Multi Carrier Modulation and OFDM

1. Multi Carrier Modulation and OFDM

2. Frequency Spread Time Spread Transmission of Data Through Frequency Selective Time Varying Channels We have seen a wireless channel is characterized by time spread and frequency spread.

3. Single Carrier Modulation in Flat Fading Channels • if symbol duration >> time spread then there is almost no Inter Symbol Interference (ISI). channel time 1 0 1 0 phase still recognizable Problem with this: Low Data Rate!!!

4. … in the Frequency Domain • this corresponds to Flat Fading channel Frequency Frequency Flat Freq. Response Frequency

5. Single Carrier Modulation in Frequency Selective Channels • if symbol duration ~ time spread then there is considerable Inter Symbol Interference (ISI). channel time ? ? 1 0 phase not recognizable

6. One Solution: we need equalization channel equalizer time time 1 0 1 0 Channel and Equalizer • Problems with equalization: • it might require training data (thus loss of bandwidth) • if blind, it can be expensive in terms computational effort • always a problem when the channel is time varying

7. The Multi Carrier Approach “symbol” “symbol” 0 1 channel time 1 0 time 1 0 time • let symbol duration >> time spread so there is almost no Inter Symbol Interference (ISI); • send a block of data using a number of carriers (Multi Carrier)

8. SC Frequency Frequency 1 0 1 1 Flat Fading Channel: Easy Demod 1 One symbol channel MC 0 1 0 1 1 1 Frequency Frequency subcarriers 0 1 0 1 1 1 Each subcarrier sees a Flat Fading Channel: Easy Demod Block of symbols Compare Single Carrier and Multi Carrier Modulation

9. Structure of Multi Carrier Modulation In MC modulation each “MC symbol” is defined on a time interval and it contains a block of data OFDM Symbol data data data data data guard interval data interval with MAX channel time spread

10. Guard Time We leave a “guard time” between blocks to allow multipath TX RX Guard Time the “guard time” is long enough, so the multipath in one block does not affect the next block Data Block Data Block data+guard RX TX NO Inter Block Interference!

11. MC Signal Transmitted Signal: Baseband Complex Signal:

12. “Orthogonal” Subcarriers and OFDM guard interval data interval Choose: Orthogonality:

13. Received subcarrier Transmitted subcarrier Channel (LTI) transient response steady state response Orthogonality at the Receiver still orthogonal at the receiver!!!

14. OFDM symbols in discrete time • Let • be the sampling frequency; • be the number of data samples in each symbol; • the subcarriers spacing • Then: with the guard time.

15. Summary OFDM Symbol # samples # subcarriers guard data TIME: Sampling Interval Freq spacing FREQUENCY:

16. OFDM Symbol and FFT Where: positive subcarriers negative subcarriers unused subcarriers

17. CP IFFT{ X } CP from the periodicity Guard Time with Cyclic Prefix (CP)

18. OFDM Demodulator No Inter Block Interference See each block: with

19. Overall Structure of OFDM Comms System IFFT +CP P/S FFT -CP S/P

20. Simple One Gain Equalization To recover the transmitted signal you need a very simple one gain equalization: transm. noise received channel Use simple Wiener Filter:

21. OFDM as Parallel Flat Fading Channels OFDM Mod OFDM Demod Frequency Selective channel N Flat Fading Channels Significance: a Freq. Selective Channel becomes N Flat Fading Channels

22. OFDM Parameters guard guard guard data data time frequency • Summarize basic OFDM Parameters: • sampling rate in Hz • N length of Data Field in number of samples • L length of Cyclic Prefix in number of samples • total number of Data Subcarriers

23. IEEE 802.11a: Frequency Bands: 5.150-5.350 GHz and 5.725-5.825 GHz (12 channels) Modulation OFDM Range: 100m IEEE 802.11g Frequency Bands: 2.412-2.472GHz Modulation: OFDM Range: 300m

24. Channel Parameters: FCC Example: the Unlicensed Band 5GHz U-NII (Unlicensed National Information Infrastructure) • 8 channels in the range 5.15-5.35GHz • 4 channels in the range 5.725-5.825GHz

25. Channel Parameters: Example IEEE802.11 Typical Signal Spectrum Typical BW~16 MHz In terms of a Transmitter Spectrum Mask (Sec. 17.3.9.2 in IEEE Std 802.11a-1999)

26. In either case: CP DATA Sampling frequency FFT size Cyclic Prefix

27. IFFT Frequency Time • Sub-carriers: (48 data + 4 pilots) + (12 nulls) = 64 Pilots at: -21, -7, 7, 21

28. Frequencies: Subcarriers index DATA

29. Time Block:

30. Overall Implementation (IEEE 802.11a with 16QAM). 1. Map encoded data into blocks of 192 bits and 48 symbols: data Encode Interleave Buffer (192 bits) Map to 16QAM …010011010101… 111001111000 … 1101 +1+j3 -1+j +3-j3 … +1-j … 4x48=192 bits

31. Overall Implementation (IEEE 802.11a with 16QAM). 2. Map each block of 48 symbols into 64 samples time domain frequency domain null +1+j3 … -3-j +3-j3 … +1-j 24 data 2 pilots null 24 data 2 pilots IFFT

32. Channel Parameters: Physical Time Spread Frequency Spread Constraints on OFDM Symbol Duration: roughly!!! to minimize CP overhead for channel Time Invariant

33. Summary of OFDM and Channel Parameters • Channel: • Max Time Spread sec • Doppler Spread Hz • Bandwidth Hz • Channel Spacing Hz • OFDM (design parameters): • Sampling Frequency • Cyclic Prefix • FFT size (power of 2) • Number of Carriers

34. Example: IEEE802.11a • Channel: • Max Time Spread • Doppler Spread • Bandwidth • Channel Spacing • OFDM (design parameters): • Sampling Frequency • Cyclic Prefix • FFT size (power of 2) • Number of Carriers

35. Applications: various Area Networks • According to the applications, we define three “Area Networks”: • Personal Area Network (PAN), for communications within a few meters. This is the typical Bluetooth or Zigbee application between between personal devices such as your cell phone, desktop, earpiece and so on; • Local Area Network (LAN), for communications up 300 meters. Access points at the airport, coffee shops, wireless networking at home. Typical standard is IEEE802.11 (WiFi) or HyperLan in Europe. It is implemented by access points, but it does not support mobility; • Wide Area Network (WAN), for cellular communications, implemented by towers. Mobility is fully supported, so you can move from one cell to the next without interruption. Currently it is implemented by Spread Spectrum Technology via CDMA, CDMA-2000, TD-SCDMA, EDGE and so on. The current technology, 3G, supports voice and data on separate networks. For (not so) future developments, 4G technology will be supporting both data and voice on the same network and the standard IEEE802.16 (WiMax) seems to be very likely

36. More Applications • 1. WLAN (Wireless Local Area Network) standards and WiFi. In particular: • IEEE 802.11a in Europe and North America • HiperLAN /2 (High Performance LAN type 2) in Europe and North America • MMAC (Mobile Multimedia Access Communication) in Japan • 2. WMAN (Wireless Metropolitan Network) and WiMax • IEEE 802.16 • 3. Digital Broadcasting • Digital Audio and Video Broadcasting (DAB, DVB) in Europe • 4. Ultra Wide Band (UWB) Modulation • a very large bandwidth for a very short time. • 5. Proposed for IEEE 802.20 (to come) for high mobility communications (cars, trains …)