Spread-Spectrum Modulation

Spread-spectrum modulation (also known as code-division multiple access or CDMA) is a particularly important form of digital modulation. The term “spread-spectrum” arises from the fact that with this technique the digital signal is spread over a relatively wide bandwidth but at a lower level of RF power. Another more complete name for this approach is direct sequence spread spectrum (DSSS).1

The digital input signal input comprises the digitized and compressed (analog) voice waveform and this is fed to one input of the exclusive-OR (XOR) gate. What is known as a “chipping” signal is fed to the second gate input. The data rate of this chipping signal is two orders of magnitude higher than that of the digital signal input.

Numerical details of this system follow what is known as the CDMA IS-95 standard which uses a chipping signal at 1.2288 Mbit/s and a digitized and compressed (originally voice) signal running at 13 kbit/s. Potential recipients of spread-spectrum signals require knowledge of their specific codes in order to receive (decode) the signals meant specifically for them.

Summary

Twenty-first century RF systems–communications and radars–rely heavily on semiconductor devices: both discrete and integrated (RFICs or MMICs). The principal trend is toward silicon technology (mainly RF CMOS but also SiGe BiCMOS). However technologies such as GaAs PHEMT remain important for designing circuitry capable of operating well into the mmWave frequency bands (e.g., for 5G). GaN HEMT technology has gained acceptance for a wide variety of RF applications. For many years this technology principally applied to relatively high-power circuits such as RF power amplifiers but is now also entering applications such as LNAs. Active electronically-scanned array (AESA) radar modules increasingly adopt GaN technology.

At the systems and subsystems levels digital techniques such as spread spectrum modulation are increasingly important, notably in IOT transceivers and military systems. Interfacing between analog (RF) and digital circuits requires high-speed conversion subsystems (ADCS and DACs). Cognitive approaches are increasingly significant for communications and radar systems.

References

  1. T. Edwards, “Technologies for RF Systems,” Artech House, Norwood, Mass., 2018.
  2. A. Ghavidel, F. Tamjid, A. Fathy and A. Kheirdoost, “GaN Widening Possibilities for PAs,” IEEE Microwave Magazine, June 2017, pp. 46-55.
  3. A. Khanna, “mmWaves Hit the Highway,” Microwave Journal, August 2017, pp. 22-42.
  4. A. Gronefeld, “Ultra-Low Phase Noise Oscillators with Attosecond Jitter,” Microwave Journal, April 2018, pp. 58-74.
  5. C. Vogel and H. Johansson, “Time-Interleaved Analog-To-Digital Converters: Status and Future Directions,” ISCAS 2006, pp. 3386-3389
  6. M. El-Chammas, “The World of Time-Interleaved ADCs-From Theory to Design,” Texas Instruments Inc., June 17, 2012.
  7. Y. Duan, “Design Techniques for Ultra-High-Speed Time-Interleaved Analog-to-Digital Converters (ADCs),” Technical Report No. UCB/EECS-2017-10, May 1, 2017, www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-10.html.