New Wave Instruments, manufacturer of spread spectrum development tools, has no affiliation with the author of the following article, and assumes no credit nor responsibility for it. It is a reprint of an original posted elsewhere on the Web. To read the most recent version of the article, visit New Wave Instruments' Resources Directory, where you can find a link to the original.
This reprint is provided for your convenience in the event of a broken link. If this is the case, please contact our webmaster so the link can be corrected. Thank you.
Over the past decade, a new commercial market place has been emerging, called spread spectrum (or code-division multiple access [CDMA for short]). This field covers the art of secure communications that is now being exploited for commercial and industrial purposes. In the next decade hardly anyone will escape being involved, in some way, with spread spectrum communications. Applications for commercial spread spectrum range from wireless LAN's (Local Area Network) to integrated bar code scanner/ palmtop computer/radio modem devices for warehousing, to digital dispatch, to cellular telephone communications, to information society (city to country wide) networks for passing faxes, computer data, email, or multi media data
Spread spectrum uses wide band, noise like (pseudo-noise) signals. Since spread spectrum signals are noise like, they are hard to detect. Spread spectrum are also hard to intercept or demodulate making the secure. Further, spread spectrum transmissions are hard to jam (interfere with) then narrow band signals. Because of the above two properties, this form of telecommunications has been used by the military since its conception.
Spread spectrum signals use fast codes that run many times the information bandwidth or data rate. These special codes are called pseudo random or pseudo noise codes, since it appears to be random noise, however there is nothing random about it.
Spread spectrum transmitters use similar transmit power levels to narrow band transmitters, but because spread spectrum signals are so wide ,they can be transmitted at much lower spectral power density (measured in Watts per Hertz) , then narrow band transmitters. Since spread spectrum (SS) is transmitted with low power, but wide bandwidth, a SS and narrow band signal can occupy the same band with little to no interference.
To qualify as a SS signal, two criteria should be met:
Most SS systems transmit a radio frequency signal bandwidth as wide as 20 to 254 times the bandwidth being sent. Some SS systems have employed radio frequency bandwidths 1000 times their information bandwidth.
figure 1: SS compared to narrow band
The performance increase for very wide band systems is referred to as process gain. This term is used to describe the received signal fidelity gained at the cost of bandwidth. The numerical advantage is obtained from the Claude Shannon's equation describing channel capacity:
C=W log (1+S/N)
where; C=channel capacity, W=Bandwidth, S=signal power, and N=noise power.
From this equation the result of increasing the bandwidth becomes apparent. By increasing W in the equation, the S/N may be decreased without decreased performance. The process gain (GP) is what actually provides increased system performance without requiring a high S/N. This describes mathematically as:
Where; BWRF= Radio Frequency Bandwidth, RINFO= Information Rate.
This was merely an introduction to SS. The following subjects dealing with various aspects of SS, shows in detail other considerations needed when choosing a SS system.
Types of Spread Spectrum Systems
Acquisition: The initial process of aligning a spread spectrum receiver's local PN sequence with the corresponding sequence received from the transmitter. After acquisition, synchronization must be maintained in order to despread the RF signal, and is accomplished through one of several code tracking techniques.
Antijam (AJ): The inherent ability of a spread spectrum radio receiver to attenuate and overcome narrowband electromagnetic interference or intentional jamming transmissions. Commonly spelled with a hyphen: "anti-jam."
Appended Code: A PN sequence that is intentionally truncated and restarted after N chips, where N is longer than the natural length of the sequence. Compare this to a truncated code, where the sequence is truncated short of the natural sequence length.
Balanced QPSK Modulation: A QPSK modulation scheme where the I (in-phase) channel of an RF signal is modulated by one PN code, the Q (quadrature-phase) channel of the signal is modulated by a second PN code, and both channels are modulated by the same data source. Compare this to dual-channel QPSK modulation, where the I and Q channels are modulated by two distinct data sources.
