Performance Evaluation of WiMax Systems

1. Performance Evaluation of WiMax Systems Metehan Dikmen and Mehmet Şafak Hacettepe University Dept. of Electrical and Electronics Engineering 06532 Beytepe, Ankara, Turkey

2. Outline • Motivation • Physical layer • IEEE 802.16 extensions • WiMax System Model • Randomization • FEC • Modulation • Interleaving • OFDM • Simulation • AWGN channel • Rayleigh fading channel • Results and Conclusions

3. Motivation • WirelessMAN provides network access to buildings through exterior antennas communicating with BSs • Offers an alternative to cabled access networks, • e.g., fiber optic links, coaxial systems using cable modems, and digital subscriber line (DSL) links • Have the capacity to provide high-rate network connections over broad geographic areas without costly infrastructure required in deploying cable links to individual sites • Likely to support nomadic and increasingly mobile users

4. Motivation • Supports different transport technologies, including IPv4, IPv6, Ethernet • The standard achieves high data rates in part via FEC and OFDM techniques • Has a long transmission rangebecause • regulations allow high power transmissions • the use of directional antennas produces focused signals • Transmission range and data rate vary significantly depending on the frequency bands used

5. The Physical Layer • Three types of physical layers are defined: • WirelessMAN-SCa • WirelessMAN-OFDM • A 256-carrier OFDM. • Multiple access of different mobile terminals: TDMA • WirelessMAN-OFDMA • A 2048-carrier OFDM. • OFDMA: Multiple access is provided by assigning a subset of the carriers to an individual user.

6. IEEE 802.16 Extensions • IEEE 802.16a • used in licensed and ISM bands from 2 to 11 GHz. • At the lower ranges, the signals can penetrate barriers and thus do not require LOS between transmitter and receiver • IEEE 802.16b • increases the spectrum in 5 and 6 GHz frequency bands • provides QoS to ensure priority transmission for real-time voice and video • offers differentiated service levels for different traffic types

7. IEEE 802.16 Extensions • IEEE 802.16c • represents a 10 to 66 GHz system profile • IEEE 802.16d(also called as WiMax) • includes minor improvements and fixes to 802.16a. • creates system profiles for compliance testing of 802.16a devices • IEEE 802.16e • standard for networking between fixed BSs and MSs. • would enable the high-speed signal handovers necessary for communications with users moving at vehicular speeds.

8. IEEE 802.l6dWirelessMAN-OFDM • Designed for NLOS operation • Operating frequencies: • 2-11GHz • Channel bandwidths: • 20 or 25 MHz (U.S.) • 28 MHz (Europe)

9. Randomization • Randomization is performed on data transmitted on the downlink and uplink. • For each OFDM symbol randomizer shall be used independently • Pseudo Random Binary Sequence (PRBS) generator :

10. Randomization PRBS for data randomization

11. FEC • FEC: • Reed-Solomon, with variable block size and error correction capabilities • RS concatenated with punctured inner convolutional code • Turbo coding is optional • Space-time block codes are optional • Interleaving is also employed

12. FEC • Reed Solomon Code • Derived from a systematic RS (n=255, k=239 T=8) code using GF(m=8) • primitive polynomial specified as:[1 0 0 0 1 1 1 0 1] • Convolutional Code • Coding rate is ½ • Constraint length is 7 • Generator polynomials: • G1=171oct [1111001] • G2=133oct [1011011]

13. Convolutional Encoder

14. Puncturing • Puncturing is applied after CC • 1 means a transmitted bit 0 means a removed bit

15. Channel coding schematic • Modulation schemes: BPSK, QPSK, 16-QAM and 64-QAM • Pulse shaping:Square-root raised-cosine with a rolloff factor of 0.25

16. Interleaving • Interleaving ensures that adjacent coded bits are mapped onto nonadjacent subcarriers • Block Interleaver with a two step permutations: • mk= (Ncbps/12).kmod12+floor(k/12) k=0,1..Ncbps-1 • jk= s.floor(mk/s)+(mk+Ncbps-floor(12.mk/Ncbps))mod(s) • De-interleaver is also defined by two step permutations • mj=s.floor(j/s)+(j+floor(12.j/Ncpbs))mod(s) j=0,1..Ncbps-1 • kj=12.mj-(Ncbps-1).floor(12.mj/Ncbps)

17. OFDM • OFDM symbol is made up from 256 subcarriers: • 192 data subcarriers • 8 pilot subcarriers • 56 null subcarriers: 28 lower, 27 higher frequency subcarriers for guard bands and one DC subcarrier • Pilot subcarriers are used to aid the receiver with synchronization and channel estimation. • Designated ratios of cyclic prefix time to useful time are 1/4, 1/8, 1/16, 1/32

18. Simulations

19. AWGN Channel - QPSK

20. AWGN Channel - 16 QAM

21. Rayleigh Fading Channel - QPSK

22. Comparison • As we did not use pilot carriers for channel estimation, the use of the OFDM has harmful effects to the system varying with coherence time

23. Rayleigh Fading Channel – 16 QAM

24. Results and Conclusions • If we use OFDM without channel estimation we are encounter with serious problems in Rayleigh fading channels • Increasing the coherence time makes this worse because of the burst errors • It is obvious that QAM is useless in a Rayleigh fading because of its dependence on amplitude • Our next step will be channel estimation