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Another means of evaluating the transceiver’s linearity and spectral spreading between the main channel (useful signal) and adjacent channel (intermodulation signal) is by ACPR testing and analysis, especially in modern wireless communications systems. For output power of +23 dBm, the transceiver’s ACPR is less than -49 dBc. The evaluation was performed with 64-state quadrature-amplitude-modulation (64QAM) signals, with a reference bandwidth of 20 MHz and reference offset of 5 MHz (Fig. 7).

Unlike the transmit channel, the receive channel requires a stable output power level of around -10 dBm and IMD3 of better than -50 dBc for the baseband over a wide power range in spite of the power of the received signal. Gain fluctuation for each receiver channel is less than 1.5 dB in a 100-MHz bandwidth—slightly larger than that of the transmitter due to the ripple of two IF cavity filters. Still, this performance level can be tolerated since multicarrier modulation is commonly used in modern systems. It is also difficult to use a 100-MHz-bandwidth single-carrier modulation technique for TDD-LTE signals in a wireless communication system.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 8The error-vector-magnitude (EVM) parameter was used to appraise the whole system’s performance, since it provides information on some key system performance levels, including amplitude/phase imbalance, gain compression/flatness, and phase noise. EVM measurements were performed with the aid of a model N9030A PXI signal analyzer from Agilent and the company’s model 89600A vector-signal-analyzer (VSA) software. Due to the limits of the available test equipment, the maximum symbol rate used for testing was 20 MHz, although this was sufficient to verify system performance.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 9

The test signals include 16QAM, 64QAM, and TDD-LTE characteristics. Characterization of the test system includes test cables with insertion loss of 3.3 dB at 2.6 GHz. The measured EVM levels for 16QAM and 64QAM signals were both less than 0.9% while the transceiver works without switching, which is excellent performance. The spectra of the transmitter and receiver deteriorate somewhat with switching, as depicted in Fig. 8 for TDE-LTE test signals.

The table summarizes the performance levels for all eight transceiver channels. The slight differences among these channels and the EVM of the transmit/receive signals is around 1.7% to 2.3%. The transceiver was tested with continuous switching between transmit and receive states under the control of the N7625B signal studio software and the E4438C signal generator.

Transceiver Supports 8 × 8 MIMO Systems, table

According to the design procedure and test results, one phenomenon should be pointed out. This TDD system consists of as much as eight transceivers, and its current can fluctuate considerably when switching between transmit and receive modes. For example, the system’s current is about 13 A when the transmit state is on and 4 A when the transmit state is off. This may be a serious problem for some aspects of transceiver performance, such as phase noise.

With only one +48-VDC power supply for the full system, the DC power’s current fluctuation may influence the LO module’s performance in parameters such as stability. One phenomenon is the EVM will deteriorate at a rapid pace, particularly the constellation rotation, which is an intimation of poor phase noise (Fig. 9, left). The distortion of the constellation’s magnitude is evident, such as the constellation of the QPSK signal (Fig. 9, left). This deterioration is not only related to the current fluctuations, but is also likely affected by switching speed and some other transceiver functions.

Some applications require proper decoupling and active filter circuit for each transceiver’s LO module and the reference board’s power supply has been adopted and carefully designed in this eight-channel MIMO system. According to Fig. 9 (right), which shows a TDD-LTE constellation with treated power, the EVM deteriorates seriously only during the first several symbols in less than 1 μs, and then a stable constellation is achieved. This is in sharp contrast to the EVM for untreated power (Fig. 9, left).

Acknowledgments

This work was supported in part by National 973 project 2010CB327400 and in part by the National High-Tech Project under Grant 2010ZX03007-002-01, 2011ZX03004-001. The authors would also like to acknowledge Agilent Technologies and Rohde & Schwarz for providing test instruments for the measurements.

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