Barker Code: Barker codes, originally developed for radar, are short (13 bits or less) sequences that are normally used in one-shot schemes, as compared to most other spreading codes which run continually. For example, one might be used as a preamble to a long PN sequence for the sole purpose of simplifying synchronization. The most notable property of Barker codes is that the minor peaks of their autocorrelation functions always consist of -1,0, and +1. Barker sequences are not the natural product of linear feedback shift registers, but rather are hard-coded. Following is the complete list of Barker codes:
Bit Error Rate (BER): Numerically equal to the number of erroneous bits divided by the total number of bits received through an RF communication channel. The bit error rate always increases with lower channel signal-to-noise ratio.
Bit Error Rate Tester: Often abbreviated as either BER tester or BERT, a laboratory instrument used to measure the bit error rate of a digital signal transmitted over an RF communication channel. A bit error rate tester typically consists of a pseudorandom sequence generator at the radio transmitter to simulate a data bit stream, and an error-detector at the radio receiver to count the number of received errors.
BPSK Modulation: Biphase shift keying. Modulation of an RF carrier via phase shifting, usually at 0 and 180 degrees.
cdma2000: Also known as IMT-CDMA Multi-Carrier or IS-136, cdma2000 is a code-division multiple access (CDMA) version of the IMT-2000 standard developed by the International Telecommunication Union (ITU). The cdma2000 standard was created for third-generation (3G) mobile wireless technology. cdma2000 can support mobile data communications at speeds ranging from 144 kbps to 2 Mbps. Versions have been developed by Ericsson and Qualcomm. cdma2000 is often misspelled as CDMA 2000 (two words), or as CDMA2000 (all caps).
CDMA One: Also written as cdmaOne, CDMA One refers to the original IS-95 code-division multiple access (CDMA) wireless interface protocol that was standardized in 1993 by the International Telecommunication Union (ITU). It is considered a second-generation (2G) mobile wireless technology. Today, there are two versions of IS-95, called IS-95A and IS-95B. The IS-95A protocol employs a 1.25-MHz carrier, operated in radio-frequency bands of either 800 MHz or 1.9 GHz, and supports data speeds of up to 14.4 Kbps. IS-95B can support data speeds of up to 115 kbps by bundling up to eight channels.
CDMA Repeater: A stand-alone device that receives CDMA signals and retransmits them at a higher power level for the purpose of improving coverage in focused areas like tunnels, indoor settings, dense urban sites, and sports stadiums.
Chirping: A less common form of spread spectrum employing a swept-frequency pulse, called a chirp, to spread the signal spectrum. Chirping is more commonly used in radar and ranging applications than in data communications.
Code: A binary bit stream. In spread spectrum, code refers to the pseudorandom sequence used to spread an information signal across a frequency band. It is more specifically referred to as a pseudonoise code.
Code Division Multiple Access (CDMA): CDMA technology exploits the orthogonality property of certain families of PN codes in order to increase channel capacity. Typically, each user is given a unique spreading code. To communicate with a particular user, the sender must use the same code assigned to that user. This technique permits many users to operate simultaneously over the same frequency band. Gold codes and Walsh codes are often used in CDMA systems.
Code Inversion Modulation: Also known as phase inversion modulation and bit inversion modulation, a popular means by which a binary data stream is modulated into a spread spectrum signal. In a direct sequence system, the data is modulo-2 added with the PN sequence prior to modulation of the carrier. In theory, this is equivalent to multiplying a PN-modulated PSK signal with the data. This is an important point to recognize, as it can be used in demonstrating the fact that multiplication of the received signal by the same PN sequence at the receiver will result in a data-modulated PSK signal, and the data can be recovered through standard PSK demodulation techniques.
Correlation: The process of synchronizing the phase of a local PN sequence within an SS radio receiver with the received PN sequence in order to despread and recover the narrowband data signal from a spread signal. Sometimes referred to as a despreading in direct sequence systems, or dehopping in frequency hopping systems.
Also, the process of determining the degree of cross-correlation, or similarity, between the two sequences.
Correlator: The SS radio receiver component that synchronizes the phase of a local PN sequence with the received PN sequence in order to despread and recover the narrowband data signal from a spread signal.. Sometimes referred to as a despreader in direct sequence systems, or dehopper in frequency hopping systems. A sliding correlator is a common type of correlator.
