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Next Generation Wireless Broadband: Comparative Analysis of Smart Antennas and MC-SCDMA Technologies

This article compares CDMA, OFDM, and MC-SCDMA technologies, as well as smart antennas vs conventional antennas. It also explores the optimal integration of these technologies for creating a broadband solution, with field trial results provided.

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Next Generation Wireless Broadband: Comparative Analysis of Smart Antennas and MC-SCDMA Technologies

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  1. Smart Antenna and MC-SCDMANext Generation Technologies for Wireless Broadband Guanghan Xu, CTONavini Networks April, 12 2001

  2. Outline • Comparative Analysis of CDMA, OFDM, and MC-SCDMA • Comparative Analysis of Smart Antennas vs Conventional Antennas • Comparative Analysis of TDD vs. FDD • Optimal Integration of Technologies to Create a Broadband • Solution • Field Trial Results of the Integrated Technologies

  3. t Wireless Broadband Challenges Path Loss (Link Budget) 14.4Kbps to 1Mbps = 69 times or 18dB more power Intercell Interference Multipath Fading F1 F1 F1 F1 F1 F1 Suburban F1 Free space Time Domain F1 F1 City Rural Mixture of Broadband & Narrowband (voice) f Frequency Domain 0.1 1 2 3 5 10 20 km

  4. OFDM Multiple Access • OFDM offers very good immunity to multipath issues. • FFT is very efficient in channelization NlogN instead of O(N2). • OFDM needs much higher fade margin requiring higher signal levels and complex coding. • OFDM has high peak to average ratio that impacts link budget due to large PA backoff. • OFDMA is difficult to reliably transmitting narrowband data or voice due to the spectrum nulls. Frequency hopping does smooth out the probability of hitting the nulls. • OFDM is susceptive to intercell interference in the N=1 deployment while all the neighboring cells are fully loaded. Transmitted OFDM Spectrum f Received OFDM Spectrum Signal Threshold f

  5. Conventional CDMA + + + Received CDMA Spectrum Transmitted CDMA Spectrum Interference Signal f f Code 1 Code 2 Code 3 Code 4 Frequency Domain Frequency Domain • CDMA (1XEVDO, EVDV & WCDMA) all have asynchronous CDMA uplink. • Due to high spreading gain, CDMA (1X and WCDMA) signals are more resistant to intercell interference which enables N=1 deployment. • Since each code has sufficient bandwidth, signal fading is marginal. • Due to high intercode or intracell interference, the link budget is adversely impacted leading to the cell breathing effect. • The high intracell interference also considerably reduces the capacity or throughput of the system.

  6. Symbol Period + + + Code 1 Code 2 Code 3 Code 4 Synchronous CDMA (SCDMA) • Synchronous CDMA (SCDMA) can maintain code orthogonality and its multipath interference or intercode interference is minimized. • Due to the spreading gain, the SCDMA signals are also more resistant to intercell interference which enables N=1 deployment. • Since each code has sufficient bandwidth, signal fading is marginal.

  7. Multipath Effect to SCDMA Multipath Channel of User 1 + + + + + Multipath Channel of User 2 + + + + Symbol Period Other User Interference Self Interference User 1 Signal User 2 Signal Code 4 Code 3 Code 1 Code 2

  8. Joint Detection for SCDMA • Joint detection is the solution to effectively handle the multipath in multi-user CDMA systems. • Joint detection is computationally expensive and its complexity is O(N2L), where N is the spreading factor and L is the channel length. • Increasing N leads to more resistence to signal fading and the ability to assign lower data rates to handle the mixture of narrowband and broad applications • Increase N does increase the complexity of joint detection quadratically. • Wide bandwidth (1.2288Mcps for IS-95 or 1X, 3.84Mbps for WCDMA) also leads to small chip periods or relatively increases L which will increase the complexity and degrade the performance.

  9. Optimal Tradeoff: MC-SCDMA f f f f OFDM Best on multipath interference Bad on intercell interference Worst on signal fading WCDMA Best on signal fading Worst on multipath interference Good on intercell interference f f MC-SCDMA Optimal tradeoff among multipath interference, intercell interference, and signal fading

  10. Subcarrier Arrangement 5MHz • Subcarrier spacing =500KHz • Chip rate = 400kcps • Chip period = 2.5us

  11. Maintain Sync in Mobility • Mobile speed 250KM/hour • Worst case movement distance in 10ms is 0.69M • Time of arrival change = 0.69/3x108 = 2.3ns. • Time of arrival change for one second is 200ns • For chip period of 2.5us, the time of arrival change is only 1/12.5 chip.

  12. Competitive Analysis SUI Propagation Model (IEEE802.16)

  13. MC-SCDMA 1X WCDMA 100% loaded w/ JD 100% loaded W/O JD 25% loaded 50% loaded 100% loaded 25% loaded 50% loaded 100% loaded Downlink Performance of WCDMA, 1X, MC-SCDMA MC-SCDMA has at least 4 times improvement in performance. Sprint Model Index

  14. MC-SCDMA 1X WCDMA 100% loaded w/ JD 100% loaded W/O JD 25% loaded 50% loaded 100% loaded 25% loaded 50% loaded 100% loaded Simulations of Uplink Performance of Best Case WCDMA, 1X, SCDMA Technologies MC-SCDMA has at least 4 times improvement in performance. SUI Model Index

  15. Fade Margins of OFDM vs MC-SCDMA 32 tones or 32 codes in 500KHz bandwidth for SUI model 4 95% reliability 99% reliability • The fade margin for OFDM with 99% reliability is about 10dB • more than MC-SCDMA.

