CDMA Solutions: Smart Antenna Technology

1. CDMA Solutions: Smart Antenna Technology Wei-Chiang Wu

2. Multiple Access Techniques for Wireless Communication • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Code Division Multiple Access (CDMA) • Space Division Multiple Access (SDMA) • Orthogonal Frequency Division Multiple Access (OFDMA) • Purpose:Allow different users to communicate over the same channel

3. FDMA - Frequency divide up the spectrum into non-overlapping sub-bands called channels but can transmit all the time Orthogonal in frequency domain

4. TDMA - Time divide up the time domain into non-overlapping time slots Orthogonal in time domain GSM uses 200 kHz channels and 8 slots

5. TDMA - Time 1 2 3 Multiple transmitters are divided in time for multiple access to the same carrier

6. CDMA - Code CDMA uses a code division scheme, each user utilizes the entire bandwidth all the time with a user-specific signature sequence

7. SDMA - Spatial Frequency reuse: Same frequency is used in different geographic areas or "cells"

8. Cell Sectoring

9. Cell Sectoring • 120° cell sectoring reduces the number of co-channel base stations from 6 to 2.

10. Characteristics of CDMA (1) • A multiple access technique that each user is assigned a unique signature waveform (PN code) upon which data bits are modulated • In FDMA, all users transmit simultaneously, but use disjoint frequency bands • In TDMA, all users occupy the same BW, but transmit sequentially in time • In CDMA, users are allowed to transmit simultaneously in time and occupy the same BW

11. Characteristics of CDMA (2) • Multiple Access Interference (MAI) limited • The signature waveforms should perform • Delta-like autocorrelation for identification and synchronization • Low crosscorrelation (approximate orthogonal) to suppress MAI. • The signature waveforms among each user should be linear independent in order not to cancel each other’s transmission

12. Characteristics of CDMA (3) • Anti-multipath fading capability • Relatively easy to add additional users to the system • CDMA Capacity: How many users can coexist in a CDMA cell? • mainly determined by processing gain and the required SIR • The capacity is soft • Same frequency can be used in all cells, BW efficient for multiple users • CDMA air-interface is well-standardized

13. Multipath Fading

14. Tapped Delay Line Channel Model

15. CDMA Path Diversity_ RAKE Receiver

16. RAKE Receiver for CDMA • CDMA takes advantage of the multipath signals to improve signal quality • Using multiple correlating receivers that matched to each path • Maximum ratio combining • coherently recombine the output signals from each finger

17. Characteristics of CDMA (4) • Anti-jamming capability • Security, ECCM • FH/CDMA systems achieve their processing gain throughInterference avoidance • DS/CDMA systems achieve their processing gain throughInterference attenuation • Soft-handover • Near-Far Problem: rx powers from different users are unequal such that stronger user may degrade weak user’s transmission

18. Characteristics of CDMA (5) • Power control: counter the near-far problem by dynamically adjusting transmitted power, based on feedbackinformation from the receiver • Multiuser detection: robust signal processing technique to design near-far resistant detectors, timing estimators,…

20. Power Control • A way to solve the near-far problems such that all the mobiles’ transmitting powers are equally-received by the Base station • For the near-end mobile, it can be asked to transmit lower power • For the far-end mobile, it can be asked to increase transmit power • All mobiles are power controlled to the minimum power so as to maintain the link

21. Advantages of Cellular CDMA • Multipath diversity can be gained with a RAKE receiver • High frequency reuse efficiency • Soft handoff capability • Rejection of narrowband interference • Ability to adapt to nonstationary traffic • No need for guard intervals as in TDMA

22. Soft Handoff and Hard handoff • Hard handoff • a physical change in the assigned channel • Break before make • Soft Handoff • Soft Handoffis performed by exploiting two or more base stations as a giant diversity system such that multiple BSs simultaneously talk to the mobile during a handoff • a different BS handles the communication

23. Soft Handoff

24. Soft Handoff (1) • Power control during soft handoff • If ANY BS orders the MS to reduce its power, it must do so, but it may increase power only when directed by ALL BSs • Macro-diversity • On the reverse link, each BS receives the signal from MS and the demodulated signal are combined (fade resistant) • On the forward link, MS receives the signal from BSs by RAKE and then combined

25. Soft Handoff (2) • Advantage • improve link performance at the edge of the cell • Disadvantage • Reduce forward link capacity since each takes up a traffic channel

26. Overview of Smart Antenna System (SAS) for Mobile Communications • SAS is equipped in basestation (BS) with a pattern that is not fixed but adapts to the current radio conditions. • Traditional omnidirectional or sectored antennas not only “Waste” power but also interfere to other users • Smart antenna is a “Spatial Filter” that directs a beam toward desired user only • It consists of a number of radiating elements, a combining/dividing network, a control unit (DSP).