Also, a device or circuit that determines the degree of cross-correlation, or similarity, between the two sequences.
Cross-Correlation: The mathematically derived measure of similarity between two functions or signals. Cross-correlation also refers to the process of determining this similarity, and is accomplished by multiplying the two signals together and integrating the result over time. If the result is zero, the two signals are said to be uncorrelated, or orthogonal.
Delay-Locked Loop Tracker: A type of PN tracker where synchronization between the local PN sequence and the received PN sequence is maintained by measuring the cross-correlation levels between the received sequence and both an early and late version of the punctual (non-shifted) local sequence, and adjusting the phase of the local sequence such that the two cross-correlation levels are equal. The early sequence is always 1/2 chip early relative to the punctual sequence, and the late sequence is 1/2 chip late. Thus, maintaining equal cross-correlation levels ensures maximum correlation with the punctual sequence, since it is precisely in the middle. The delay-locked loop, or delay-lock loop, is sometimes called an early-late detection loop or early late gate synchronizer.
Direct Sequence Spread Spectrum (DSSS or DS): A modulation technique where a pseudorandom sequence directly phase modulates a (data-modulated) carrier, thereby increasing the bandwidth of the transmission and lowering the spectral power density (i.e. the power level at any given frequency). The resulting RF signal has a noise-like spectrum, and in fact can intentionally be made to look like noise to all but the intended radio receiver. The received signal is despread by correlating it with a local pseudorandom sequence identical to and in synchronization with the sequence used to spread the carrier at the radio transmitter.
Dual-Channel QPSK Modulation: A QPSK modulation scheme where the I (in-phase) channel of an RF signal is modulated by one PN code, the Q (quadrature-phase) channel of the signal is modulated by a second PN code, and where the I and Q channels are modulated by two distinct data sources. Compare this to balance QPSK modulation, where both channels are modulated by the same data source.
Eb: The energy of an information bit. Eb is expressed in Joules, or equivalently in Watts per Hertz.
Feedback Taps: The taps of a linear feedback shift register that are fed back to the input of the register. Also, a specification of which taps are fed back. The latter sense of the term is also known as feedback tap set or feedback pattern.
Fibonacci Form LFSR: A form of linear feedback shift register where multiple taps from the register are modulo-2 summed and the result fed back to the shift register's input. Also known as a simple shift register generator (SSRG). Compare this to the Galois form LFSR, where the shift register's output is fed back at multiple points along the shift register.
Frequency Hopping Spread Spectrum (FHSS or FH): A spread spectrum modulation technique whereby the radio transmitter frequency-hops from channel to channel in a predetermined but pseudorandom manner. The RF signal is dehopped at the radio receiver using a frequency synthesizer controlled by a pseudorandom sequence generator synchronized to the transmitter's pseudorandom sequence generator. A frequency hopper may be fast-hopped, where there are multiple hops per data bit, or slow-hopped, where there are multiple data bits per hop.
Galois Form LFSR: A form of linear feedback shift register where the shift register's output is fed back to multiple inputs along the shift register. At each of these inputs, the sequence being fed back is modulo-2 summed with the output of the prior register. Also known as a multiple-return shift register generator (MRSRG) or modular shift register generator (MSRG). Compare this to the Fibonacci form LFSR, where multiple taps of the shift register are modulo-2 summed and fed back to the input of the shift register.
Gold Code: One of a family of pseudonoise codes that exhibits minimal, well defined, cross-correlation levels with all other members of the family. This property is often exploited in CDMA spread spectrum systems. A Gold code is generated through modulo-2 addition of two PN codes of equal length. Distinct members of a Gold code family are determined by the chip (bit) offset of one code relative to the other. Selection of preferred pairs of PN codes, which results in optimal Gold code performance, has been thoroughly studied and documented. A balanced Gold code is one in which the number of ones exceeds the number of zeros by one, a trait shared by all m-sequences. An orthogonal Gold code is one in which an extra zero is appended to the end of the naturally-generated sequence in order to make the number of ones and zeros the same. Without this extra zero, the code would not be perfectly orthogonal with other members of the family.
