The Transmitted Bit Stream

As shown in Figure 5.1, a transmitted digital signal must contain more than just the digitized data. A data frame is shown in the drawing.

• There is typically a block of bits that provide frame synchronization.

• In many systems, for example, a command link to an unmanned aerial vehicle (UAV), the information bits may need to be sent to one of several destinations at the receiver location. In a UAV, this could be the UAV navigation system, one of several payloads, and so forth. Thus, there would need to be a block of address bits.

• The information bits carry the actual transmitted information.

• 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 received signal. The parity or error detection and correction (EDC) block of bits support this function.

Figure 5.1 A transmitted digital signal contains synchronization, address, information, and parity or EDC bits.

5.2.1 Transmitted Bit Rate Versus 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.

5.2.2 Synchronization

There are two aspects of synchronization: bit synchronization and frame synchronization. The digital signal arrives at the 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 bit synchronizer produces a clean digital bit stream with 1s and 0s determined from the demodulated received signal. At this point, some of the bits may be wrong (bit errors) because of degradation in the received RF signal, but the output is a series of bits that can be processed in digital circuitry. As

shown in Figure 5.2, the bit synchronizer, in addition to generating the code clock, also determines when the RF signal is sampled to decide whether a received bit is a 1 or a 0. Figure 5.2 A bit synchronizer circuit creates binary bits from the demodulated output of the receiver’s discriminator. When information is transmitted digitally, the transmitter sends a typically continuous series of bits (1s and 0s) that is meaningless unless the receiver can determine the function of each bit. The information is organized into frames of many bits, and the receiver must be able to determine the beginning of each frame. The position of each bit in the frame then identifies its function. This process is called synchronization. In some data transfer systems, there is a separate modulation value for a synchronization pulse at the start of the data frame. However, typically, there is a unique series of bits in the digital bit stream that the receiver can compare against a stored bit sequence to identify the beginning of the frame.

Figure 5.3 shows the thumb-tack correlation of a series of bits. A digital signal will have approximately the same number of 1s and 0s, and they will be close to randomly distributed. The correlation value of the two signals is determined by comparing their states. If, at any instant, the two signals are equal (e.g., both 1s) the correlation is one. If they are not equal (i.e., a one and a zero), the correlation is zero. Because the bits are randomly distributed, averaging the correlation value over a block of bits will yield 0.5 correlation value. If the received code is moved in time against the reference code [by changing the frequency of the code clock (slightly) to slide one signal against the other], the correlation of the two signals will start to increase as soon as the received code is within one bit period of the reference code and will have a correlation value of 1 (100% correlation) when the two bit streams are exactly aligned. The receiver will store (as the reference code) the unique series of random 1s and 0s in the synchronization block (refer to Figure 5.1). It will average the correlation over a series of bits as long as the synchronization block, and stop delaying the received code when the average correlation pops up to 100%. Then the receiver can identify the function of each received bit from its position in the frame.

Figure 5.3 A received digital signal must be synchronized so that the information in its bits can be recovered.

Note that there is no reason synchronization bits must be in a contiguous group at the beginning of the frame. They could just as well be distributed pseudo-randomly through the frame to make it more difficult for a hostile receiver or jammer to recover the frame or interfere with frame synchronization. Because preventing synchronization is a highly efficient method of jamming digital communication, an important communication link can be expected to have a very robust synchronization scheme.

5.2.3 Required Bandwidth

The thumb-tack synchronization diagram in Figure 5.3 shows a very sharp correlation triangle that is two bit periods wide. This requires that the bits be square, which, in turn, requires an infinite bandwidth. When the link bandwidth is narrowed, the bits become rounded, which dulls the correlation as shown in Figure 5.4. Dixon stated that the 3-dB bandwidth of the main lobe of the digital signal frequency spectrum is adequate to support the recovery of the transmitted digital signal [1] (see Figure 5.5).

The 3-dB bandwidth is also given in [1] as 0.88 × the transmitted bit rate for most digital RF modulations, but is only 0.66 × the transmitted bit rate for minimum shift keying (MSK). MSK is an efficient modulation that is widely used in digital links because this reduced bandwidth versus bit rate allows improved receiver sensitivity. In Section 5.4, we will be discussing a number of modulations and their implications in detail.

Figure 5.4 The shape of the correlation curve is dependent on the bandwidth of the link over which the digital signal is carried.

Figure 5.5 The digital signal spectrum includes a main lobe and side lobes with clearly defined nulls spaced at

multiples of the clock rate from the carrier frequency.

5.2.4 Parity and EDC

The final block of bits in the frame of Figure 5.1 is to preserve information fidelity by detecting or correcting bit errors. For systems designed to operate in very hostile environments, these bits, or other techniques for fidelity preservation, can significantly increase the bandwidth required to pass a given amount of data in the required amount of time.