Jamming Tolerance of an Orthogonal Frequency Division Multiplexing Based System

1. Jamming Tolerance of an Orthogonal Frequency Division Multiplexing Based System kari.pietikainen@tkk.fi5.4.2005 supervisor: Sven-Gustav Häggman, Communications Laboratory, TKK

2. Contents • Background • OFDM basics • IEEE 802.11g • Measurement setup • Results • Conclusions • Summary

3. 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 • Possible to use in rural and hostile (interference) environment?

4. OFDM Basics (1/3) • Can be considered as multicarrier modulation or multiplexing technique • Used in: WLAN, DAB, DVB, xDSL, 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

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

6. OFDM Basics (3/3) • Advantages • spectral efficiency • simple implementation • tolerant to ISI • Disadvantages • bandwidth loss due guard time • prone to frequency and phase offset errors (ICI) • peak-to-average power problem

7. 4 pilot subchannels 48 data subchannels Subchannels • Frequency-selective channel is divided into flat fading subchannels • Pilot subchannels are used to correct frequency and phase offsets. • Channel estimation.

8. IEEE 802.11g WLAN • Currently IEEE 802.11b is the most common WLAN standard. • Recent addition to the 802.11 family • 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 802.11b) • MAC layer is the same for both 802.11b and 802.11g standards. • interoperability

9. 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

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

11. 4 pilot subchannels 48 data subchannels Results (1/4) • Vendor 1’s operation is interesting. • Two ’lobes’. • Subchannels from -XX to XX have no influence on the system. • Pilot subchannels have minor influence.

12. 4 pilot subchannels 48 data subchannels Results (2/4) • Vendor 2’s operation is erratic • Also two ’lobes’ • Every Xth subchannel has no influence • Subchannels from -X to X, no influence

13. 4 pilot subchannels 48 data subchannels Results (3/4) • Vendor 3’s interface operates as expected. • Data subchannels have minor influence. • Interfering the pilots deteriorates the connection.

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

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

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

17. Thank you. Questions?