GPS: Global Positioning System. Known also as NAVSTAR, a satellite-based radio positioning systems that provide 24-hour three-dimensional position, velocity and time information to suitably equipped users anywhere on or near the surface of the Earth, and sometimes off the earth. The system employs spread spectrum technology in a 24-satellite constellation, 20,000 Km above the earth in six orbital planes. NAVSTAR is operated by the U.S. Department of Defense, and was the first global positioning system widely available to civilian users.
ISM Band: Industry, Scientific and Medical frequency band, as designated by the FCC. Unlicensed 902 - 928 MHz, 2.4 - 2.4835 GHz and 5.725 - 5.850 GHz bands, with RF power up to 1 watt at the lower band. Frequency hopping, direct sequence, and other spread spectrum transmissions are allowed. The ISM band frequencies are often abbreviated as 902 MHz or 915 MHz, 2.4 GHz, and 5.7 GHz or 508 GHz, respectively.
Jam: To intentionally or maliciously interference with another radio signal.
Jammer: A device that transmits an energetic RF signal with the intention of interfering with another radio signal.
JPL Code: Named after Jet Propulsion Laboratories, where it was invented, a pseudonoise code generated through the modulo-2 addition of two PN codes of differing lengths. (Compare this to Gold codes, where the two summed codes are of identical length.) Certain properties of JPL codes can be exploited to attain fast acquisition at the radio receiver.
Kasami Code: Kasami codes are similar to Gold codes in that they are produced by exclusive-ORing two distinct sequences. The twist in the case of Kasami codes is that both these sequences are produced by a single linear feedback shift register. One sequence is the output of the LFSR, whereas the other is derived from the first by decimating it by a factor of N, and then repeating it N times. For example, if the original sequence is 15 chips long, and it is decimated it by a factor of 5, this results in 3 chips (every fifth chip). Then these three chip are repeated 5 times to produce the second sequence of 15 chips. Finally, this sequence is exclusive-ORed with the original to obtain the Kasami sequence.
Linear Feedback Shift Register (LFSR): A logic shift register using feedback and XOR (exclusive-or, or modulo-2 addition) elements that produces linear recursive sequences. Two practical implementations of LFSR are the Fibonacci form and Galois form.
Linear Recursive Sequence (LRS): A periodic sequence of bits generated through the use of a logic shift register with linear feedback, known as a linear feedback shift register. The most common type of sequence used in spread spectrum systems. Given a proper set of feedback taps, the sequence produced can be of maximal length and have certain desirable properties. Such a sequence is referred to as an m-sequence.
Local Area Network (LAN): Relatively small (building-wide) network of computers connected together via transmission cable and using one of various RF communication protocols.
Low Probability of Intercept (LPI): The property of a transmitter which, because of its low power, high directivity, frequency variability, or other design features, is difficult to detect or identify. In the case of spread spectrum, LPI is achieved either through the lowering of the power spectrum at any given frequency by means of spectrum spreading, or through the frequency agility provided by frequency hopping.
Low Probability of Intercept Radar(LPIR): A radar system which, because of its low peak power output, the way in which it is operated, or other design features, is difficult to detect or identify. In the case of spread spectrum, LPI is achieved either through the lowering of the power spectrum at any given frequency by means of spectrum spreading, or through the frequency agility provided by frequency hopping.
Maximal Length Sequence (M-Sequence, MLS): A linear recursive sequence of period 2n-1 chips (bits), where n is the number of stages in the linear feedback shift register generating the sequence. Since this constitutes every possible state of the register, it is the longest sequence that can be generated. Only certain combinations of feedback taps will produce an m-sequence, also referred to as a maximal sequence. M-sequences, also known as pseudonoise (PN) sequences and pseudorandom bit sequences (PRBS), have favorable noise-like properties that make them particularly useful in spread spectrum applications.
