Jamming Tolerance of an Orthogonal Frequency Division Multiplexing

Jamming Tolerance of an Orthogonal Frequency Division Multiplexing Based System kari. pietikainen@tkk. fi 5. 4. 2005 supervisor: Sven-Gustav Häggman, Communications Laboratory, TKK

Contents Background ¡ OFDM basics ¡ IEEE 802. 11 g ¡ Measurement setup ¡ Results ¡ Conclusions ¡ Summary ¡

Background IEEE 802. 11 based WLAN systems gaining popularity ¡ Flexible system, high data rates, easy to use ¡ Intented for indoor office and urban outdoor use ¡ l Possible to use in rural and hostile (interference) environment?

OFDM Basics (1/3) ¡ ¡ ¡ Can be considered as multicarrier modulation or multiplexing technique Used in: WLAN, DAB, DVB, x. DSL, PLC Divides frequency selective channel into several parallel flat fading subchannels Data is serial-to-parallel converted before transmission Symbols are transmitted on parallel subchannels

OFDM Basics (2/3) ¡ Signal processing made digitally in the frequency domain l ¡ FFT/IFFT pair Guard time is added between consecutive l reduces the effects of multipath propagation Tolerant to frequency selective fading ¡ Flexible data rates ¡ l l convolutional coding modulation methods

OFDM Basics (3/3) ¡ Advantages l l l ¡ spectral efficiency simplementation tolerant to ISI Disadvantages l l l bandwidth loss due guard time prone to frequency and phase offset errors (ICI) peak-to-average power problem

Subchannels ¡ ¡ Frequency-selective channel is divided into flat fading subchannels Pilot subchannels are used to correct frequency and phase offsets. l Channel estimation. 4 pilot subchannels 48 data subchannels

IEEE 802. 11 g WLAN ¡ ¡ Currently IEEE 802. 11 b is the most common WLAN standard. Recent addition to the 802. 11 family l l l ¡ 2, 4 GHz frequency band (U-NII, unlicensed) adds OFDM to the physical layer faster data rates 6 -54 Mbit/s (1 -11 Mbit/s in 802. 11 b) MAC layer is the same for both 802. 11 b and 802. 11 g standards. l interoperability

Measurement setup (1/2) ¡ ¡ Ad-hoc WLAN connection is interfered with single tone and raised cosine signal 10000 1000 -byte UDP-packets sent (Client >> Server) PER is calculated desired signal interference ¡

Measurement setup (2/2) Purpose is to test receiver functionality. ¡ Single tone interference in AWGN channel ¡ ’spectra’ Wideband interference in AWGN channel l l 50 MHz raised cosine signal 5 MHZ raised cosine signal

Results (1/4) 4 pilot subchannels 48 data subchannels ¡ ¡ Vendor 1’s operation is interesting. Two ’lobes’. Subchannels from -XX to XX have no influence on the system. Pilot subchannels have minor influence.

Results (2/4) 4 pilot subchannels 48 data subchannels ¡ ¡ Vendor 2’s operation is erratic Also two ’lobes’ Every Xth subchannel has no influence Subchannels from -X to X, no influence

Results (3/4) 4 pilot subchannels 48 data subchannels ¡ ¡ ¡ Vendor 3’s interface operates as expected. Data subchannels have minor influence. Interfering the pilots deteriorates the connection.

Results (4/4) ¡ ¡ Under wideband interference Vendor 2 & 3 operate similarly The effect of modulation and coding is clearly visible

Conclusions ¡ When narrowband interference is present, there are major differences between interfaces. l ¡ ¡ channel estimation, channel equalization, receiver architechture. Interfaces operate similarly when wideband interference is applied Vendor 1 operates erraticly l used modulation or coding rate has no effect, connection is easily clogged.

Summary ¡ OFDM is very effective physical layer technology l l ¡ high data rates tolerance to multipath propagation effects Major differences between manufacturers (transceiver chipsets) l l receiver implementation price

Thank you. Questions?