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Quick Reference Guide


System Description

This provides an introduction to Code Division Multiple Access (CDMA) communications, covering a Radio Carrier Station (RCS) and a Fixed Subscriber Unit (FSU).

This introduction to CDMA proceeds heuristically, we use very little mathematics in developing the theories, and do not assume a deep mathematical or engineering background. If you would like further information on the math and communication theories behind this introduction, please consult the following references:

Viterbi, A. CDMA: Principles of Spread Spectrum Communication Addison-Wesley Wireless Communications Series, 1995

Pickholtz, R. L., Schilling, D. L., and Milstein, L. B. “Theory of Spread-Spectrum Communications—A Tutorial” IEEE Trans. Commun., vol. COM30, no. 5, May 1982, pp 855-884.

Pickholtz, R. L., Schilling, D. L., and Milstein, L. B. Revisions to “Theory of Spread-Spectrum Communications—A Tutorial” IEEE Trans. Commun., vol. COM32, no. 2, Feb 1984, pp 211-212.


Introduction to Spread Spectrum Communications

CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements:

1. The signal occupies a bandwidth much greater than that which is necessary to send the information. This results in many benefits, such as immunity to interference and jamming and multi-user access, which we’ll discuss later on.

2. The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes in which the data modulation will always spread the spectrum somewhat.

3. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.

In order to protect the signal, the code used is pseudo-random. It appears random, but is actually deterministic, so that the receiver can reconstruct the code for synchronous detection. This pseudo-random code is also called pseudo-noise (PN).

700132

Figure 1. Direct Sequence Spread Spectrum System

Three Types of Spread Spectrum Communications

There are three ways to spread the bandwidth of the signal:

Direct Sequence Spread Spectrum

CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on 64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels, modem data, etc.

Figure 1 shows a simplified Direct Sequence Spread Spectrum system. For clarity, the figure shows one channel operating in one direction only.

Signal transmission consists of the following steps:

1. A pseudo-random code is generated, different for each channel and each successive connection.

2. The Information data modulates the pseudo-random code (the Information data is “spread”).

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and broadcast.

Signal reception consists of the following steps:

1. The carrier is received and amplified.

2. The received signal is mixed with a local carrier to recover the spread digital signal.

3. A pseudo-random code is generated, matching the anticipated signal.

4. The receiver acquires the received code and phase locks its own code to it.

5. The received signal is correlated with the generated code, extracting the Information data.


Implementing CDMA Technology

The following sections describe how a system might implement the steps illustrated in Figure 1.

Input data

CDMA works on Information data from several possible sources, such as digitized voice or ISDN channels. Data rates can vary, here are some examples:

Data Source

Data Rate

Voice

Pulse Code Modulation (PCM)

64 kBits/sec

 

Adaptive Differential Pulse Code Modulation (ADPCM)

32 kBits/sec

 

Low Delay Code Excited Linear Prediction (LD-CELP)

16 kBits/sec

ISDN

Bearer Channel (B-Channel)

64 kBits/sec

 

Data Channel (D-Channel)

16 kBits/sec

The system works with 64 kBits/sec data, but can accept input rates of 8, 16, 32, or 64 kBits/sec. Inputs of less than 64 kBits/sec are padded with extra bits to bring them up to 64 kBits/sec.

For inputs of 8, 16, 32, or 64 kBits/sec, the system applies Forward Error Correction (FEC) coding, which doubles the bit rate, up to 128 kbits/sec. The Complex Modulation scheme (which we’ll discuss in more detail later), transmits two bits at a time, in two bit symbols. For inputs of less than 64 kbits/sec, each symbol is repeated to bring the transmission rate up to 64 kilosymbols/sec. Each component of the complex signal carries one bit of the two bit symbol, at 64 kBits/sec, as shown below.


Generating Pseudo-Random Codes

For each channel the base station generates a unique code that changes for every connection. The base station adds together all the coded transmissions for every subscriber. The subscriber unit correctly generates its own matching code and uses it to extract the appropriate signals. Note that each subscriber uses several independant channels.

In order for all this to occur, the pseudo-random code must have the following properties:

1. It must be deterministic. The subscriber station must be able to independently generate the code that matches the base station code.

2. It must appear random to a listener without prior knowledge of the code (i.e. it has the statistical properties of sampled white noise).