Multipath: The presence of multiple copies of a single RF signal arriving at a radio receiver's antenna simultaneously. Signals that are in phase will add to one another, and signals that are out of phase will cancel.
Multipath Fading: Multipath fading, a.k.a. Rayleigh fading, occurs when a direct-path transmitted wave destructively interferes with reflections of itself at the receiving end. The destructive interference is a result of the reflected waves arriving at the receiving end out of phase with the direct-path transmitted wave. Multipath interference can vary in intensity depending on the amount of destructive interference that takes place.
No: The amount of noise energy accumulated over one period of an information bit. No is expressed in Joules, or equivalently in Watts per Hertz.
OQPSK Modulation: Offset Quadriphase Shift Keying. Similar to QPSK, but with an initial phase offset, of usually 45 degrees, in one of its two binary channels. As a result, the phase never jumps by more than 90 degrees at any given data transition. OQPSK, also known as staggered QPSK (SQPSK), has a lower envelope modulation than does QPSK.
Orthogonal Code: A PN code is said to be orthogonal with another if their cross-correlation, a mathematical measure of similarity, is zero. Orthogonality ensures that the two codes will not interfere with one another when present on the same communication channel.
Orthogonality: A property exhibited between two PN codes whose cross-correlation, a mathematical measure of similarity, is zero. Orthogonality ensures that the two codes will not interfere with one another when present on the same communication channel.
Part 15 Rules: That part of the Federal Communication Commission's (FCC) regulations which regulates unlicensed use of the ISM bands for wireless networking and other uses, and that includes spread spectrum in certain bands.
PCN: Personal Communication Network. PCNs are usually short range (100s of feet to 1 mile or so) and involve cellular radio type architecture, sometimes utilizing spread spectrum. Services include digital voice, FAX, mobile data and national/international data communications.
PCS: Personal Communication System (or Services). Usually associated with cordless telephone-like devices, and personal data assistant devices. These services are typically digital and often employ spread spectrum technologies. Within the U.S., the 1.9 GHz band has been allocated for PCS systems; the allocated spectrum is 120 MHz wide and is licensed as two 30 MHz segments for the 51 major trading areas, and three 10 MHz segments for the 493 basic trading areas.
Processing Gain: Also known as process gain, the ratio of the bandwidth of a spread spectrum signal to the data rate of the information signal being spread. As a rule-of-thumb, this ratio determines the level of interference rejection exhibited by the system, and thus the anti-jam performance.
Pseudonoise Code (PN Code): Also called pseudonoise (PN) sequence, any of a group of binary sequences that exhibit random noise-like properties. PN sequences are distinguishable from truly random sequences in that they inherently or deliberately exhibit periodicity (i.e. they repeat). An integral part of all spread spectrum systems, PN sequences are usually generated using a liner feedback shift register. Often spelled with a hyphen: "pseudo-noise code."
In the strict sense, pseudonoise sequence and pseudorandom sequence are synonymous with maximal sequence. However, the terms are often used informally to include both maximal and nonmaximal sequences.
Pseudonoise (PN) Code Generator: Also called a pseudonoise sequence generator or pseudorandom sequence generator, a hardware or software device that generates a pseudonoise code. Often implemented in the form of a linear feedback shift register.
Pseudorandom Sequence Generator: Same as pseudonoise code generator.. Commonly spelled with a hyphen: "pseudo-random sequence generator." Also known as pseudorandom bit sequence generator and pseudorandom binary sequence generator.
QPSK Modulation: Quadriphase shift keying. Modulation of an RF carrier via phase shifting, usually at 0, 90, 180, and 270 degrees. Also known as quaternary phase shift keying.
Rake Receiver: A receiver technique which exploits multipath phenomenon to improve system performance. Multiple baseband correlators are used to individually process multiple multipath components. The correlator outputs are then added to increase total signal strength.
