The study was conducted by Aalborg University in Denmark on behalf of the Nordic Council of Ministers. It stated:
For voice service the weakest link is typically the link from the mobile phone to the base station, called the uplink by mobile network operators.For data services, the weakest link is the downlink according to information received from the Danish Energy Agency. Therefore, the current study focuses on the transmitter performance for voice service and the receiver performance for data mode, as these are the crucial links in weak radio signal conditions
The study used the Total Isotropic Sensitivity (TIS) to evaluate each smartphone's receiver performance:
The lower the value of the TIS, the smaller a signal the phone requires for operation, and the better the phone is to receive in weak signal areas.
Along with the Lumia 640 and 650, the study evaluated the Lumia 950 from Microsoft. The study also included the iPhone 6, 6s, 6s Plus and SE from Apple, the Samsung Galaxy S7, S7 edge and S6 edge+, the Nexus 6P and 5X from Google, the LG G5 and the HTC 10, among many others.
The study checked out voice signals when a phone is held in either the right or left hand side. In this case, the Lumia 640 was fourth on the right hand side, using the GSM 900 band and came in second on the left hand side, followed by the Lumia 650 in third place.
In terms of data service, the Lumia 640 was on top of all 26 smartphones on the LTE 800MHz band, with the Lumia 650 in fifth place behind the Galaxy 6s edge+, the S7 edge and the S7. The Lumia 950 came in 10th in the study. The Lumia 640 also came in fourth for data in the LTE 1,800MHz band.
Thanks to Claus Jægerlund for the tip!