Wireless Radio Technologies

1. Wireless Radio Technologies

2. Learning Objectives Upon successful completion of this lesson, you will be able to: • State the FCC regulation 15.247 regarding the synchronization of frequencies. • Define Direct Sequence Spread Spectrum (DSSS), and its uses. • Define Frequency Hopping Spread Spectrum (FHSS), and its uses. • Define Orthogonal Frequency Division Multiplexing (OFDM) • Discuss the co-location issues with regards to DSSS and FHSS systems.

3. Communication Layers • Physical Layer • Frequency Hopping Spread Spectrum (FHSS) • Direct Sequence Spread Spectrum (DSSS) • Infrared • Data Link (MAC) Layer • CSMA/CA • Virtual Carrier Sense (RTS/CTS) • ARQ algorithm

4. Cellular Concept & Frequency Re-Use • Using the same frequency over and over again. • For some of our products, the concept of different Hopping Sequences is the same as different Frequency Channels.

5. Co-channel Interference & Cell Planning • In a cellular system, frequency reuse is achieved by confining service in a hexagonal region to a specific set of frequencies. Then, at the appropriate distance from the original cell, these frequencies are reused. • Co-channel interference is the interference caused by this reuse.

6. Co-channel Interference & Cell Planning, cont. • Why Is Co-channel Interference Avoidance Important? • It limits the capacity density a system may achieve. • Reuse allows the same frequency to be used multiple times, increasing capacity density (Bits/Hz/Sq km). • It may set limits on cell radius (interference limited).

7. Modulation/Demodulation • Encodes/Decodes Information To/From Transmitted Waveform For Transmission: • Encoding varies waveform characteristics in deterministic ways • Amplitude / Frequency / Phase • Two Basic Types – Depending On Information Type • Analog • Digital • Alvarion uses Gaussian Frequency Shift Keying (GFSK) in Frequency Hopping systems • OFDM is the advanced modulation used on 20 MHz channels

8. Digital Modulation • Discrete waveform elements carry information - symbols • Number of different symbols depends on number of Bits per Symbol • # Symbols = 2# Bits Example:

9. Modulation Schemes • Spread Spectrum vendors commonly use 2 techniques to spread the RF signal: • Direct Sequence • Frequency Hopping • Each has specific advantages and disadvantages.

10. Direct Sequence Spread Spectrum (DSSS)

11. Direct Sequence • A broadband signal, which is uniquely keyed based on a scrambling (spreading) algorithm. • By spreading the power across the band, it is designed to disappear into the RF noise floor and still be identified by an identically keyed radio.

12. Direct Sequence Spread Spectrum (DSSS) • Signal power is spread over 22 MHz bandwidth. • Same center frequency is used for each transmission.

13. Power 1 6 11 Freq 2.412 2.437 2.462 6 1 6 11 1 1 6 Choosing DSSS Channels (2.4 GHz) • 11 Channels • 3 Non-Overlapping Channels. • Allows for 2-Dimensional Channel Plan. • 1 Mbps Cell Size dictates frequency reuse limits.

14. Choosing Channels (5.8 GHz) • 20 MHz Channels, 4 or 5 non-overlapping* • ISM rules 5.725 – 5.850 • * UNI rules 5.725 – 5.825 Power 5.740 5.760 5.780 5.800 5.820

15. Frequency HoppingSpread Spectrum (FHSS)

16. Frequency Hopping • A narrowband signal which rapidly hops within a specified range of frequencies. • By concentrating all of the RF power into individual narrow band transmissions, it can overcome heavy RF noise in short links.