3. The cross-correlation between any two codes must be small (see below for more information on code correlation).

4. The code must have a long period (i.e. a long time before the code repeats itself).

Code Correlation

In this context, correlation has a specific mathematical meaning. In general the correlation function has these properties:

Intermediate values indicate how much the codes have in common. The more they have in common, the harder it is for the receiver to extract the appropriate signal.

There are two correlation functions:

The receiver uses cross-correlation to separate the appropriate signal from signals meant for other receivers, and auto-correlation to reject multi-path interference.

700133

Figure 2a. Pseudo-Noise Spreading

700134

Figure 2b. Frequency Spreading

Pseudo-Noise Spreading

The FEC coded Information data modulates the pseudo-random code, as shown in Figure 2a. Some terminology related to the pseudo-random code:

Figure 2b shows the process of frequency spreading. In general, the bandwidth of a digital signal is twice its bit rate. The bandwidths of the information data (fi) and the PN code are shown together. The bandwidth of the combination of the two, for fc>fi, can be approximated by the bandwidth of the PN code.

Processing Gain

An important concept relating to the bandwidth is the processing gain (Gp). This is a theoretical system gain that reflects the relative advantage that frequency spreading provides. The processing gain is equal to the ratio of the chipping frequency to the data frequency:

There are two major benefits from high processing gain:

So the higher the PN code bit rate (the wider the CDMA bandwidth), the better the system performance.

700143

Figure 3a. Complex Modulator

700135

Figure 3b. Complex Modulation

Transmitting Data

The resultant coded signal next modulates an RF carrier for transmission using Quadrature Phase Shift Keying (QPSK). QPSK uses four different states to encode each symbol. The four states are phase shifts of the carrier spaced 90_ apart. By convention, the phase shifts are 45, 135, 225, and 315 degrees. Since there are four possible states used to encode binary information, each state represents two bits. This two bit “word” is called a symbol. Figure 3 shows in general how QPSK works.

First, we’ll discuss Complex Modulation in general, applying it to a single channel with no PN-coding (that is, we’ll show how Complex Modulation would work directly on the symbols). Then we’ll discuss how we apply it to a multi-channel, PN-coded, system.

Complex Modulation

Algebraically, a carrier wave with an applied phase shift, Y(t), can be expressed as a sum of two components, a Cosine wave and a Sine wave, as:

I(t) is called the real, or In-phase, component of the data, and Q(t) is called the imaginary, or Quadrature-phase, component of the data. We end up with two Binary PSK waves superimposed. These are easier to modulate and later demodulate.

This is not only an algebraic identity, but also forms the basis for the actual modulation/demodulation scheme. The transmitter generates two carrier waves of the same frequency, a sine and cosine. I(t) and Q(t) are binary, modulating each component by phase shifting it either 0 or 180 degrees. Both components are then summed together. Since I(t) and Q(t) are binary, we’ll refer to them as simply I and Q.

The receiver generates the two reference waves, and demodulates each component. It is easier to detect 180_ phase shifts than 90_ phase shifts. The following table summarizes this modulation scheme. Note that I and Q are normalized to 1.

Symbol

I

Q

Phase shift

00

+1

+1

45

01

+1

-1

315

10

-1

+1

135

11

-1

-1

225

For Digital Signal Processing, the two-bit symbols are considered to be complex
numbers, I +jQ.

Working with Complex Data

In order to make full use of the efficiency of Digital Signal Processing, the conversion of the Information data into complex symbols occurs before the modulation. The system generates complex PN codes made up of 2 independent components, PNi +jPNq. To spread the Information data the system performs complex multiplication between the complex PN codes and the complex data.

Summing Many Channels Together

Many channels are added together and transmitted simultaneously. This addition happens digitally at the chip rate. Remember, there are millions of chips in each symbol. For clarity, let’s say each chip is represented by an 8 bit word (it’s slightly more complicated than that, but those details are beyond the scope of this discussion).

At the Chip Rate

For each component (I or Q):

Since I and Q are no longer limited to 1 or -1, the phase shift of the composite carrier is not limited to the four states, the phase and amplitude vary as

A2 = I2 + Q2

Tan((Y) = Q/I

At the Symbol Rate

Since the PN-code has the statistical properties of random noise, it averages to zero over long periods of time (such as the symbol period). Therefore, fluctuations in I and Q, and hence the phase modulation of the carrier, that occur at the chip frequency, average to zero. Over the symbol period the modulation averages to one of the four states of QPSK, which determine what the symbol is.