27. Difference of BS radiation pattern between a traditional antenna and SAS

28. Principle of a SAS

29. Sharing the radio spectrum: SDMA (Space Division Multiple Access) • Users in the same cell can use the same physical communication channel (carrier frequency, time slot, spreading code) as long as their angles are different • The next step in an evolutionary path toward increasing the capacity of cellular systems • Dr. Andrew Viterbi

30. Advantages of Smart Antenna Technology • Enhance coverage through Range extension • Improve building penetration and Hole filling • Reduce delay spread (time dispersion) because fewer scatters are illuminated • Reduce co-channel interference (CCI) and multiple access interference (MAI) • Link quality can be improved through multipath management • Improve system capacity • Helps to isolate the uplink signals from different users, reduce the power control burden

31. Level of intelligence • Switched Beam system • A switch is used to select the best beam to receive a particular signal • Rx power level fluctuates as a subscriber travels in an arc • Unable to take advantage of path diversity of multipath signal • Dynamically phased array • Adaptive antenna system • Interference nulling • Multipath diversity • Main beam steering

32. Different levels of SAS

33. Challenges of optimizing CDMA capacity • Traffic load balancing • Handoff overhead management • Interference control

34. Traffic loadingChallenge • The time-varying traffic load is usually distributed unevenly among the cell sites or even within the sectors of an individual cell. Consequently, the capacity limit may be attained in the heavily loaded sectors (cells) even though quite a few channels (codes) remain available in the lightly loaded sectors (cells). The problem arises that as the capacity limit is reached in a specific heavily loaded sector, the unused capacity in some more lightly loaded sectors is inaccessible to subscribers.

36. Load Balancing: SAS rotates sectors and changes beamwidth

37. Traffic load balancing • Consider a conventional 3-sector cell, one has to climb the antenna tower, physically rotate the orientation of the antennas such that the traffic is redistributed, which seem to be time-consuming and inefficient • Smart antennas not only redirect the sector orientation, the beamwidth or coverage area of each sector is also changed. Furthermore, to reflect the fact that the load distribution pattern is time-varying, a software controlled sector orientation and beamwidth shouldbe applied.

38. Dynamic sector synthesis

39. Handoff Overhead Challenge • In the existed or third generation CDMA network, e.g. IS-95, cdma2000, and W-CDMA, Soft/softer handoff is a well-published mean to enhance the link quality in the cell edge. It is a giant diversity system that several base stations (BSs) simultaneously communicate to a single mobile station. Reliable link performance and fade resistance can be attained in the handoff process. However, since a MS contacts to several BSs via different channels (codes), Soft/softer handoff does exact a significant cost in capacity.

40. Handoff Overhead Management • Goals • Reduce the size of handoff zones • Shift the handoff zone from heavy to light traffic area • Cannot be overcome by conventional antennas, since the off-the-shelf antennas typically display very gradual roll off characteristics • Best solution: apply array of antennas to produce narrower beams • Smart antennas can create beams with sharper roll off especially in the high traffic area, the unnecessary handoff zones can be reduced and handoff overhead is lessened. Thereby, system capacity is increased.

41. Handoff Overhead plot

42. InterferenceChallenge • Pilots interference: The MS usually receives various pilot signals from the surrounding cell sites and always chooses the strongest one to register. In some environment, the strength of the received pilot signals are approximately equal. This leads to unreliable handoff, synchronization problem, and moreover, reduced system capacity. It is not uncommon since many high elevation sites with RF coverage footprints much larger than normal sites due to line-of-sight. To reduce the transmitting power, downtilt or reduce the elevation height to the offending antenna seem to be the solution to prevent interference from too many pilots. However, the overall coverage is reduced. • Multiuser interference and near-far problem

43. Pilot pollution: results from too many dominant servers

44. Interference Control • It is required to propose a per-beam gain control scheme • the beam that is dedicated to provide service to the in-building or tunnel should have a larger gain • the beam that is directed to an open area should have lower gain in order not to induce interference to neighboring sites

45. Per-beam gain control

46. The CDMA Spatial Processing RAKE receiver • Each RAKE finger uses the adaptive antenna to reject multipath component to which the finger is locked • Diversity combining (MRC) is then used to combine the output from each RAKE finger to maximize the SINR for that finger

49. Downlink Beamforming for CDMA (1) • Downlink Beamforming is used to significantly reduce the overall transmitted power of the BS, since it focuses only sufficient power needed to meet FER requirements for a subscriber receiver • CDMA BS transmitter uses a broad beam to provide universal pilot, sync, paging channels throughout coverage (otherwise, MSs in those areas may not detect that the cell is available to provide service)

50. Downlink Beamforming for CDMA (2) • Individual traffic channel is focused where they are required using Downlink Beamforming technique • Coherent demodulation is degraded • Traffic signals (narrow beam) are phase-modulated relative to pilot (broad beam) • They may encounter phase differences, since broad beam encounter more scatters • Add auxiliary pilot in cdma2000