JED - April 2009 - (Page 52)

EW101 Communications EW – Part 23 Communication Jamming By Dave Adamy After last month’s brief discussion of cell phone networks, we now will consider some cell phone jamming situations. We will use the propagation formulas discussed in the July and August 2007 JEDs and the jamming-to-signal ratio (J/S) formulas discussed in the December 2008 JED. Because any propagation loss model might be appropriate to any link, it is necessary when approaching a communication jamming problem first to determine the appropriate loss model for each of the links involved. Since cell phones and cell towers are near the ground, the uplink (i.e., cell phone to tower) and the downlink (i.e., tower to cell phone) either will be “line of sight” or “two-ray,” depending on the range, frequency and antenna heights. This also applies to the link from a jammer (regardless of its location) to the cell phone or to the cell tower. Thus, the first step in analyzing cell phone jamming is to determine the Fresnel zone (FZ) distances for the cell phone and jamming links. Then the J/S can be calculated. We will consider four cases – jamming from the ground and from the air against the uplink and the downlink. In each of these cases, the cell system is operating at 800 MHz and we are jamming the whole RF channel. If the cell system is analog, this will jam one signal. If the system is digital, this will jam all user channels using this RF channel. To jam only one user channel in a digital system, it is necessary to limit the jamming to the appropriate time slot (for GSM systems) or apply the code for one user (for CDMA systems). (continued) phone transmit power can be reduced to as little as 6 milliwatts, providing only the amount of power needed for adequate signal-to-noise ratio (SNR) in the tower receiver. However, we can assume that our jamming will cause very low SNR in the jammed link, so the cell phone would remain at its maximum power during jamming. First, let’s calculate the FZ distance for the cell phone and jamming links using the following formula: where FZ = (hT x hR x F)/24,000 FZ = the FZ distance (in km) hT = the transmitter height (in meters) h R = the receiver height (in meters) F = the link frequency (in MHz) The FZ distance for the cell phone to tower link is FZ = (1 x 30 x 800) / 24,000 = 1 km The cell phone is 2 km from the tower, which is greater than the FZ distance, so two-ray propagation applies to the cell phone link. For the jamming link: FZ = (3 x 30 x 800) / 24,000 = 3 km Because the link distance is greater than the FZ distance, the propagation is two-ray. As in all communication jamming, when the receiving antenna has approximately the same gain in all directions, the J/S is calculated from J/S = ERPJ - ERPS - LOSS J + LOSS S where ERPJ = the ERP of the jammer (in dBm) ERP = the ERP of the desired signal transmitter (indBm) LOSS J = the loss from the jammer to the receiver (in dB) LOSSS = the loss from the desired signal J = the ERP of the jammer (in dBm) Converting the two ERP values to dBm is 100 Watts = 50 dBm and 1 Watt = 30 dBm 52 | April 2009 The Journal of Electronic Defense UPLINK JAMMING FROM THE GROUND As shown in Figure 1, the cell phone is 1 meter from the ground and 2 km from a 30-meter high cell tower. The cell phone has a maximum effective radiated power (ERP) of 1 Watt. The jammer is 4 km from the cell tower, 3 meters above the ground and generates 100 Watts ERP. Because the uplink goes from the cell phone to the tower, we must jam the link receiver, which is in the tower. The cell 30 m high 4 km 2 km 1 Watt ERP 1 m high base station cell phone The loss from the jammer (two-ray propagation model) is LOSS J = 120 + 40 log(4) - 20 log(3) - 20 log(30) = 120 + 24 - 9.5 - 29.5 = 105 dB The loss from the cell phone to the tower (two-ray propagation model) is LOSS S = 120 + 40 log(2) -20 log(1) -20 log(30) = 120 + 12 - 0 - 29.5 = 102.5 dB 100 Watts ERP 3 m high JAMMER Figure 1: Jamming a cell phone uplink requires broadcasting to the cell tower.

Table of Contents for the Digital Edition of JED - April 2009

JED - April 2009
Contents
The View From Here
Calendar
From the President
The Monitor
Washington Report
World Report
Lethal SEAD
Airborne SIGINT Forecast
RF Power Sources for IED Jammers
Roost Profile
EW 101
AOC News
JED Sales Offices
Index of Advertisers
JED Quick Look

JED - April 2009

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