Benefits of Adaptive Antennas in CDMA Networks

1. Benefits of Adaptive Antennas in CDMA Networks Steve Kuo

2. Conventional Transmission • Omnidirectional/sectorized radiation and reception • Only a tiny fraction of radiated power on uplink or downlink is available to the receiver • The rest - the vast majority - becomes interference for other users sharing the same frequencies

3. Adaptive Array Transmission • Combines multiple antennas, digital signal processing hardware and spatial processing software • Constantly optimizes the transmission and reception patterns of the base station to the radio environment • Significantly increases coverage, capacity, spectral efficiency and practical system bandwidth

4. Network Coverage Conventional Network (Black = low C/I ratio) User 2 Cell Station 2 Cell Station 1 User 1 Intended User Adaptive Antenna Network (Cell adapts to user) User 2 Intended User User 1 Cell Station 2 Cell Station 1

5. Adaptive Antennas Defined • Adaptive Antennas (AA) • Multiple antennas • Each antenna has own separate radio • Signals from all antennas are processed simultaneously • Adaptive antennas increase C/I ratio by mitigating interference and fading

6. Array Gain • Gain associated with coherent combining of multiple antennas • Antenna array elements assumed to have no spatial diversity except a potential difference in phase • Due to imperfect knowledge of the downlink channel the downlink array gain is typically somewhat smaller then the uplink array gain

7. Uplink Array Gain • The amount of gain is independent of the antenna array configuration • An array with M uncoupled elements will typically result in an uplink array gain of 10log(M) dB

8. Downlink Array Gain • Ideal downlink gain in delivered power is equal to the number of antennas M, when total transmitted power is preserved • Imperfections in channel estimations and feedback from mobile can degrade downlink gain performance • Practical downlink gains ranges from close to 100 down to 50 percent of ideal values

9. Diversity Gains • Diversity gain is achieved through the use of multiple receive and transmit antennas provide resistance to fading in multipath propagation environments • Amount of gain achieved depends on the inherent diversity in channel

10. Uplink Diversity Gain Factors • Largest gains achieved in flat fading channels with slow fading • Gains are greater when the channels to each antenna fade independently • Depends heavily on propagation environment and antenna geometry • Gains in the range of 1-6 dB are reasonable for uplink diversity gain

11. Downlink Diversity Gain • Downlink diversity gains depend on same factors as uplink • Additionally, if a transmit diversity scheme is used, then the D/L gain will depend on which such scheme is used

12. Nulling • Coherent combination of uplink signal can be done so as to place an interferer in a null of antenna pattern • In CDMA nulling can be used to mitigate interference to and from high data rate users resulting in lower latency and higher data rates

13. Array and Diversity Gain Benefits With constant loading of 55% and 4 antennas, AA’s can increase capacity by 4X and coverage by 2X simultaneously (Not counting diversity gain)

14. Array Gain Effect on Capacity and Coverage • Increasing the number of antennas by a factor of M can ideally increase the capacity by a factor of M and coverage area by a factor of M2/ (where  is the pathloss exponent) This is not counting diversity gain • In addition, the gain of adaptive antenna arrays can be enhanced by diversity and nulling effects

15. Auxiliary Pilots for CDMA 2000 • Although fully adaptive antennas can be implemented without auxiliary pilots, actual implementation of adaptive antennas is simplified and performance is enhanced if auxiliary pilots are available