The symbol only sees the QPSK, and obeys all the statistical properties of QPSK transmission, including Bit Error Rate.

Receiving Data

The receiver performs the following steps to extract the Information:

Demodulation

The receiver generates two reference waves, a Cosine wave and a Sine wave. Separately mixing each with the received carrier, the receiver extracts I(t) and Q(t). Analog to Digital converters restore the 8-bit words representing the I and Q chips.

Code Acquisition and Lock

The receiver, as described earlier, generates its own complex PN code that matches the code generated by the transmitter. However, the local code must be phase-locked to the encoded data. The RCS and FSU each have different ways of acquiring and locking onto the other’s transmitted code. Each method will be covered in more detail in later sections.

Correlation and Data Despreading

Once the PN code is phase-locked to the pilot, the received signal is sent to a correlator that multiplies it with the complex PN code, extracting the I and Q data meant for that receiver. The receiver reconstructs the Information data from the I and Q data.


Automatic Power Control

The RCS gets bombarded by signals from many FSUs. Some of these FSUs are close and their signals are much stronger than FSUs farther away. This results in the Near/Far problem inherent in CDMA communications. System Capacity is also dependant on signal power. For these reasons, both the RCS and FSU measure the received power and send signals to control the other’s transmit power.

Near/Far Problem

Because the cross-correlation between two PN codes is not exactly equal to zero, the system must overcome what we call the Near/Far problem.

The output of the correlator consists of two components:

Mathematically, if we are trying to decode the kth signal, we have:

Where:

Aj is the amplitude of the jth signal,
rjk is the cross-correlation between the kth and jth signal, and
S is the sum over all the j signals (excluding k).

Since the cross-correlation is small (ideally, it is zero), the sum of cross-correlation terms should be much less than the amplitude of the desired signal. However, if the desired signal is broadcast from far away, and undesired signals are broadcast from much closer, the desired signal may be so small as to be drowned out by the cross-correlation terms.

Note that this problem only exists in the reverse direction. The RCS is receiving signals from many FSUs at different distances, but the FSU is receiving all signals from one RCS. The RCS controls the power of each FSU so that the signals received from all FSUs are the same strength.

System Capacity

The capacity of a system is approximated by:

where:

is the maximum number of simultaneous calls
is the processing gain
is the total signal to noise ratio per bit, and
is the inter-cell interference factor.

Notice, as we said earlier, the capacity is directly proportional to the processing gain. Capacity is also inversely proportional to the signal to noise ratio of the received signal. So, the smaller the transmitted signal, the larger the system capacity (as long as the receiver can detect the signal in the noise!). Both the RCS and FSU control the power transmitted by the other so that the received signal is as small as possible while maintaining a minimum signal to noise ratio. This maximizes system capacity.

700137

Figure 4. Multi-Path Interference Rejection


Interference Rejection

CDMA technology is inherently resistant to interference and jamming. A common problem with urban communications is multi-path interference.

Multi-path interference is caused by the broadcast signal traveling over different paths to reach the receiver. The receiver then has to recover the signal combined with echoes of varying amplitude and phase. This results in two types of interference:

Combating Interference

Two methods are commonly used to combat multi-path interference:


System Operation

The following sections describe a hypothetical implmentation of CDMA technology. A connection can be one of many types of data, but for simplicity we will refer to any connection as a “call”.

These sections cover the following system states:

But first, in order to understand system operation, you must understand the Pilot codes and communication channels the system uses.

Pilot Codes

At each phase of operation, the system broadcasts pilot signals. These pilot signals are the unmodulated PN codes associated with each channel, used to synchronize and track the locally generated PN codes for despreading. The system uses the following pilot signals.

Communication Channels

In order to understand system operation, we need to introduce the system communication channels. The system has the following channel groups:

Each logical channel in each group is realized by assigning a unique PN code to it.

Channel Group

Channel Name

Direction

Number of Channels

Description

Broadcast

Global Pilot

F

One

An unmodulated PN code that the FSU can synchronize to.

 

Fast Broadcast Channel

F

One

A single message indicating which services and access channels are available. This information may change rapidly.

