C0 C7C6C5C4C3C2C1

C01

C07

C47C03

C67C45C23

C0 C7C6C5C4C3C2C1

C01

C07

C47C03

C67C45C23

C0 C7C6C5C4C3C2C1

C01

C07

C47C03

C67C45C23

C0 C7C6C5C4C3C2C1

C01

C07

C47C03

C67C45C23

Leaf test codes are secure sine they would not be jammed by jammers. When few normal users are present, many leaf code tests are wasted since absent

users cannot not report jamming. Optimal height for test codes in the tree is dependent on the number of normal

users present, absent users, and jammers. A subtree behaves according to its leaves:

- A subtree is jammed if any of the leaves of the subtree are jammers.- A subtree is absent if all of the leaves of the subtree are absent.- A subtree is normal otherwise.

These designations reflect how the network will react when the root of that subtree is chosen as a test code.

Given there are 2n total users, of which A are absent, J > 0 are jammers, and N > 0 are normal, The probability of detecting jamming when a test code is chosen at height m is:

The expected number of tests until detection:

The optimal height minimizes the expected number of tests until detection

We performed MATLAB simulations with 20 normal users and 0 to 10 jammers. Frequency hopping CDMA with 127 channels and 63 hops per bit Noise across the entire spectrum is 15dB higher than the total signal power. Power emitted by each jammer equals that emitted by the transmitter. We simulated 10 tests where the base station transmitted 10,000 6-bit messages Three scenarios simulated: - Ignore jamming: transmitter uses only one spread spectrum code - Our scheme: network follows our protocol - Best possible: jammers only emit noise

Our scheme delivers almost 100% of packets when there are 5 jammers or fewer and delivers more than 90% of packets between 6 to 10 jammers.

When the jammers have no knowledge of the spread spectrum code used by the system, the network can delivery almost 100% of the packets, but spread spectrum does not mitigate jamming in wireless broadcast networks if the network ignores jammers who have obtained the code.

Cross-Layer Jamming Mitigation in Wireless Broadcast NetworksJerry T. Chiang, Yih-Chun Hu

Illinois Center forWireless Systems

Jamming is hard to efficiently prevent in broadcast wireless networks. Transmitting to all receivers using only one spread spectrum code allows any

legitimate receiver to jam. Transmitting to all different receivers using individual codes is not power efficient.

Problem Statement

Proposed Solution – Tree Coding

Optimization – Test Height

Research Results

Background Mobile communication networks are susceptible jamming attacks. Jamming introduces noise, lowering the signal-to-noise ratio and reducing the

probability of successful packet reception. Jamming can be mitigated using spread spectrum where the transmitter redundantly

encodes information using a code allowing the receiver to reject signals that do not come from the transmitter.

Total transmit power is divided between codes in use. Power efficiency is directly related to number of codes in use.

Broader Impact Our work allows all existing broadcast channels used in spread spectrum wireless

access technologies to defend against jamming attacks. Our work has broad applicability since there already exist a wide variety of spread

spectrum wireless access technologies such as IEEE 802.11, Bluetooth, IS-95, and CDMA2000.

Example:

User N2 holds codes C2, C23, C03, C07

N0 N7N6N5N4N3N1

C0 C7C6C5C4C3C2C1

C01

N2

C07

C47C03

C67C45C23

An example run of our protocol that isolates the effect of jammer, N5

Disjoint covers shown in blue, detectable codes shown in cyan, and test codes shown in red

We propose a power efficient scheme to mitigate jamming in wireless broadcastnetworks using balanced binary tree: Each element of the tree corresponds to a spread spectrum code. Each user is assigned the codes corresponding to a leaf and all its ancestors.

A disjoint cover is a set of spread spectrum codes such that any user can decode using exactly one code in the set.

Each packet sent simultaneously on a disjoint cover and a set of test codes. The ancestor of a test code in the disjoint cover is called a detectable code. If any receiver receives packets on a test code but not the corresponding detectable

code, the receiver reports JAMMING DETECTED to the transmitter using his leaf code. The transmitter stops using the detectable code on which jamming is detected and

instead uses its two children codes. Thus the transmitter always sends packets on the minimal disjoint cover on which no

jamming had been previously detected.

Our protocol mitigates jamming by isolating jammers on individual codes while conserving power efficiency by keeping normal users in groups thereby transmitting on fewer spread spectrum codes.