This article is intended to gently introduce the reader to the rapidly growing world of spread spectrum, wireless local and wide area networks, as well as introduce the evolution (some may call it explosion) in new communications technologies such as PCN/PCS. We will also try to thoroughly define new terms and concepts, the first time we use them. We plan to share some of our personal 25 plus years experience in this field with the reader and try to highlight the significant and exciting developments in these evolving technology fields.
As an introduction, a little history lesson and a few definitions seem to be in order. Spread spectrum (SS) dates back to World War II. A German lady scientist was granted a patent on a simple frequency hopping CW system. The allies also experimented with spread spectrum in World War II. These early research and development efforts tried to provide countermeasures for radar, navigation beacons and communications. The U. S. Military has used SS signals over satellites for at least 25 years. An old, but faithful, highly capable design like the Magnavox USC-28 modem is an example of this kind of equipment. Housed in two or three six foot racks, it had selectable data rates from a few hundred bits per second to about 64 kBits per second. It transmitted a spread bandwidth of 60 MHZ. Many newer commercial satellite systems are now converting to SS to increase channel capacity and reduce costs.
Over the last twenty years, many spread spectrum signals have appeared on the air. The easiest way to characterize these modulations is by their frequency spectra. These SS signals occupy a much greater bandwidth than needed by the information bandwidth of the transmitted data. To rate being called an SS signal, two technicalities must be met:
- The signal bandwidth must be much wider than the information bandwidth.
- Some code or pattern other than the data to be transmitted, determines the actual on-the-air transmit bandwidth.
In today's commercial spread spectrum systems, bandwidths of 10 to 100 times the information rates are used. Military systems have used spectrum widths from 1000 to 1 million times the information bandwidth. There are two very common spread spectrum modulations: frequency hopping and direct sequence. At least two other types of spreading modulations have been used: time hopping and chirp.
One way to look at spread spectrum is that it trades a wider signal bandwidth for better signal to noise ratio. Frequency hop and direct sequence are well-known techniques today. The following paragraphs will describe each of these common techniques in a little more detail and show that pseudo noise code techniques provide the common thread through all spread spectrum types.
Frequency hopping is the easiest spread spectrum modulation to use. Any radio with a digitally controlled frequency synthesizer can, theoretically, be converted to a frequency hopping radio. This conversion requires the addition of a pseudo noise (PN) code generator to select the frequencies for transmission or reception. Most hopping systems use uniform frequency hopping over a band of frequencies. This is not absolutely necessary, if both the transmitter and receiver of the system know in advance what frequencies are to be skipped. Thus a frequency hopper in two meters, could be made that skipped over commonly used repeater frequency pairs. A frequency hopped system can use analog or digital carrier modulation and can be designed using conventional narrow band radio techniques. De-hopping in the receiver is done by a synchronized pseudo noise code generator that drives the receiver's local oscillator frequency synthesizer.
The most practical, all digital version of SS is direct sequence. A direct sequence system uses a locally generated pseudo noise code to encode digital data to be transmitted. The local code runs at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added (an EXOR operation) with the faster pseudo noise code. The composite pseudo noise and data can be passed through a data scrambler to randomize the output spectrum (and thereby remove discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed carrier modulate the carrier frequency to be transmitted. The resultant DSB suppressed carrier AM modulation can also be thought of as binary phase shift keying (BPSK). Carrier modulation other than BPSK is possible with direct sequence. However, binary phase shift keying is the simplest and most often used SS modulation technique.
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code.
The use of these special pseudo noise codes in spread spectrum (SS) communications makes signals appear wide band and noise-like. It is this very characteristic that makes SS signals possess the quality of Low Probability of Intercept. SS signals are hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times the information bandwidth.
The spread of energy over a wide band, or lower spectral power density, makes SS signals less likely to interfere with narrowband communications. Narrow band communications, conversely, cause little to no interference to SS systems because the correlation receiver effectively integrates over a very wide bandwidth to recover an SS signal. The correlator then "spreads" out a narrow band interferer over the receiver's total detection bandwidth. Since the total integrated signal density or SNR at the correlator's input determines whether there will be interference or not. All SS systems have a threshold or tolerance level of interference beyond which useful communication ceases. This tolerance or threshold is related to the SS processing gain. Processing gain is essentially the ratio of the RF bandwidth to the information bandwidth.
