Enabling generation and analysis of non-standard OFDM waveforms

Author : Rick Overdorf

21 January 2013

The structure of a typical OFDM waveform created using a system modeling tool

Orthogonal Frequency Division Multiplexing (OFDM) is a digital multicarrier modulation method that has quickly gained in popularity for wideband digital communications.

Providing a combination of spectral efficiency, flexibility and robustness, it is now considered the modulation scheme of choice for many current and emerging commercial standards.

OFDM is today used in such diverse applications as digital television and audio broadcasting, wireless networking and mobile communications. OFDM is also now seeing adoption in the military arena.

There are a number of factors driving this adoption. To begin with, many military communications applications demand the highest level of security. OFDM enables secure wireless data transmission and its PHY layer is inherently resistive to interference and jamming. Moreover, future military radios seem likely to incorporate the software-defined radio (SDR) enhancement known as cognitive radio. OFDM, with its proven, scalable and adaptive technology, is expected to play an important role in the realisation of that concept.
 
While the use of OFDM in military communications applications brings with it significant benefits, it also presents a significant challenge in terms of generating and analysing waveforms. The OFDM modulation scheme is very complicated and has a great deal of idiosyncrasies, which means that complex mathematics are needed for the creation and demodulation of OFDM transmission. Things get even more complicated when it comes to military communications applications because the waveforms needed for this market segment are often custom or non-standard. To bring the benefits of OFDM to today’s military communications applications, engineers will require the traditional combination of flexible signal generation and analysis to support a unique and proprietary set of air interfaces.

Algorithm design and the challenges ahead
Algorithm design is an essential step in the development of a custom OFDM system. Ideally, when designing an OFDM signal, the system architect’s goal is to create a robust waveform that is not plagued by a lot of transmission issues and offers a high quality of service. In the case of military applications, they also want to ensure high throughput and resistance to interference, as well as compliance to standards or creation of a waveform that it is unique enough (proprietary) so as to prevent its unauthorised demodulation.

To achieve these goals, many design options at the physical layer (PHY) must be considered. For example, how many sub-carriers will be used, should encoding or interleaving be employed and what will be the mapping type. The system architect also needs to decide what the preamble and pilot structure will look like. Once these decisions have been made and the waveform has been designed, the system architect must then evaluate its system performance. Did he/she design it the way they wanted to and did it turn out right? In the initial stages of algorithm design, the answer will likely be no since there are many challenges—like fading problems and channel effects—that are associated with custom OFDM communication systems. Some of the other challenges include:

? Interference issues–OFDM is vulnerable to frequency offset errors which can destroy the subcarrier orthogonality, resulting in intercarrier interference and degrading system performance. Adjacent channel interference can also prove problematic.

? Distortion—Intermodulation distortion between OFDM subcarriers can raise the noise floor both in-channel and out of channel and must therefore be minimised. Phase distortion can also occur that reduces the maximum achievable data throughput rate.

? Synchronisation issues—Issues like imperfect frequency synchronisation can cause a loss in subcarrier orthogonality, severely degrading system performance.

? High peak-to-average-power ratio (PAPR)—OFDM signals typically have a higher PAPR than single-carrier signals. This can be very problematic for the power amplifier as it results in nonlinearities and loss of efficiency, as well as in-band and out-of-band distortions.

Agilent’s SystemVue creates a custom OFDM waveform that can interface with test equipment

In each case, a range of techniques (e.g., symbol windowing, higher order modulation, or adjustments to the preamble or pilot structure) can be employed to ensure the quick resolution of these issues in the waveform.

A practical COTS approach
Fortunately for today’s system architecture, the process of custom OFDM waveform creation and analysis can be greatly aided with traditional signal generation and analysis using a practical commercial off-the-shelf (COTS) approach. The approach utilises both a system modeling tool and signal analysis software like Agilent Technologies’ SystemVue and 89600 VSA solutions.

The system modeling tool provides reference models for exploring trade-offs in OFDM system architectures. It enables scenario modeling by adding fading, noise, interferers, and the RF effects necessary for realistic system analysis and early R&D verification using connections to live test equipment.

Using the tool, the system architect can quickly and easily create the custom OFDM waveforms they need. A graphical user interface simplifies the process by enabling the user to enter parameter data to configure the structure of the OFDM waveform without even requiring in depth knowledge of OFDM. They can even configure their own OFDM frames from specific requirements (Figure 1). The waveforms can then be evaluated and tweaked, as necessary, to ensure the result is exactly what the system architect wants and will work for the specific application in question. The waveforms can be inserted anywhere in the block diagram (e.g., baseband, IF, RF, analogue, and digital) and used to help optimise system performance against specific requirements.

Once a custom OFDM waveform is created, it can be demodulated with signal analysis software. Demodulating the waveform provides a clue to modulation quality and aids in the identification and resolution of any potential physical layer signal problems. The signal analysis software assists in this troubleshooting by providing advanced tools and measurements for evaluating signal spectrum, modulation and time characteristics.

A prime example of a practical COTS solution for generation and analysis of custom OFDM waveforms comprises the SystemVue environment for electronic system level (ESL) design and the 89600 VSA signal analysis software. As a system modeling tool, SystemVue can be used to create custom OFDM communications signals that can be used to test Layer 1 PHY architectures, both in test equipment as well as simulation (Figure 2). The VSA software demodulates the signal and performs any associated analysis. A link between SystemVue and the VSA software enables users to very quickly setup the demodulation, since the actual waveform parameters can just be imported into VSA for implementation. 

Conclusion
Generating and analysing custom OFDM signals for military communications systems can be a difficult task, in large part due to OFDM’s inherent complexity. There is no way to make OFDM perfectly simple, but today’s engineers can employ the traditional combination of flexible signal generation and analysis to significantly ease the burden. A practical COTS approach that utilises a system modeling tool and signal analysis software like the SystemVue and 89600 VSA solutions provides an ideal means of quickly and efficiently designing and analysing custom OFDM systems for today’s military applications.

Figure 1. Shown here is the structure of a typical OFDM waveform created using a system modeling tool. One frame consists of idle, preamble (Preamble 1 and Preamble 2) and payload data (Data 1 and Data 2). A user can configure these fields from user-specified parameters using a simple graphical user interface or configure their own OFDM frames according to their specific requirements

Figure 2. Agilent’s SystemVue creates a custom OFDM waveform that can interface with test equipment in the test environment (simulation) or be downloaded into a source to interrogate it with a DUT or analyser to see how it performs in an analogue sense or with any part of the block diagram.


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