JED - October 2011 - (Page 58)

EW 101 Spectrum Warfare – Part 6 Digital Communication By Dave Adamy continued and a latency of 5.5 times a data block duration introduced. However, this approach improves the data fidelity. You can retransmit the received data back to the transmitter, check the returned data bit for bit, and repeat the data PROTECTING CONTENT FIDELITY One very important requirement for networking is to ensure that correct information will arrive at a remote location. Because most information is sent digitally, this means that the bit error rate must be low enough to allow proper function of the networked activity. BASIC FIDELITY TECHNIQUES There are several approaches to assuring the fidelity of information sent over a transmission link. You will note that each technique makes trade-offs among data rate, latency, level of fidelity assurance and system complexity. You can use majority encoding, which involves sending the data multiple times as shown in Figure 1. Let’s assume that each data block is sent three Figure 1: Majority encoding requires multiple transmissions of a block of code and the receiver times. At the receiver, the received data selects the block that is received the same the most times for output. blocks are compared. If all three agree, the data is passed to an output register. If two of the three block if there were any errors as shown in Figure 3. Only coragree, their version of the data is passed to the output. If none rect data blocks are placed into the output register, and the agree, the data can either be rejected or some arbitrary decitransmitter is authorized to send the next data block. If a data sion can be made. The fidelity is improved, but the throughput block has errors, that block is re-sent until an error free block data rate is reduced by a factor of three and the output data is is received. This approach assures that every data block will be delayed by three times the duration of a data block. Sending correctly transmitted (eventually). However, the complexity of more repetitions would increase the fidelity in a hostile envia return transmission link is added. Consider a wideband data ronment, but would further reduce the throughput rate and link from a remote sensor to a control station. Typically, the increase the latency. command link from the control station to the remote sensor You can also send the data blocks multiple times, but with has far less bandwidth than the data link – a command link multiple parity bits added to each block as shown in Figure 2. As discussed below, the parity bits for each data block can be checked and any block containing bit errors can be rejected. The first data block received without errors is passed to the output register. In this case, the data throughput rate is reduced and the latency increased by both the percentage of parity bits per block and the number of block repetitions sent. For example, if each block were sent five Figure 2: Repetitive transmission with many parity bits requires that each information code block times and there were 10 percent parity be sent with enough parity bits that a block with errors can be dependably detected. The receiver bits in each block, the throughput data rejects any block that does not pass parity check and outputs the first error free information block rate would be reduced by a factor of 5.5 received. 58 The Journal of Electronic Defense | October 2011

Table of Contents for the Digital Edition of JED - October 2011

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
RF EW Program Forecast
EW Ideas and Innovation
Technology Survey: Portable and Flightline EW Testers
EW 101
AOC News
Index of Advertisers
JED Quick Look

JED - October 2011