17. Sequence # 3 Hop # 1 2 3 4 5 6 7 8 9 . . . 76 77 78 79 Channel # 5 28 67 13 48 21 76 52 24 . . . 56 43 35 51 Hopping Sequences • Hopping sequences are grouped in 3 Sets. Each One Includes: • 26 Sequences in US, Canada, and Mexico (Canada and Mexico use the upper half of the band) for a total of 79 frequencies. • Hopping Synchronization (2.4/5.8 GHz) is NOT allowed in the U.S. • IEEE 802.11 specifically defines every hopping sequence. • e.g. Sequence # 3 is:

18. Frequency f7 f6 f5 f4 f3 f2 f1 t1 t2 t3 t4 t5 t6 Time Frequency Hopping • Transmitted signal is “spread” over a wide range of frequencies. • Transmission hops 8 to 32 times per second.

19. Frequency f7 f6 f5 Jamming / Interference f4 f3 f2 f1 t1 t2 t3 t4 t5 t6 Time Frequency Hopping, cont. • Low probability of interception. • Jamming & interference immunity.

20. Frequency f7 f6 f5 f4 f3 f2 f1 t1 t2 t3 t4 t5 t6 Time Frequency Hopping, cont. • Co-existence of similar systems.

21. FHSS vs. DSSS Co-location • DSSS Systems • DSSS systems support a limited number of co-located access points without interfering with each other. • Using more than the non-overlapping channels, either disturb one another or, if signals are strong enough, share only one single CSMA/CA domain. (Throughput of one system). • FHSS Systems • Co-location is based on the use of different hopping sequences. IEEE 802.11 defines three sets of 26 sequences each, allowing the co-location of up to approximately 13 FHSS systems.

22. 900 MHz ISM

23. Rules for 900 MHz • FCC rules for ISM band apply (15.247, 1.1307, 15.203) • 902MHz - 928MHz, 26MHz, no restricted bands at edges • Frequency Hopping (Hybrid Digital Modulation) • Max RF Power, 30 dBm permitted. • We can do ~23 dBm based on Power Spectral Density Limits • 36 dBm EIRP (no 3:1 PtP gain vs. power rule) • No limit on min. hops • 0.4 sec max. dwell • Unique RF connection to the antenna is required (15.203) if product is user installed. • FCC MPE (max. permissible exposure) limit is 2 m.

24. Orthogonal Frequency Division Multiplexing (OFDM)

25. time OFDM - TECHNOLOGY OVERVIEWHigh Speed Digital Communication - the curse of Multipath • The traditional way of sending information is serially • This type of communication in wireless communication systems is affected by Multipath

26. Sent data Data and the echoes Resulting waveform OFDM - TECHNOLOGY OVERVIEWInter Symbol Interference (ISI) - Definition • Each bit becomes distorted by echoes • The symbols disturb each other

27. Sent signal Received signal OFDM - TECHNOLOGY OVERVIEWInter Symbol Interference (ISI) - Solution Long symbols • Only the edges are corrupted • But.. longer symbols mean less symbols per second (Lower throughput)

28. Sent signal Received signal OFDM - TECHNOLOGY OVERVIEWInter Symbol Interference (ISI) - SolutionFrequency Division Multiplexing (FDM) • Long symbols => low data rate • FDM – Multiplexing by sending data in a few carriers (each in a different frequency) in parallel. • Bit rate = number of carriers * bit rate of each carrier

29. The Guard Band Issue A Guard Band Guard Band F FDM Channel Width OFDM - TECHNOLOGY OVERVIEWInter Symbol Interference (ISI) - Guard band • A signal carrier looks like this: • A guard band is needed between carriers to prevent mutual interference • Conclusion - Using simple FDM will give very low spectral efficiency

30. OFDM Channel Width OFDM - TECHNOLOGY OVERVIEWOFDM – The innovation • OFDM = Orthogonal FDM • Carrier centers are put on orthogonal frequencies • ORTOGONALITY - The peak of each signal coincides with nulls of other signals • The result => high spectral efficiency

31. symbol GI Data period Sent signal Received signal OFDM - TECHNOLOGY OVERVIEWGuard time and Multipath • The Multipath corrupts the Guard Interval • The demodulated region (data period) remains undistorted

