System-Level Model Of Analog Devices Agile RF Transceiver – AD9361

This System-Level Model Analog Devices AD9361 Agile RF Transceiver model has been co-developed by MathWorks and Analog Devices and is validated with measurements in the lab. The archive contains:

- Model of the AD9361 RF Transmitter - Model of the AD9361 RF Receiver

To run these models you will need:

• The following MathWorks products (version 2015a):

− SimRF™ − Stateflow® − Communications System Toolbox™ − Fixed-Point Designer™ −

For detailed information and examples visit:

− https://www.mathworks.com/programs/trials/trial_request.html

If access to above products are needed, please use the Contact Sales or Request Trial Links to the top right on the page linked above.

• To only install the AD9361Filter Wizard App and supporting files:

- http://www.mathworks.com/matlabcentral/fileexchange/45843-ad9361-filter-design-wizard

MathWorks LTE System Toolbox™ is recommended for generating LTE standard compliant test signals and decode the transmitted / received LTE signals.

More information on the transceiver architecture and functionality can be found here:

− http://www.mathworks.com/hardware-support/analog-devices-rf-transceivers.html

General Transmitter and receiver models are available after installation. You can open them using the Simulink library browser and opening “SimRF Models for Analog Devices RF Transceivers”, or by typing the following in the MATLAB Command Window:

>> aditxrxmod

You can inspect the models and modify them as needed.

For the Simulink models of the transmitter and the receiver, the top level parameters are defined in initialization call-backs of the models. The call-backs are executed when the models are updated. One way of accessing the call-backs consists of opening the mask of the models and modifying the MATLAB functions, ad9361_tx_initifcn and ad9361_rx_initfcn.

Model parameters derived from their characterization in the lab are directly defined in the masks of sub-systems, RF transmitter and RF receiver. You can see this by right-clicking on the model, selecting the menu “mask” and then the sub-menu “edit mask”. In the initialization pane you will find code to derive and manipulate local parameters to the sub-system.

For questions on the model, please contact us using http://www.mathworks.com/company/aboutus/contact_us/?s_tid=gn_cntus .

AD9361 Transmitter Testbench The transmitter testbench consists of:

− signal sources, − the AD9361 transmitter which is the Device Under Test, − a spectrum analyser, − a power meter.

Signal Source You can choose an ideal test signal (CW, dual tone) or an LTE-like signal (OFDM with 64QAM and a bandwidth of 5, 10 or 20 MHZ). During a simulation, toggle the manual switch to change the type of signal injected into the transmitter chain. You can also change the power of the test signal. By design, the sample rate of the ideal test signals is set to LTE test signal.

You can also use a custom test signal. To define a custom signal, represent the I and Q components of the desired test signal with an array of complex numbers. Specify this array in the Tab “Custom Test Signal” of the Test Source block mask. When you specify a custom test signal make sure that the sample time parameter is consistent with the signal characterization. Also the sample time settings of the ideal source and the sampling rate of the digital up-conversion filters must be consistent.

Signal Visualization The spectrum analyser plots the signal power or the signal power density. The power meter computes the average power (over 1024 samples) of the received signal.

AD9361 Transmitter If you double click on the AD9361_tx block, the mask parameters of the transmitter are exposed. You can change the configuration of the digital up-conversion filters and the central frequency of the RF front-end.

If you look under the mask of the transmitter, you will find three stages:

- Digital up-conversion filters - Analog filters - RF transmitter

Digital Up-Conversion Filters The digital up-conversion filters convert the baseband signal to a higher rate. These digital filters are described using finite precision arithmetic.

When you use the default configurations specified in the top level mask of the transmitter, the default parameters supplied by Analog Devices are used to design the digital up-conversion filter.

If you want to design these filters using different specifications, for example a custom test signal, simply choose the Filter Configuration “custom” in the top level mask of the transmitter. If you click on the button “Design Filter” the App for designing the custom digital filters opens. After designing the filters, export them to the workspace. The simulation will then use the custom configuration for the filters.

When you run the simulation, the sample time of the input signal must be consistent with the sample rate expected by the digital up-conversion filters.

Analog Filters The analog filters shape the noise floor introduced by the digital filters, and provide a continuous time signal processed by the RF Front end.

RF Transmitter The RF transmitter up-converts the baseband signal around the carrier frequency using direct conversion. The carrier frequency is specified in the top-level mask of the transmitter

The transmitter attenuation is tuneable and can be changed during simulation. Double click on the constant block during simulation to change this. The attenuation can have any value between 0dB and 80dB in steps of 0.25dB. Many of the imperfections modelled by the transmitter are dependent on the attenuation setting.

The RF transmitter models the following behaviour:

- Tuneable attenuation - Oscillator phase noise - Carrier dependent noise floor - Attenuation and carrier dependent output referred IP3 - Attenuation dependent gain imbalance - Attenuation dependent LO (carrier) leak

By disabling “Enable RF Impairments”, in ADI9361-TX/RF-TX block mask, the transmitter behaves as an ideal transmitter. In this case no imperfections are added to the signal other than the noise generated by the digital filters.

AD 9361 Receiver Testbench The receiver testbench consists of:

− signal sources, − the AD9361 receiver which is the Device Under Test, − a spectrum analyser, − a power meter.

Signal Source You can choose an ideal test signal (CW, dual tone) or an LTE-like signal (OFDM with 64QAM and a bandwidth of 5, 10 or 20 MHZ bandwidth). During the simulation toggle the manual switch to change the type of signal injected into the receiver chain. You can change the power of the test signal using the source mask parameter “Power”. By design, the sample rate of the ideal test signal is set to selected LTE test signal. The signals are oversampled by a factor of 1.5.

