Simulation of Proprietary Low Power Wireless Systems

A designer faced with the task of wirelessly connecting two or more devices will often look to these standards first for a solution. However, depending on the application requirements, the available wireless standards are not always the best fit.

One reason is that, in the main, these standards specify operation in the license-free band at 2.4 GHz due to its worldwide nature and approximately 84-MHz bandwidth. However, this has the drawback of non-trivial co-existence issues and lower propagation distances for a given power budget. Because of this, interest has increased in the lower UHF bands—common frequencies include 868 MHz and 433 MHz in Europe, 902 to 928 MHz in the United States and 426 MHz in Japan. Along with other unlicensed bands below 1 GHz, these are usually collectively termed the sub-GHz bands. Due to the shortage of wireless standards below 1GHz, designers tend to use a proprietary physical layer (PHY) and communication protocol stack, which can then be tailored to their specific needs. Figure 1 shows a world map listing most of these unlicensed sub-GHz bands worldwide.

Simulation of sub-GHz wireless connectivity systems
The advantage of using a wireless standard like WiFi or Bluetooth is that the standards working groups have already defined the data-rate, modulation type, output power, and frequency plan, so designers need not be concerned with the underlying national regulations. Bluetooth designers, for example, can be confident that standard reference designs meet the maximum allowed radiated power, maximum modulation bandwidth, emission mask, and minimum number of hop channels to meet EN 300-440 and FCC Pt.15 regulations covering the 2.4 GHz ISM band.

At sub-GHz, the problem is a little different. First of all, the fragmented nature of the bands results in fewer standards at sub-GHz. That means that most system designers operating at sub-GHz tend to use proprietary wireless protocols, thus having the freedom to choose the various system parameters themselves. The risk here is that a given set of parameters may not meet the regulations. Thus, it can be helpful to use a simulation tool which allows users to simulate various scenarios before going to the lab, guiding them through the design process, while keeping the underlying regulations in mind. A graphical overview of the main design tasks is shown in Figure 2.

The range of subsystems or parameters to be considered in the development process include phase-locked loop (PLL) optimization, RF filtering and matching, data-rate and modulation type, demodulation process, packet data formatting, and average power consumption. System designers typically rely on a combination of spreadsheet-based tools and iterative lab work to help with the optimization of these parameters. Time domain analysis can be performed using a Spice based simulator, but performing accurate phase noise simulations in the frequency domain is usually only possible using specialized software. Alternatively, designers can make multiple trips to a local regulatory test-house to optimize the system, but this can be expensive.

To help with these challenges Analog Devices has released a free software package called ADI SRD Design Studio to allow real-time simulation and optimization of various system parameters using the ADF7xxx family of transceivers and transmitters. The development tool is based on the company's ADIsimPLL software, and has been enhanced to allow users to view modulation in both the time and frequency domains using a virtual spectrum analyzer. A summary of the design workflow tasks are listed in Table 1.

Basic overview of operation

The core of ADI SRD Design Studio is a model library of ADF70xx devices that contains parameterized data for each device including, for example, the VCO and synthesizer phase noise, VCO gain, frequency range, available data filter types, sensitivity performance, and noise figure. Using these models a Non-linear Time Domain Analysis is performed with the baseband data used to modulate the RF carrier and obtain a time series output of the VCO. The baseband data can be chosen to be pseudo-random (PRBS) or a periodic (010101) data pattern. Unlike conventional linear analysis, nonlinear effects like VCO pulling, nonlinear VCO gain curves and charge pump saturation are accurately modeled. An FFT is then performed on the time domain waveform to obtain the spectrum analyzer output.

The spectrum analyzer is very versatile, allowing the user to adjust resolution bandwidth, detector type, and number of sweeps like a real spectrum analyzer. The resolution bandwidth can be set from 100 Hz to 300 kHz, while the span is selectable from1 kHz to 3 MHz. Users can also choose whether to use peak or average detectors, telling the analyzer to take the maximum or average number, respectively, in each FFT bin. Having these parameters adjustable is useful as each regulatory standard specifies different measurement conditions including the resolution bandwidth, span and detector type that should be used in the measurement equipment. The simulator takes all of these into account in the various preset tests available in the spectrum analyzer mode. These useful preset tests, listed in Table 2, allow users to quickly test to the relevant standard without trawling through the documentation.

In addition to the transient and spectrum analyzer modes, a PLL frequency domain analysis is also performed to calculate PLL loop filter components and estimate phase and gain margin. By adjusting the PLL loop bandwidth in the simulation, users can see the effect on the transmit modulation spectrum and eye opening. This allows a proper optimization of the loop filter without having to rely on a small subset of vendor filter tables or basic guidelines. All of these three main simulations run in less than two seconds for a typical setup.

Propagation models
The ADI SRD Design Studio package has a link analysis worksheet, which is used to estimate link budget and range under various conditions. Like all of the other tasks, this is integrated into the main simulator. A change in the data-rate to meet an emission mask will cause a corresponding change in sensitivity, affecting the link budget and, ultimately, the propagation range. This provides an advantage over a set of standalone tools, as changing one parameter such as the data-rate will ripple through to the other worksheets.

The link analysis works by first calculating the link budget, that is the difference between the transmit power and the receive sensitivity taking any filter or antenna loss into account. The device setup for the simulation is shown in Figure 3.

The range can then be determined by increasing the distance between the antennas in the simulation so that the path loss equals the link budget, i.e., the point at which there is 0 dB link margin. The path loss is calculated using the selected propagation mode. Three different propagation models are supported.

Free Space Propagation Model
The free space model assumes that there are no obstructions between the transmitter and receiver, or any significant reflecting objects (including the ground). The spacing between the transmitter and receiver is R. This formula tends to give over-optimistic propagation ranges for most practical emitter/receiver placements.

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