Test of micro cell Base Station in TD-LTE

1 Introduction

Micro-honeycomb is similar to Home Node B of a Home base station. It is a small, low-cost access point for improving the network coverage of personal residences and small-sized enterprise buildings. Generally, it is directly connected to the carrier's core network in a way similar to DSL or wired broadband. Its operation mode is basically the same as that of the base station, but its power level is lower than that of the base station, the RF performance can also be slightly reduced.

At present, China is moving towards the 3G era. With the advent of the 3G era, it provides a wealth of imagination for its subsequent LTE technology market. As the time-division mode TD-LTE of LTE technology, because it is fully compatible with TD-SCDMA and can provide a very smooth Technology for TD-SCDMA technology, it has aroused widespread attention of China's communication manufacturers. As an improvement of the final level of coverage of the combination of micro cellular and TD-LTE, will undoubtedly greatly improve the coverage of TD-LTE in personal residences and small enterprise buildings, so as to have more physical resources that can be allocated to customers, this greatly improves user experience.

2 frame structure of TD-LTE

In the long-term evolution of umts lte, there are two modes: FDD and TDD. TD-LTE is its time-division mode, also known as LTE-TDD; its high-level protocol and FDD are completely consistent, in the frame structure, it fully considers and inherits the design idea of TD-SCDMA, in addition to the difference between the FDD mode and the special sub-frame, other sub-frames also maintain good compatibility with the FDD mode.

In two frame structures in LTE, FDD uses Type 1 and TDD uses Type 2. For the Type 2 frame structure used by the TD-LTE, there are many similarities with the TD-SCDMA, it divides the 10 ms wireless frame into two 5 ms half frame; each half frame consists of 5 1 ms sub frame, includes one special sub-frame and four normal sub-frames. A special sub-frame consists of three special time slots: DwPTS, GP, and UpPTS; DwPTS is the downstream frequency slot, and UpPTS is the uplink frequency slot, GP is used for the Protection interval between the downstream and upstream. Normal sub-frames are used to transmit data information like FDD.

Different from the LTE-FDD rely on frequency to distinguish between upper and lower rows), LTE-TDD is in the same frequency, rely on time that is different time slot to distinguish between upper and lower rows, therefore, you can easily adjust the upper-and lower-Line Time Slot ratio to meet the needs of many asymmetric services in the real network. Seven different upstream/downstream matching relationships are defined in the Specification, from, which has more downstream support to, which can be selected as needed, this provides good support for the download or upload business.

3 TD-LTE micro honeycomb Test

Lte tdd and FDD use air interfaces completely different from CDMA, which are mainly used in third-generation mobile communications. OFDMA orthogonal frequency division multiplexing (multiplexing) technology is used in the downlink, in the uplink, the use of peak-to-peak ratio optimization of SC-FDMA Single Carrier Frequency Division Multiple Access) technology, to the test of LTE equipment brings a huge challenge. Figure 1 is a diagram of the RF Test of the TD-LTE micro-cell Base Station by using rod & Schwarz vector signal source SMU200A and vector signal analyzer FSQ according to the specification TS36.141.

Figure 1 use SMU and FSQ from R & S to test the microcell

At present, due to the development of LTE technology, the corresponding technical specifications have not been completely frozen. In TS 36.141 V 8.2.0, the tests on the base station transmitter have defined the time comparison relationships between different antennas, such as power and transmit signal quality, including frequency error, vector amplitude error EVM, downstream reference power, etc.), the spectrum includes the occupied bandwidth OBW, adjacent channel leakage ratio ACLR and stray, etc.) and the transmitter intermodulation. The FSQ of R & S can be used to test all the above metrics. In addition, the EVM, power time template, frequency flat, and constellation diagrams of each carrier and each sub-frame can be tested, CCDF and bit stream. These testing functions further expand the test breadth and depth of R & D personnel and accelerate the development progress.

The biggest difference between the test and the traditional mode is that it needs to support MIMO technology. The test items related to the above test items have time comparison between different antennas; the principle is to test the time difference between two or more antennas in normal operation. Using the R & S vector signal analyzer FSQ (or FSV), a single instrument can achieve this test result, see table 1, you can also test the signal quality of each antenna. During the test, you only need to link the two antennas of the base station to the fsq rf port through the bonding method, and select the corresponding test result through the function key in the FSQ.

TD-LTE receiver and performance test, vector signal source to generate the corresponding upstream signal SC-FDMA), through the single-ended receiver test method to verify the receiving situation of the base station. The so-called single-ended signal is the signal sequence sent by the base station known signal source. After receiving the signal, it compares and computes the test result BER/BLER. The following test items are defined in the corresponding test specification:

1) Reference sensitivity.

2) dynamic range.

3) in-band selectivity.

4) adjacent channel selectivity and narrow band blocking.

5) blocking.

6) receiver stray.

7) receive calls and so on.

8) HARQ retransmission and multi-path fading performance tests.

9) performance tests for regular uplink adjustment during HARQ retransmission.

Receiver stray data can be tested using a frequency spectrometer. The above other tests can be achieved using a dual-channel vector signal source SMU200A. SMU200A can be configured as a dual-channel, and one channel generates upstream tdd lte signals, the other channel generates interference signals required by specifications. These interference signals can be single-tone or different standard vector modulation signals, which can separately add fading and AWGN noise to each channel. In performance testing, SMU200A can re-transmit upstream signals in real time based on the feedback from the base station, which cannot be implemented by any waveform generator.

In addition, the multi-input and multi-input MIMO technology is an indispensable part of LTE. The high-speed implementation of LTE cannot provide the support of the MIMO technology, therefore, it is necessary to use the corresponding MIMO signal to test the performance of the base station receiver. In this case, the dual-channel SMU200A built-in four fading modules can also be used to test 2 × 2 MIMO. For details, see Figure 2.

Figure 2 SMU generation of 2x2 MIMO test signal

4 Conclusion

At present, although most of the technical specifications related to LTE have been frozen, there are still some indicators that are constantly changing and developing. For testers, then, you need to use devices that can keep up with the specifications to test the LTE product and maintain the adequacy of the product. Rode & Schwarz company and 3GPP relationship is very close, and is one of its members, always keep up with the development of LTE, launched the corresponding test function, fully support the evolution of China's TD-SCDMA to TD-TLE.