  16. Comparison among WCDMA/1X, OFDM, & MC-SCDMA • With respect to intercell interference, MC-SCDMA has similar performance as WCDMA/1X and outperforms OFDM significantly due to spreading gain. • With respect to intercode interference, MC-SCDMA with low complexity joint detection has similar performance as OFDM and outperforms WCDMA/1X significantly in the presence of multipath. • With respect to signal fading, MC-SCDMA with low complexity joint detection has similar performance as WCDMA/1X and outperforms OFDM significantly in the presence of multipath. • With respect to mixture of narrowband and broadband, the MC-SCDMA performs similarly as WCDMA and has the similar low complexity as OFDM (leveraging FFT).

  17. Adaptive Antenna Array Conventional Smart Antenna Power Distribution Legacy RF System Patented Smart Antenna Software Power Distribution Low Capacity High Capacity Interference from other users Signal High Complexity Signal Interference from other users 128W Low Complexity 2 W Power Level Power Amplifier Power Amplifier Module Power Level

  18. Link Budget Advantages Adaptive Phased Array 2 Watts + 18 dB Gain Conventional 2 Watts + 0 dB Gain Same scale, same terrain, same clutter, same location

  19. Interference Nulling Desired Signal 1 Desired Signal 2

  20. Interference Nulling I/C=18dB I/C=15dB Desired Signal 1 Lost Desired Signal 2 Lost

  21. Interference Nulling Simple CDMA with a Single Antenna

  22. Interference Nulling Simple Beamforming

  23. Interference Nulling Interference Nulling

  24. Interference Nulling Example Actual Signal Measured at 2.4GHz Signal Without Interference BTS Receive Period BTS Transmit Period

  25. Interference Nulling for N=1 Deployment • Simulation Assumptions: • 3 sectors linear array with 8 elements • Each sector has 10 simultaneous users each has the same data rate

  26. FDD vs TDD Frequency division duplex (FDD) requires at least 30-40 MHz guard band between up and down streams to make the duplexer feasible. >30MHz Unusable Spectrum Up/down stream Down/up stream Duplexer filtering Profiles

  27. FDD vs TDD • Summary of FDD Advantages: • Guard time of TDD fundamentally limits the communication distance while FDD does not have such a restriction. • TDD may not be backward compatible to existing FDD wireless communication systems such as cellular phones. • FDD has 3dB more link budget than TDD in uplink link budget for symmetric separation. • Summary of TDD Advantages: • Flexibility of selecting a carrier for providing services. • Flexibility of providing dynamic asymmetric services for both uplink and downlink. • Exploitation of full benefits of smart antenna technologies leading to high capacity, high performance, and low cost.

  28. Optimal Integration of Smart Antennas, MC-SCDMA, and TDD • Smart Antennas with TDD • Smart uplink and smart downlink (same carrier frequencies) • Smart Antennas with MC-SCDMA • Simple smart antenna algorithms and robust performance • Low complexity joint detection algorithms • TDD and MC-SCDMA • Simple open-loop power control scheme for mobile communications • Smart Antennas + TDD + MC-SCDMA • Require simple signaling protocol • Multiple antennas lead to high redundancy • Can localize the terminal and predict handoff

  29. Baton Handoff • Determine distance from uplink synchronization • Determine direction-of-arrival (DOA) from smart antennas Determine the terminal location from DOA and distance • Location based handoffBaton handoff Close to base station Downlink Uplink Far from base station Downlink Uplink q Handset Antenna Array

  30. Base Station • Antenna installation on 30m PCS tower • Outdoor cabinet

  31. Frequency Planning E2 F2 E2 F2 F2 E3 E2: 2608-2614MHz F2: 2614-2620MHz E3: 2620-2626MHz Frequency Reuse

  32. Outdoor Tests Omni antenna used for drive test, CPE located inside car

  33. Indoor Tests

  34. Test Items in the Trials • Technology • Beamforming Gain Stability (Up and Down Link) • C/I Comparative Performance (Up and Down Link) • Effectiveness of Interference Rejection Technology • Product and Network Deployment • Data Rates vs Distance • Coverage Prediction Accuracy • Service Level Agreement Stability Across System Load Levels • System Stability with Large Number of Simultaneous Users • System Stability with under High User Contention Load • CPE Portability (Roaming) Between Cells • System Recovery Speeds • Cell Coverage Stability Across System Loads • Quality of Service/Grade of Service 10) Indoor Penetration Loss

  35. Beam Pattern • Many beam patterns suggests high levels of multipath during data rate tests • This multipath is exploited by adaptive antennas. • This multipath would severely degrade conventional systems.

  36. Beamforming Results • Average Downlink beamforming gain was 21 dB • 92% of Non Line Of Site (NLOS) locations had a downlink beamforming gain of 18dB or better

  37. Drive Test Result of One of Six BTS Sectors

  38. Prediction vs Field Measurement

  39. FTP Raw Downlink Data Rates

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