JED - March 2012 - (Page 44)

EW 101 Spectrum Warfare – Part 11 Digital Communication By Dave Adamy continued I   44 The Journal of Electronic Defense | March 2012 The received signal power is a function of the effective ran order to pass data from one location to another, the diated power, propagation losses and receiving antenna gain. digital data link must have adequate link margin. This margin includes some elements that are clearly measurable (like link distance and system gains and losses). PR = ERP – L + GR It also includes some elements that are statistical (like Where: ERP is the effective radiated power from the transweather). The link availability is related to the link marmitting antenna (dBm) including adjustments for transmitgin. The greater the margin, the higher the probability that ting antenna pointing error gain reduction and radome loss; the link will be performing up to full specifications at any L is the propagation loss between the transmitting and regiven time. ceiving antennas, including line of sight or 2-ray propagaThe link, including a few elements ERP that have not been discussed in earlier Line “EW 101” columns, is Propagation Loss Line Loss shown in Figure 1. RCVR XMTR Loss LINK SPECIFICATIONS Typical specifications for an overall digital link are shown in Table 1. Data In PT Radome PR Radome Data Out Figure 1: The received power in a data link receiver is a function of all of the gains and losses between the transmitter and receiver. LINK MARGIN The link margin is the amount that the received signal power exceeds the receiver sensitivity. M = PR – S Where: M = Link margin (dB). PR = Signal strength at the receiver system input (dBm). S = Receiver system sensitivity at output of receiving antenna including the effects of any cable losses from the antenna (dBm). tion loss, diffraction loss, atmospheric loss, and rain loss (all in dB); GR is the receiving antenna gain including radome loss and antenna gain reduction caused by pointing error. The three important propagation loss models used to predict general future performance of systems in dynamic conditions are discussed in the “EW 101” columns in the July, August and September 2007 JED issues. Figure 2 shows the antenna pointing error in the transmitting antenna. This same geometry applies to the receiving antenna not perfectly pointed at the transmitter. In our previous radio propagation discussions related to intercept and TABLE 1: TYPICAL LINK SPECIFICATIONS Specification Max Range Data Rate Bit Error Rate Angular Tracking Rate Weather Anti-Jam Capability Anti-Spoof Capability Definition Maximum operating range of link Transmission data bit or symbol rate Ratio of bits incorrectly received Maximum angular tracking rate and angular acceleration of transmit or receive antennas Rain conditions under which link will meet its other specifications The jamming-to-received-signal ratio under which the link will meet full performance specifications The authentication measures of the system to prevent hostile insertion of false data

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
US Rotorcraft EW Programs
Technology Survey: DRFMs for EW Applications
Book Review
EW 101
AOC News
2012 AOC Industry/Institute/ University Member Guide
Index of Advertisers
JED Quick Look

JED - March 2012