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Adaptive Antennas (or “doing more with less”)

This article discusses the challenges of spectrum management in telecommunications networks and introduces the concept of adaptive antennas. Adaptive antennas can reduce adjacent band emissions, mitigate in-band interference, and increase capacity and range. The article also explores the fundamentals of adaptive antennas and their coexistence with other systems.

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Adaptive Antennas (or “doing more with less”)

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  1. Adaptive Antennas(or “doing more with less”) Spectrum Management 2002 Marc Goldburg CTO, Internet Products Group ArrayComm, Inc. marcg@arraycomm.com

  2. base station cell sector Telephony Networks Backhaul Network Switching/Routing Switching/Routing Data Networks Cellular Technology • Coverage area divided into cells • Each with infrastructure and users • Typical of two-way wireless • “cellular” (1G, 2G, 3G, …) • MMDS • Wireless LANs • LMDS 2

  3. Spectrum Management Challenges • Inter-System management • co-channel at service boundaries • adjacent channel within coverage area • Intra-system management • balancing service quality and capacity • self-interference reduces capacity • Preview of adaptive antennas • reduce adjacent band emissions • reduce sensitivity to in-band emissions • mitigate in-band interference, increasing capacity • provide gain, increasing range and quality 3

  4. Outline • Spectral efficiency • Adaptive antenna fundamentals • Coexistence 4

  5. Spectral Efficiency Defined • Information delivered per unit of spectrum • Measured in bits/second/Hertz/cell, includes effects of • multiple access method • modulation methods • channel organization • resource reuse (code, timeslot, carrier, …) • “Per-Cell” is critical • primary spectral efficiency limitation generally self-interference • isolated base station results not representative of real-world • Spectral Efficiency Interference Management 5

  6. offered load (bits/s/km2) number of cells/km2 = available spectrum (Hz) x spectral efficiency (bits/s/Hz/cell) Why Is Spectral Efficiency Important? • For given service and grade of service, determines • required amount of spectrum (CapEx) • required number of base stations (CapEx, OpEx) • required number of sites and associated site maintenance (OpEx) • and, ultimately, consumer pricing and affordability • Quick calculation (capacity limited system) • Affects radiated power per km2, too 6

  7. Designing For Spectral Efficiency • Spectral/Temporal tools • multiple access method and data compression • optimize efficiency based on traffic characteristics • modulation, channel coding, equalization • optimize efficiency based on link quality • Spatial tools, interference management • cellularization • mitigate co-channel interference by separating co-channel users • sectorization • mitigate co-channel interference through static directivity • power control • use minimum power necessary for successful communications 7

  8. interference cells cells serving sector sectors sectors serving sector interference user user sectorized adaptive antennas Self-Interference and Capacity 8

  9. Outline • Spectral efficiency • Adaptive antenna fundamentals • Coexistence issues 9

  10. Adaptive Antennas Defined • Systems comprising • multiple antenna elements (antenna arrays) • coherent processing • processing strategies that adapt to environment • Providing • gain and interference mitigation • improved signal quality and spectral efficiency • improved coexistence behavior 10

  11. User 2, s2(t)ejt User 1, s1(t)ejt as1(t)+bs2(t) as1(t) as1(t)-bs2(t) as1(t) +1 -1 +1 +1 2as1(t) 2bs2(t) Adaptive Antenna Concept • Users’ signals arrive with different relative phases and amplitudes • Processing provides gain and interference mitigation 11

  12. In-Band Uplink Gain • Signal s, M antennas, M receivers with i.i.d. noises ni • Adaptive antennas improve uplink SNR by factor of M • M=10, 10x SNR improvement, examples • double data rate if single antenna SNR is 10 dB • reduce required subscriber transmit power by 10 dB • increase range by 93% with R3.5 loss received signal s + ... + s = noise n1 + … + nM (Ms)2 s2 therefore, Uplink SNR = = M Ms2 s2 = M x single antenna SNR 12

  13. (P/M s + … + P/M s)2 ( Ps)2 In-Band Downlink Gain • Similar to uplink calculation, • except dominant noise is due to (single) receiver at user terminal • With same total radiated power P in both cases • Adaptive antennas improve downlink EIRP by factor of M • M=10, 10 dB gain examples • 10 elements with 1 W PA’s, same EIRP as single element with 100 W PA • 90% reduction in total radiated power for same EIRP EIRP (Adaptive Antenna) = = M EIRP (Single Antenna) 13

  14. Out-of-Band Downlink Gain • Out-of-band gain different from in-band gain • non-linearities that create out-of-bands destroy coherency • With same total in-band radiated power P in both cases • Ratio of in-band:out-of-band gains approximately M • very different from conventional systems • M=10, 10 dB gain examples • out-of-band gain up to 90% less than in-band gain (MP/M s)2 In-band gain (Adaptive Antenna)  = M Out-of-band gain (Adaptive Antenna) M(P/M s)2 14

  15. In-Band Interference Mitigation • Directive gain results in passive interference mitigation • Active interference mitigation additive (in dB) to gain • Gain and interference mitigation statistical quantities • Theoretical gain closely approached (within 1 dB) in practice • Theoretical interference mitigation, , harder to achieve • limited by calibration, environment, scenario • active mitigation in excess of 20 dB can be reliably achieved 15

  16. In-Band Benefits Processing Gain Operational Significance Selective Uplink Gain Increased Range & Coverage Increased Data Rates Reduced System – Wide Uplink Noise Improved Uplink Multipath Immunity Improved Signal Quality Maintained Quality with Tightened Reuse Increased Range & Coverage Increased Data Rates Reduced System–Wide Downlink Interference Improved Co–existence Behavior Reduced Downlink Multipath Maintained Quality with Tightened Reuse Uplink Interference Mitigation Selective Downlink Gain Downlink Interference Mitigation • Actual level of benefits depends on implementation details 16

  17. Outline • Spectral efficiency • Adaptive antenna fundamentals • Coexistence issues 18

  18. Co-Channel Coexistence • Adaptive antennas (AA’s) reduce in-band emissions • factor of M less total radiated power for same EIRP • peak interference remains the same • average interference significantly improved • RF safety/exposure benefits • AA’s less sensitive to in-band interference • active interference mitigation can “null” interferers • reduced planning (frequency, separation) requirements 19

  19. Adjacent Channel Coexistence • AA out-of-bands have reduced directionality • generating non-linearities destroy element-to-element coherence • lack of coherence reduces directionality • out-of-band gain roughly a factor of M less than in-band gain • peak out-of-band gain closer to average out-of-band gain • AA’s less sensitive to out-of-band interference • active interference mitigation can “null” interferers • reduced planning (frequency, separation) requirements 20

  20. Summary • Adaptive antennas lead to • increased spectral efficiency, better use of spectrum • affordable, diverse services • improved coexistence behavior • Adaptive antennas are becoming pervasive • more than 100,000 deployments worldwide • as a backwards compatible upgrade to existing networks • as a fundamental element of new broadband networks 21

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