Juha Ylitalo

Performance evaluation of different antenna array approaches for 3G CDMA uplink

One approach to solve the coverage problem in 3/sup rd/ generation CDMA macro cells is to increase the number of receiving antennas at a base station. Antenna gain, diversity gain or both can be increased compared to conventional solutions. The relative performance of maximal-ratio combining, MRC, interference-rejection combining, IRC, fixed beam method (analog beamforming) and higher-order sectorization were studied. It was found out that the link level performance of different approaches depends mainly on the radio channel characteristics and on the particular multiple access interference scenario. In case of a large number of low-bit-rate users sectorization performs best in the macro cell channel giving 2.7-5.3 dB gain compared to 2-antenna MRC with 4-8 antennas. If only very few high-bit-rate interfering users are present and fractional loading is 50%. IRC gives best results giving a relative gain of 2.5-4.8 dB.

Channel estimation study of CDMA downlink for fixed beam BTS

The quality of channel estimation made by the user terminals has a crucial impact on downlink capacity of a CDMA network. In current CDMA systems the downlink channel estimation is performed by the help of common pilot signal. This primary pilot is transmitted to the entire cell at all times. In case of multi-beam base station (grid of fixed beams) each beam may have a secondary common pilot. This paper studies such radio environments in which the user terminal could employ both the primary and the secondary pilot signals in channel estimation in order to reduce the overhead due to multiple pilot signals. It was found out that it is possible to employ both the primary and the secondary pilot signals in typical macro cell radio environments. In flat fading channels the gain from using both pilots ranges from 0.3 dB to about 0.8 dB depending on the angular spreading of the desired signal at the base station antenna array.

On RAKE allocation for CDMA receiver employing antenna arrays

Utilization of antenna arrays allows capacity enhancement in cellular CDMA networks. For wideband receivers employing antenna arrays a 2D-RAKE receiver is required and a search for best RAKE taps has to be performed in spatio-temporal domain. This simulation study focuses on 2D-RAKE allocation in the mean energy sense assuming a digital beamforming approach. The scheme permits also a straightforward way to downlink beamforming. In the current method a 2D-channel impulse response is evaluated first and then a two-dimensional search gives the RAKE taps with most energy. A wideband spatio-temporal channel model for urban macro cells is employed. The simulations produce the relative signal-to-interference-and-noise ratio (SINR) as a function of the number of allocated RAKE taps and the beamwidth of the antenna array. The results show that SINR gains depend strongly on the angular spread of the desired signal. The enhancement in SINR varied approximately between 6-9 dB for a 10-element antenna when four 2D-RAKE taps were exploited. This improvement can be interpreted as an approximate increase in capacity. Increasing the system bandwidth from 1.25 MHz to 5 MHz only 1-3 additional 2D-RAKE taps were required for maintaining the desired SINR level. In the downlink the performance was about 1-3 dB worse than in the uplink.

Performance of a UMTS uplink MIMO scheme

In this study, uplink (UL) performance was studied in the 3/sup rd/ generation UMTS (universal mobile telecommunications system) system assuming different SIMO/MIMO (single/multiple input multiple output) approaches. Link simulations and theoretical analysis were used to investigate the uplink capacity and coverage of the considered antenna configurations. Link simulations assumed that the user equipment (UE) has a maximum of two transmit antennas while the base station can employ up to four antennas. The considered MIMO scheme can be classified as diversity MIMO, which means that the UE transmits the same data symbols via two antennas. Results of the performance evaluation demonstrate that receive diversity is a dominant factor in improving uplink capacity and coverage. For example, in terms of isolated cell capacity, a MIMO technique with 2 transmit (Tx) and 2 receive (Rx) antennas performed approximately 50% worse than a traditional approach with 1 Tx and 4 Rx antennas. Results show also that MIMO transmission will provide some benefit (up to 2 dB) for UL coverage and UE battery consumption. However, doubling the number of receiving antennas is still much more efficient than introducing the 2/sup nd/ transmit antenna at the UE. Based on this study, the MIMO concept does not seem feasible for improving UMTS uplink performance.

Performance of a Digital Beamforming Concept in a GSM1800 Downlink: A Simulation Study

A smart antenna concept was studied in the context of a GSM1800 downlink. In practice, it is not possible to estimate the actual downlink channel when using frequency division duplexing (FDD). Therefore, the current approach is based on uplink direction-of-arrival (DoA) estimation. The downlink beam is then directed to the DoA obtained from the uplink. The purpose was to study the dependence of downlink performance on the angular and delay distribution of the desired signal. The downlink beamforming was studied in this work using simulations with 2D channel models for different test environments. The probability of error in DoA estimation of the desired mobile station was evaluated in conjunction with DoA averaging. In addition, downlink BER performance was examined in a noise-limited situation and compared to the results of the corresponding uplink and a conventional 2-branch GSM receiver. With a 1*8-element antenna configuration, downlink performance degradation compared to the corresponding uplink case was approximately 1.3 dB, 2.4 dB, and 0 dB in typical Urban, Suburban, and Rural environments. The losses were greater in environments with very large angular spread. It was also found that increasing accuracy in uplink DoA estimation does not necessarily correspond to improved downlink performance.

A digital beamformer concept for GSM1800 uplink: simulation results for various test environments

Performance of a digital beamforming concept for GSM1800 uplink was studied by simulation. The receiver algorithm is based on direction-of-arrival (DOA) estimation and digital beam steering. The beamformed signal is processed by a multidimensional MLSE equalizer which allows interference suppression. The principal DOA is estimated using the training sequence in order to better separate the desired user and the co-channel interfering users. Simulation results for urban, suburban and rural environments showed that using 8-element antenna arrays offers a gain of 5-6 dB in SNR at 8% BER compared to a conventional 2-branch GSM-receiver (noise limited environment). In interference limited case the receiver performance varied a lot depending on the number of signal paths and their azimuth direction from the co-channel interference.