During the development of an LTE base station, also referred to as eNB, a series of different tests are necessary to prove that it is operating correctly. After verifying the transmitter and receiver branch, the performance is evaluated, in order to satisfy compliance with the requirements covered by section 8 of the 3GPP technical specification (TS) 36.141. In particular, the conformance test specification defines two challenging closed-loop performance tests to characterize the performance of LTE base station receivers: hybrid automatic repeat request (HARQ) feedback and uplink (UL) timing adjustment, a newly added feature for LTE. HARQ makes it possible to correct data packet errors by requesting retransmissions. Correctly received data packets are acknowledged by the eNB. Data packets containing errors are requested for retransmission with a different redundancy version. Therefore, various test scenarios are specified, including challenging multipath fading propagation profiles along with different predefined signal settings.

UL timing adjustment enables the eNB to align the transmission signals of different user equipment (UEs) in a radio cell to maintain orthogonality. This minimizes interference between consecutive subframes. The test in line with 3GPP TS 36.141 requires simulation of two UEs: one stationary UE as reference and one moving UE that is time-adjusted by the eNB. Figure 1 shows the setup for testing the UL timing adjustment capabilities of an eNB with the R&S SMU200A.

Equipped with the R&S SMU-K69 option, the R&S SMU200A signal generator processes HARQ feedback as well as uplink timing adjustment commands from the base station in real-time, and then adapts the LTE signal dynamically in line with the 3GPP TS 36.141 test specification. This means that during testing, the signal generator not only sends RF signals with the correct channel coding in compliance with the LTE standard, but must also be able to respond to the feedback information from the base station. These tests previously required special test user equipment and fading simulators.

The R&S SMU200A behaves in a way that is analogous to the HARQ processes of real user equipment. The signal generator can also be installed with options that introduce fading and additive white Gaussian noise (AWGN) to simulate transmission conditions as stipulated by the test specification. This makes it possible to verify the base station's response to the received data packets with respect to the standard. Since all the required functionality is available with a single instrument, the test setup is dramatically simplified. Not only is the rack space minimized, but it also makes the tedious and error-prone calibration of the inter-connection between test system components (signal generator/test mobile and fading simulator) a thing of the past.

Due to the R&S SMU200A's dual-path architecture, even the demanding uplink timing adjustment test can be performed with a single unit: The signal generator simulates a stationary UE on one path and – by means of its optional fading simulator – a moving UE on the other. The moving UE can transmit with or without a time offset relative to the stationary UE.

“Rohde & Schwarz now offers a unique one-box solution to replace complex test setups – the R&S SMU200A combines RF signal generation and fading simulation with realtime feedback processing capability all in a single instrument,” said Wolfgang Kernchen, director of subdivision signal generators, audio analyzers and power meters at Rohde & Schwarz. “Furthermore, the R&S SMU200A features excellent RF performance that in many cases surpasses that of test UE. This provides base station developers with more accurate and reliable results.”

An LTE closed loop BS test option (R&S AMU-K69) is also available for the R&S AMU200A baseband signal generator and fading simulator.