LabVIEW and PXI Enhance Communications Design and Test
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Never before in history has the need for new wireless and communications standards been so prevalent. While chip manufacturers are working to pack as much functionality into their latest chipsets as possible, device manufacturers are scrambling to implement this new functionality into their latest products and test these devices. To combat this accelerated product development cycle, a flexible, software-based architecture is essential to rapidly prototype, design, and test devices using current and emerging wireless and communications technologies. By stepping through a simplified functional block diagram of a typical communications system, you quickly learn how to use features in National Instruments LabVIEW software, the NI Modulation Toolkit for LabVIEW, and the PXI RF hardware platform to design and test such devices.
A Typical Communications System Block Diagram
Figure 1 represents the major functional blocks in a typical communications system. You can use these blocks for source coding, channel coding, modulation, and upconversion on the transmit side and the reverse of this process on the receive side. A real-world communication link contains a physical channel over which the transmission occurs. Physical channel examples include air (wireless), fiber optic, and copper.
Source Coding and Decoding
The primary operation of source coding is to represent your message in as few bits as possible to minimize resources. You can think of source coding as data compression; the smaller the message, the faster the transmission time, which translates into more efficient use of precious resources. With source coding, you can send more information using the same bandwidth. You may be familiar with some of the more common source coding algorithms, such as jpeg compression, zip (a combination of the LZ77 and Huffman coding algorithms), MP3 (part of MPEG-1 for sound and music compression), and MPEG-2 (used in DVDs).
Channel Coding and Decoding
Unlike source coding, channel coding actually can add bits to the data, which increases the message size. Added bits ensure that the original message can better withstand the effects of any channel impairments, including noise and fading, to be properly decoded to obtain the original transmitted message.
Many channel coding algorithms have been developed to balance the need to correctly encode and transmit data while minimizing message size. Figure 2 lists the channel coding algorithms available in the NI Modulation Toolkit.
Modulation and Demodulation
The strict definition of modulation is the process of varying one or more properties (amplitude, frequency, and/or phase) of an electromagnetic wave or signal. You can use modulation to transmit information that originates at a low frequency signal to a signal operating at a higher frequency. Why would you want to transmit at a higher frequency as opposed to a lower frequency?
Transmitting a baseband audio signal (from 20 Hz to 20 kHz) in a wireless fashion would require an antenna, power source, and electronic equipment of substantial size, which would be impractical because of the large wavelength that is inversely proportional to the frequency. Therefore, if you could transmit this same signal at a higher frequency, the wavelength would become smaller, and you could reduce the size of the equipment and the amount of power you need. This fact signifies the prevalence and importance of modulation. With modulation, you can piggyback your baseband signal on a higher-frequency signal. The lower-frequency signal that contains the information or message you want to transmit is the modulating signal. The higher-frequency signal is referred to as the carrier signal because it “carries” the baseband information. The resultant combined signal is called the modulated carrier signal.
You also can use modulation when you want several signals to share the same channel or if you want to transmit more information without increasing the signal bandwidth. You achieve more efficient bandwidth use because more information can be carried in the same amount of space. You can choose a specific modulation format depending on the application and the amount of data you need to transmit.
Figure 2 lists the various modulation formats with which the Modulation Toolkit works. Additionally, with the Modulation Toolkit, you can develop custom formats; this is particularly useful for proprietary and/or military applications that require custom formats.
Upconversion and Downconversion
You can use an upconverter and downconverter to shift an input frequency either up or down, respectively. You conduct upconversion and downconversion with a device called a mixer. Mixers “multiply” two signals with different frequencies to produce a sum and difference signal.
National Instruments provides separate upconverters and downconverters for these specific design needs – the NI PXI-5600 2.7 GHz RF downconverter and the NI PXI-5610 2.7 GHz upconverter. You also can use the NI PXI-5660 RF vector signal analyzer and NI PXI-5671 RF vector signal generator in these applications.
Multiple Communications Standards – One Test Platform
With the inherent flexibility of LabVIEW and PXI, you only need one platform to design and test many of the new and emerging wireless and communications standards. Figure 3 shows that the same functional blocks in a communications system also correspond with the LabVIEW VIs, or functions, in the Modulation Toolkit. This way, you can quickly prototype and evaluate a new communications system or wireless standard. With PXI hardware, you can perform hardware-in-the-loop (HIL) testing.
As society’s need for communications continues and new wireless standards are developed, LabVIEW software and the PXI platform can respond and adapt to these growing and changing needs. LabVIEW and PXI provide all the tools you need – in one platform – to respond to current and future wireless and communications demands.
Joseph E. Kovacs
RF and Communications
Marketing Manager
joseph.kovacs@ni.com
Learn more about the latest NI technical innovations in RF and communications.
Obtain more information on the NIWeek 2006 RF and Wireless Communications Summit.
This article first appeared in the Q2 2006 issue of Instrumentation Newsletter.
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