Field Trial Results in Coordinated Multi-Point Downlink Systems
von Jörg HolfeldThe extension of cellular communication standards with multiple transmit and receive antennas is constantly crossing new technological frontiers from point-to-point towards spatially distributed communication links. This thesis investigates a coordinated multi-point transmission system in the downlink to overcome the limitations from spatial interference observed by several terminals. It will be shown that especially cell-edge users profit from the joint-transmission amongst the base stations if the cellular network operates on the same carrier frequency. The central results are gained from field trials within a cellular testbed where different spatial configurations are compared to interference limited systems in terms of the spectral efficiency.
At first, the system model of OFDM is introduced and the implications of distributed radio channel links especially onto the synchronization mechanisms are emphasized. Then, the model for the spatial transmission system is outlined where also the performance of precoding algorithms is related to spatial channel parameters. This derives the connection to the field trial experiments.
The second part concentrates on the measurement campaigns. Implementation details are presented for the transmit Wiener filter as an order-recursive precoding algorithm. It provides a low-complex solution and enables spatial configurations from multiple transmission streams over several terminals down to the single-user case. Then, the entire measurement system and experiments are portrayed including a comparison to the downlink of 3GPP LTE Rel. 8 as reference technology. But also the shortcomings of the prototyping hardware are illustrated which restrict the closed-loop real-time experiments to intra-site measurements.
The results within this thesis are gained by a two-fold approach: It is shown that data-rates over multiple modulation and coding schemes match to observed post-equalization signal-to-interference-and-noise ratios and enable the prediction of SINRs solely based on physical channel coefficients. Further field trials gathered physical channel coefficients to anticipate the spectral efficiency at the cell-edge.
From these field trials, a spectral efficiency gain by base station cooperation at the cell-edges of at least 100 % could be observed, but also the relative cell-edge size must be considered. The results confirm the trade-off between the amount of spatial streams to the interference of multiple users and additional streams especially under line-of-sight conditions. Furthermore, an observation is made that the transmission concepts without full cooperation are valuable alternatives only if the cell-edge will not be considered. Then, the achieved performance gains may not justify the large implementation and signaling overhead of the realized precoding scheme. Although the system achieved homogenous performance results over the entire cell, appropriate scenarios with precise synchronization and finegranular channel knowledge are required.
At first, the system model of OFDM is introduced and the implications of distributed radio channel links especially onto the synchronization mechanisms are emphasized. Then, the model for the spatial transmission system is outlined where also the performance of precoding algorithms is related to spatial channel parameters. This derives the connection to the field trial experiments.
The second part concentrates on the measurement campaigns. Implementation details are presented for the transmit Wiener filter as an order-recursive precoding algorithm. It provides a low-complex solution and enables spatial configurations from multiple transmission streams over several terminals down to the single-user case. Then, the entire measurement system and experiments are portrayed including a comparison to the downlink of 3GPP LTE Rel. 8 as reference technology. But also the shortcomings of the prototyping hardware are illustrated which restrict the closed-loop real-time experiments to intra-site measurements.
The results within this thesis are gained by a two-fold approach: It is shown that data-rates over multiple modulation and coding schemes match to observed post-equalization signal-to-interference-and-noise ratios and enable the prediction of SINRs solely based on physical channel coefficients. Further field trials gathered physical channel coefficients to anticipate the spectral efficiency at the cell-edge.
From these field trials, a spectral efficiency gain by base station cooperation at the cell-edges of at least 100 % could be observed, but also the relative cell-edge size must be considered. The results confirm the trade-off between the amount of spatial streams to the interference of multiple users and additional streams especially under line-of-sight conditions. Furthermore, an observation is made that the transmission concepts without full cooperation are valuable alternatives only if the cell-edge will not be considered. Then, the achieved performance gains may not justify the large implementation and signaling overhead of the realized precoding scheme. Although the system achieved homogenous performance results over the entire cell, appropriate scenarios with precise synchronization and finegranular channel knowledge are required.