Sinc Function: Defined as sin(x)/x, the sinc function is mathematically equivalent to the Fourier transform of a rectangular function. Consequently, a rectangular pulse in the time domain appears as a sinc function in the frequency domain. Accordingly, in digital radio communications where rectangular waveforms dominate, sinc-like power spectra are observed. However, in an effort to prevent the sinc function's sidelobes from interfering with neighboring frequency bands, pulse shaping is usually performed in an effort to attenuate all but the central, or main, lobe of the function. In the time domain, this appears as a smoothing or rounding of the discontinuous edges of the pulse.
Signal-to-Noise Ratio (SNR or S/N Ratio): The dimensionless ratio Eb/(No+Io), or bit energy divided by the noise-plus-interference energy accumulated over one bit period. Usually expressed in dB.
Sliding Correlator: A simple type of PN correlator where the local PN sequence in an SS radio receiver is slid relative to the received PN sequence until the phase of the two sequences match. Sliding is usually done in discrete steps so the cross-correlation, or similarity, between the two sequences can be measured before the local sequence is again slid.
Spectrum Spreading: The act of spreading the bandwidth of an information signal to be transmitted to a remote radio receiver. The receiver despreads the transmission to recover the original information signal.
Spread Spectrum (SS): A wideband modulation technique which imparts noise-like characteristics to an RF signal. This communication technique spreads a signal over a wide range of frequencies for transmission and then despreads it to the original data bandwidth at the radio receiver. Spread spectrum's advantages and properties include low probability of intercept, antijam capabilities, CDMA multiplexing, and FCC Part 15 license free operation.
Spread Spectrum Generator: A laboratory instrument used to generate or simulate spread spectrum signals. Spread spectrum generators are used in both the development and testing of spread spectrum systems, such as CDMA, PCS, cellular, and wireless LAN systems.
Spreading Code: Any pseudonoise code used to spread the data signal's frequency spectrum within a direct sequence spread spectrum system. Spreading codes are usually generated with a linear feedback shift register.
Synchronization: The process within a spread spectrum radio of maintaining alignment between the local PN sequence and the received PN sequence. Synchronization is broken down into two steps: initial acquisition followed by tracking.
Synchronizer: The component within a spread spectrum radio that maintains alignment between the local PN sequence and the received PN sequence. Synchronization is broken down into two steps: initial acquisition followed by tracking.
Tau-Dither Tracker: A type of PN tracker where synchronization between the local PN sequence and the received PN sequence is maintained by intentionally shifting, or dithering, the local sequence back and forth relative to the received sequence by a small amount, and measuring the change in their cross-correlation level, for the purpose of maximizing the level.
Time Division Multiple Access (TDMA): A method of digital multiplexing whereby each signal is sent and received at predesignated time slots, in a series of time slots shared by multiple signals. The radio transmitter and receiver must be time-synchronized. Public telephone networks typically use TDMA.
Tracking: After initial acquisition, the process of maintaining alignment of the local PN sequence of an SS radio receiver relative to the corresponding sequence received from the radio transmitter, in order to despread the spread signal.
Truncated Code: A PN sequence that is intentionally truncated and restarted after N chips, where N is shorter than the natural length of the sequence. Compare this to an appended code, where the sequence is truncated long of the natural sequence length.
Walsh Code: One of a group of specialized PN codes having good autocorrelation properties but poor cross-correlation properties. Walsh codes are the backbone of the CDMA One and cdma2000 cellular systems, and are used to support the individual channels used simultaneously within a cell. Walsh codes are generated in firmware by applying the Hadamard transform on 0 repeatedly.
Wideband CDMA (WCDMA or W-CDMA): A form of CDMA technology where the bandwidth is appreciably greater than that provided by the digital cellular systems introduced in the 1990s. The bandwidth of WCDMA, which will be the standard for third-generation (3G) cellular systems of the early 2000s, is expected to be around 5 MHz.
Wireless: An all-encompassing buzzword which describes what traditionally has been called "radio", but which typically also implies inclusion of some of the newer cellular or digital radio technologies, including spread spectrum.
Wireless Local Area Network (WLAN or W-LAN): A short range computer-to-computer wireless data communications network. In the United States, operation is in the 2.4 GHz and 5.8 GHz unlicensed ISM bands using spread spectrum technology.