A typical commercial direct sequence radio, might have a processing gain of from 11 to 16 dB, depending on data rate. It can tolerate total jammer power levels of from 0 to 5 dB stronger than the desired signal. Yes, the system can work at negative SNR in the RF bandwidth. Because of the processing gain of the receiver's correlator, the system functions at positive SNR on the baseband data.
Besides being hard to intercept and jam, spread spectrum signals are hard to exploit or spoof. Signal exploitation is the ability of an enemy (or a non-network member) to listen in to a network and use information from the network without being a valid network member or participant. Spoofing is the act of falsely or maliciously introducing misleading or false traffic or messages to a network. SS signals also are naturally more secure than narrowband radio communications. Thus SS signals can be made to have any degree of message privacy that is desired. Messages can also, be cryptographically encoded to any level of secrecy desired. The very nature of SS allows military or intelligence levels of privacy and security to be had with minimal complexity. While these characteristics may not be very important to everyday business and LAN (local area network) needs, these features are important to understand.
Over the last three or four years a new commercial marketplace has been emerging. Called spread spectrum, this field covers the art of secure digital communications that is now being exploited for commercial and industrial purposes. In the next five years hardly anyone will escape being involved, in some way, with spread spectrum communications. Applications for commercial spread spectrum range from "wireless" LAN's (computer to computer local area networks), to integrated bar code scanner/palmtop computer/radio modem devices for warehousing, to digital dispatch, to digital cellular telephone communications, to "information society" city/area/state or country wide networks for passing faxes, computer data, E-mail, or multimedia data.
As the technologies of spread spectrum and networking mature, many opportunities will be created for the entrepreneur. Right now there is a need for knowledgeable consultants and contract workers in software, technical and user manual copy preparation, hardware design and test, field service and installation and system planning and marketing. Other opportunities are presenting themselves in the service sectors of the spread spectrum and networking technologies. For instance, the author just three months ago started a monthly newsletter "Spread Spectrum Scene" that covers PCN/PCS, LAN/MAN/WAN and CDMA/TDMA technology. The reader and advertiser response to this new publication has surprised even its publisher. Other service sector possibilities exist, such as: on call service and installation, repair and maintenance, software development, test, or integration and verification and finally, small manufacturing or product development start-ups.
These exciting opportunities will provide an outlet for a lot of smart, ambitious people who will also help create job opportunities for other well prepared, knowledgeable individuals. While very few entrepreneurs will get rich in their own small businesses, many people can make comfortable livings in their own spread spectrum-based small businesses.
Since so many different fields will be affected by spread spectrum, a real plethora of career opportunities will soon exist in spread spectrum. Jobs now available in this new field range from assembler, test technician, QA engineer, field service, customer service and support, computer programmer, production co-ordinators, hardware design and development, RF/radio engineering associates, programmers, BETA testers, manufacturing engineering assistants, technical writers, tele-marketeers, various administrative jobs and many other functions that require some technical knowledge of spread spectrum and networking technologies. Truly, the opportunities will only be limited by imagination and the economy. Jobs now available for people with two year technical degrees range from entry level positions at $5 or $6 per hour through $12 to $18 per hour for more experienced hands-on people.
The author has tried to hire experienced and knowledgeable people, with some spread spectrum experience, over the last three years in line with his professional employment and met with very little success. The author has been forced to hire people with amateur radio or aerospace or microwave engineering and development experience in lieu of people who know something about modern digital communications. This causes employers to heavily invest in on-the-job training and reduces starting wages for new employees.
If even one introductory lecture and lab course were taken by people with two year degrees in electronics, the author feels that they could increase their starting wages by $2 or more per hour. The author believes that this kind of incentive should be sufficient for many colleges, trade schools and universities to start designing new curricula to address these new career opportunities.
Our world is rapidly changing -- computers have gone from mainframes to palmtops. Radio communications has gone from lunchbox sized (or trunk mounted/remote handset car phone) to cigarette pack sized micro-cellular telephone technology. The technical challenges of this technical progress are significant. The new opportunities created by this new technology are also significant. We've talked here about some of the very basic principles in spread spectrum and talked about evolving career opportunities -- isn't it time some of us did something about getting ready for the next millennium?
Spread spectrum technology seems to present an alphabet soup to most newcomers. We define some of the more commonly used terms in this field in the following text box.
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