QAM modulation levels rising: 1024 QAM and beyond

Tremendous growth in smartphone, web, video and audio applications has increased the demand for high-speed systems. As more and more users demand high-speed data, operators require increased capacity base station networks with higher modulation and bandwidth support capability. Base station networks supporting microwave backhaul at frequencies around 6 to 42 GHz send data from numerous users to the central backbone network. Data from these networks is an order of magnitude, more than from one user or from one sector of users, and requires support capability of higher modulation levels.

Such high modulation schemes impose increased challenges both in transmitters and receivers. Due to the increased number of symbols, the system requires higher signal-to-noise-ratio (SNR), or lower noise. Better linearity is required since the peak-to-average ratio of such signals is relatively more.

Measuring the performance of a transceiver’s quality in the presence of impairments such as phase noise, system thermal noise and clock jitter is mainly described in terms of error vector magnitude (EVM). In the case of direct conversion modulators and demodulators, additional impairments exist such as local-oscillator (LO) leakage in transmitters, DC offset in receivers, and IQ magnitude and phase imbalance. EVM measures the difference between measured and ideal symbol locations of a digitally-modulated waveform. EVM is related to the system’s SNR. An ideal system with zero noise, non-linearity distortion, frequency error, IQ imbalance will have excellent SNR (theoretically infinite) and zero EVM. Degradation in EVM is due to longer error distance between the referenced and measured symbol locations, which is due to system noise and distortions.

The TRF2443, a full-duplex IF (intermediate frequency) transceiver, supports wireless microwave backhaul, point-to-point microwave and broadband wireless applications. It is integrated with transmit (Tx) and receive (Rx) chains. Tx integrates a quadrature modulator, highly linear DVGA, and synthesizer. Rx chain integrates IQ demodulator, analog VGA, digital VGA and synthesizer. TRF2443 also supports XPIC (cross-polarization interference cancellation) through separately integrated IQ demodulator receive chain.

Figure 1 below shows the measurement set-up to evaluate TRF2443 transmitter. 20 MHz onboard crystal is used to lock all the instruments. CDCM7005 on TSW3003 is used to divide 20MHz crystal reference to 10MHz. Figure 2 shows the set-up photograph. Figure 3 below shows 56 MHz 1024 QAM TRF2443 transmit chain EVM 1.1%.

Figure 1: TRF2443 transmitter evaluation set-up

Figure 2: TRF2443 transmitter set-up photograph

Figure 3: TRF2443 Tx. 56MHz 1024QAM error vector magnitude

Figure 4 below shows an experimental set-up block diagram of TRF2443 receiver using the TSW6011, a single RX channel reference design with ADS5282 and DAC34H84.

Figure 5 below shows the set-up photograph and Figure 6 shows 0.945% 56MHz 1024 QAM signal EVM at the cascaded output of the TRF2443 + ADS5282 + DAC5672

Figure 4: TRF2443 receiver evaluation set-up

Figure 5: TRF2443 receiver set-up photograph

Figure 6: TRF2443 Rx. 56MHz 1024 QAM error vector magnitude

Larger modulation levels impose increased system challenges and require better SNR, evaluate the performance on your systems and let me know what you see.