You can also use a custom test signal. To define a custom signal, represent the I and Q components of the desired test signal with an array of complex numbers. Specify this array in the Tab “Custom Test Signal” of the Test Source block mask. When you specify a custom test signal make sure that the sample time parameter is consistent with the signal characterization. Also the sample time settings of the ideal source and the sampling rate of the digital up-conversion filters must be consistent.

CW and wideband interferers can be included with the “Test Source” Block tuneable power. If you add a wideband LTE interferer, make sure that the sample time used for the RF simulation is sufficiently small to capture the entire desired bandwidth. You may need to increase the oversampling of the signal.

Signal Visualization The spectrum analyser plots the signal power or the signal power density at the input of the receiver in dBm and at the output of the receiver scaled in dBFS. The time domain scope allows for inspecting the I and Q components of the received signal over time.

AD9361 Receiver If you double click on the AD9361_rx block, the receiver mask parameters are exposed. You can change the configuration of the digital down-conversion filters and the central frequency of the RF front-end.

The RF Receiver model consists of seven functional stages closed in a feedback loop:

- RF receiver - Analog filters - Analog to Digital Converter (ADC) - Digital down-conversion filters (DDC)

- Receiver Signal Strength Indicator : three power meters to detect the strength of the received signal at different section of the chain

- Automatic Gain Control (AGC) state machine - Gain table

RF Receiver The RF receiver down-converts the signal centered on the carrier frequency to baseband using down conversion. Specify the parameter in the model mask.

The receiver consists of three stages: the Low Noise Amplifier, the demodulator (Mixer) and the Trans-Impedance Amplifier; the chain is indicated as LMT. The gain of each stage is tuneable and controlled by the AGC. Many of the imperfections modelled by the receiver are dependent on the gain setting.

The RF receiver models the following behaviour:

- Tuneable LMT gains - Carrier dependent noise floor - Gain and carrier dependent input referred IP3 - Gain and carrier dependent input referred IP2 - Gain dependent gain imbalance - Gain dependent LO (carrier) leakage

By disabling “Enable RF Impairments”, in ADI9361-RX/RF-RX block mask, the receiver behaves as an ideal receiver. In this case, no imperfections are added to the signal other than the noise generated by the digital filters.

When you run the simulation, the sample time of the input signal must be consistent with the step size of the RF simulation.

Analog Filters The analog filters (and the frequency selectivity of the TIA) provide a continuous time signal to the ADC for discretization. The gain of the low pass filters is tuneable and controlled by the AGC.

Analog to Digital Converter The analog to digital converter models a high-sampling rate third order delta-sigma modulator. The quantized output signal ranges between -4 and 4.

Digital Down-Conversion Filters The low-pass digital filters convert the highly sampled signal at the output of the ADC to a lower baseband rate. These filters are described using finite precision arithmetic.

When you use the default configurations specified in the top level mask of the receiver, the default parameters provided by Analog Devices are used to design the digital down conversion filter.

If you want to design these filters using different specifications, for example a custom test signal, simply choose the Filter Configuration “custom” in the mask of the top level block AD19361-rx. When you push

the “Design Filter”, the App for the custom digital filters opens. After designing the filters, export them to the workspace. The simulation will then use the custom configuration for the filters.

Received Strength Signal Indicator The power of the received signal is measured at three stages in the receiver:

- At the output of the LMT – LMT peck detector - At the output of the ADC – ADC peak detector - After the half-band filters (before the programmable FIR) – Average Power Detector

The first meter reacts instantaneously to overload of the signal received by the RF front end. The thresholds (inner and outer) of this peak detector are programmable.

The second meter integrates the output of the ADC over 4 cycles. Given the high rate of the ADC, essentially it also provides an almost instantaneous detection of the signal peak. The thresholds (inner and outer) of this peak detector are programmable.

The third meter measures and controls the maximum and the minimum of average power. This information fed to the AGC that allows to increase the gain of the receiver in automatic gain control mode. The meter measures the power of the signal (in dBFS) over a programmable number of cycles. Thresholds can also be programmed. When the AGC is enabled, the signal power will remain within the designated boundaries.

When a threshold is crossed, a flag (event) is passed to the AGC state machine that reacts accordingly.

Automatic Gain Control (AGC) The AGC changes the index in the gain table according to the flags of threshold crossing reported by the RSSI (Received Signal Strength Indicator).

The AGC implements manual model and slow attack mode. To toggle between the two modes, change the position of the manual switch connected to the “mode” input port in the AGC.

In manual mode, the position of the index in the gain table is directly set by setting the value of the constant block “Manual Index”.

In slow attack mode, the AGC receives the results of the power measurements at the output of the LMT, ADC and DDC. The AGC then counts the number of times a threshold was crossed. For example, the mask of the AGC allows specifying the number of times the LMT upper threshold must be crossed before having a change of state. The change of state delivers a decrease of the position of the index in the gain table. The index decrement is also a tuneable parameter in the AGC mask. There is no change of gain for LMT and ADC inner threshold crossing. In slow attack mode, a certain (user defined) interval of time is waited before re-measuring the power and changing the setting.

The output of the AGC is the index in the gain table.

Gain Table In the current model only the full table mode is implemented and not split mode. In full mode, there is a single table to control the gains of the LMT and the analog low-pass filters. The table is currently read from a mat file, and is completely customizable.