 was adopted by 3GPP Release 7. When comparing the performance improvements achieved with 3GPP Release 6 advanced receiver techniques, it was concluded that the big- gest improvement is achieved with plain receive diversity. By having an additional receive antenna at the terminal, a 3 dB array gain is added to the realized signal- to-noise ratio (SNR) for all terminals, due to which the VoIP DL capacity is sig- nificantly boosted. Further, the utilization of an LMMSE chip equalizer improves VoIP system performance as well by reducing the intracell interference of the users in the cell, but the achieved performance improvement is much smaller compared to utilization of receive diversity, as the equalizer is not able to improve the perfor- mance of the cell edge users, that is, the users that most probably are in the outage, as the performance of those users is mainly limited by the intercell interference. On the basis of system level simulations, the utilization of receive diversity provides approximately 80% gains in HSDPA capacity compared to the case with 1-antenna RAKE receivers at the terminals. For a 1-antenna LMMSE chip level equalizer, the corresponding gain is in the order of 20-30%. In LTE, the baseline receiver technique is a 2-antenna receiver with either MRC combining or Interference Rejection Combining (IRC), where the diversity anten- nas are used to reject the co-channel interference by estimating the interference © 2009 by Taylor & Francis Group, LLC Radio Access Network VolP Optimization covariance matrix and selecting combining weights accordingly to maximize SNR. According to [17], IRC combining provides approximately 0.5 dB gain in the received SNR over MRC combining, and this would correspond to 10-15% gains in VoIP LTE DL capacity. 10.5 Practical Constraints in VolP Concept Design and Solutions When optimizing air interface for VoIP traffic, several practical constraints are to be considered in VoIP concept design on HSPA and LTE radio systems. This section provides descriptions for the most