM SYNCHRONIZER BIT SYNCHRONIZER TO DIGITAL LOGIC
There are two aspects of synchronization: bit synchronization and frame synchronization. The digital signal arrives at the
Figure 2: A bit synchronizer circuit creates binary bits from the demodulated output of the receiver’s discriminator.
Table of Contents for the Digital Edition of JED - September 2011
The View From Here
From the President
Detecting and Defeating IEDs
Developing Critical EW Technologies: Digital Devices Move Into the Analog Space
Index of Advertisers
JED Quick Look
JED - September 2011