32. OFDM Frame Structure • Carrier spacing is 312.5 KHz • Fourier transform performed over 3.2 μsec • 0.8 microsecond Guard Interval for ISI rejection

33. Waveform Structure RF Channel Profile • 20 MHz channel spacing • Bandwidth divided into multiple carriers • Carriers are orthogonal, hence can be spaced close together • Each carrier is narrow band processed, hence longer period to integrate multipath signals while preventing inter-symbol interference OFDM Profile • 64 carriers • 48 payload • 4 pilots • 1 center (unused) • 5 unused carriers at bottom end • 6 unused carriers at top end

34. Multipath Symbol Period Original Signal Single Carrier ISI Original Signal Symbol Period Multipath OFDM overcomes ISI Single Carrier Issue: • A single carrier waveform creates a narrow symbol period in time. • The slightest delay from a multipath signal leaks into the following symbol, causing inter symbol interference. OFDM Solution: • An OFDM waveform is made of several narrow band carriers. • Each carrier creates a long symbol period • Multipath delay is so minor compared to symbol, it creates no ISI • Additionally, multipath is actually integrated, creating more signal strength. OFDM

35. OFDM overcomes elective fading Original OFDM waveform With OFDM, the baseband signal or information is carried over multiple carriers. If one or two carriers are degraded or sucked out by selective fading, the impact is minimal since the information is spread across the remaining carriers. Selective fading Carriers affected

36. Carriers are individually modulated • Data is interleaved and encoded with FEC on each sub-carrier with the resulting OFDM symbol being all the combined carrier symbols and the bit rate is the combined bit rate of each sub-carrier.

37. Where to use Direct Sequence vs. Frequency Hopping

38. Where to use Frequency Hopping • Harsh environment • i.e. large amount of interference. • Mission critical application • Immunity to interference. • Indoors or outdoors, where high number of clients must be served • More clients can be served per unit area with FH. • FH is best in environments where deep fading occurs or large multi-path delays • Big open buildings. • Coverage planning for DS is more complicated than FH (if you’re trying to increase the aggregate throughput or # of clients) • FH is easy, set the hopping sequence and place it.

39. Direct Sequence Radios Greater Distances for Point-to-Point Links Excellent Backhaul Solution Up to 11 Mbps More Susceptible to Interference Inefficient Spectrum Use; Prohibits Expansion Limited Number of Subscribers Frequency Hopping Radios Greater Distances for Point-to-Multipoint Links Excellent Last-Mile Solution Up to 3 Mbps Less Susceptible to Interference Efficient Use of Spectrum; Promotes Expansion Increased Number of Subscribers Unlicensed Technologies Summary

40. Radio Technologies • TDD • Time Division Duplex • One band (Uplink/Downlink) • Frequency Hopping • FDD • Frequency Division Duplex • Two separate bands (Uplink/Downlink) • Frequency Hopping

41. SU-Tx Uplink (AU-Rx) AU-Tx Downlink (SU-Rx) Guard/Duplex (100 MHz) FDD – Frequency Division Duplex • Duplex Separation Uplink/Downlink (Tx/Rx) Bands. • Frequency Hopping in each band. • Usually the uplink is in the lower band.

42. Licensed vs. Unlicensed • Licensed • Interference Protection • High Transmit Power Limits • Burdensome Auctions • Known Competition – Restricted Number Of Licenses • May Have Coverage/Operation Rules For Continued License • Unlicensed • Easy Access To Spectrum • No Interference Guarantees • Lower (than Licensed) Transmit Power Limits • May Have Additional Technical Requirements (e.g. Processing Gain) • No Coverage/Operation or Competition Rules(*) * This does NOT mean FCC rules

43. Summary In This Lesson, We Discussed: • FH and DS Spread Spectrum • OFDM and how it works • DS Channel Assignments for 2.4 & 5.8 GHz • Where to use FH or DS Systems • Advantages & Disadvantages of FHSS & DSSS

44. Any Questions?