JED - August 2012 - (Page 44)

EW 101 Spectrum Warfare – Part 16 Bandwidth Trade-offs By Dave Adamy Bit Error Rate B 44 The Journal of Electronic Defense | August 2012 andwidth is an important parametric trade-off for any communication network. In general, the greater the bandwidth, the faster information can be transported from one location to another. However, the greater the bandwidth, the greater received signal power required to provide adequate received signal fidelity. In digital communication, received signal fidelity is measured in terms of the accuracy of the received signal bits. The bit error rate (BER) is the ratio of incorrectly received bits to total bits received. As previously discussed in this series, digital data cannot be directly transmitted; it must be modulated onto a radio frequency carrier. For a typical modulation scheme, Figure 1 (repeated from the January 2012 “EW 101” column) shows the received bit error rate as a function of E b /No. E b /No is the predetection signal to noise ratio (RFSNR) adjusted for the bit rate to bandwidth ratio. In typical transmitted digital links, the received BER varies between 10-3 and 10-7. From the figure, you can see that in this range, the BER increases about an order of magnitude for each dB reduction in the RFSNR. This rate of change of BER with RFSNR is the same for all modulations used for digital data. In cases where the BER must be less than this range, error correction techniques are used to correct bit errors. 10 2 10 -3 Note slope of About 1 order of magnitude Of BER per dB of E b / N 0 10 -4 10 5 4 6 6 10 0 12 14 16 18 Eb / N (dB) Figure 1: The bit error rate in a received signal is an inverse function of E b /N0 . a. Echo-back error correction INPUT MESSAGE COMPARE XMTR b. Majority encoding ECHO BACK RCVR OUTPUT WHEN ECHO MATCHES MESSAGE MESSAGE (n times) INPUT XMTR PICK MAX AGREEMENT OUTPUT RCVR c. Forward error correction MESSAGE + EDC CODE INPUT BIT-ERROR CRITICAL CASES In the June 2012 “EW 101” column, we discussed video compression. With each of the techniques discussed, the presence of bit errors reduces the fidelity of the recovered imagery. In some cases, the impact of even a ENCODE XMTR DECODE RCVR OUTPUT Figure 2: Bit errors can be corrected by several techniques. http://www.crows.org

Table of Contents for the Digital Edition of JED - August 2012

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
EW and SIGINT Payloads for UAVs
EW Careers: The Changing Market
Technology Survey: FPGA Boards
EW 101
AOC News
Index of Advertisers

JED - August 2012

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