 

Slow Broadcast Channel

F

One

Paging messages and other system information that does not need to be updated rapidly.

Call Setup

Short Pilot

R

Four

Alerts the RCS that an FSU is requesting access.

 

Long Pilot

 

Four

Allows the RCS to synchronize to the FSU to setup a call.

 

Access Channel

R

Four

Used by the FSUs to access an RCS and get assigned channels.

 

Control Channel

F

Four

Used by the RCS to reply to access attempts from FSUs.

 

Control Channel APC

F

Four

Controls FSU power during initial access.

Assigned

Assigned Pilot

R

One per FSU

An unmodulated PN code that the RCS can synchronize to.

 

APC Channel

F

One per FSU

Controls FSU power during call.

   

R

 

Controls RCS power of assigned FSU channels.

 

Traffic Channels

F

Up to 3 per FSU

Signal data from RCS to FSU.

   

R

 

Signal data from FSU to RCS.

 

Order wire

F

One per FSU

Control signals: CDMA and Telco messages.

   

R

   

Note on Direction: F - Forward - From RCS to FSU

R - Reverse - From FSU to RCS

Pilot Ramp Up

When the FSU transmits its Short and Long Access Pilots, it ramps the power up to determine what power level it should transmit. When the RCS detects the Short Access Pilot, it acknowledges over the Fast Broadcast Channel. The FSU then knows that it is being received, and switches to the Long Access Pilot code. The Long Access Pilot code ramps up more slowly, until the RCS locks and starts transmitting Automatic Power Control signals.

System Idle

On startup, the RCS places one of its modems in broadcast mode, in which state it broadcasts the following Global Channels continuously:

In addition, the RCS sets aside 4 modems for Call Setup channels. These modems continuously listen for access attempts by the FSUs. We’ll discuss the operation of the modems in more detail later.

Paging Groups and Sleep Cycles

The RCS divides all the FSUs associated with it into paging groups. The RCS assigns each paging group a particular time slot on its Slow Broadcast Channel (the first time slot is reserved for general Slow Broadcast information). When the RCS pages an FSU, the RCS will only page it during the time slot of that FSU’s paging group.

The Slow Broadcast Channel cycles through all the paging groups. The cycle takes approximately one second to complete. Each FSU remains powered down for most of the cycle. When the Slow Broadcast Channel reaches the time slot of the FSU’s paging group, the FSU powers up, synchronizes to the Global Pilot, and checks for its address in the paging group. If it recognizes its paging address, it requests access; if not, it powers down. This results in a duty cycle of less than 10%, and saves considerable power at the FSU.

700142

Figure 5. Call Setup

Call Setup

Two events can initiate a call:

Once either of these events occur, call setup proceeds as follows:

1. FSU requests access.

2. RCS assigns channel group to FSU.

Note that the RCS now tracks the Assigned Pilot; the FSU continues to track the Global Pilot.

Call Processing

Call processing puts together everything we’ve covered so far. There are slight differences in the way the RCS and FSU process calls, so we will cover both the Forward link (RCS to FSU) and Reverse link (FSU to RCS). Note that the system uses Frequency Division Duplexing for the Forward and Reverse links: they transmit over different frequencies.

In the forward direction, the RCS:

1. Generates CDMA data signal for each traffic channel:

2. Generates other signal channels:

3. Adds all signals together:

4. Adds together the signals for all currently active FSUs.

5. Modulates and transmits carriers

The FSU:

1. Extracts the I and Q data:

2. Filters the I and Q data:

3. Extracts the CDMA data signal for each traffic channel:

In the reverse direction, the FSU:

1. Generates CDMA data signal for each traffic channel:

2. Generates other signal channels:

3. Adds all signals together:

4. Passes the signal through a pulse shaping digital filter.

5. Modulates and transmits carriers

The RCS:

1. Extracts the I and Q data:

2. Filters the I and Q data:

3. Extracts the CDMA data signal for each traffic channel, for each subscriber connection:

Call Teardown

An on-hook signal causes the RCS to release the resources, and the FSU returns to its idle state.



Quick Reference Guide


A - C D - F G - I J - L
M - O P - R S - U V - Z


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.

AJ: See Antijam.

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:

R2: 10 (or 11)
R3: 110
R4: 1011 or (1001)
R5: 11101
R7: 1110010
R11: 11100010010
R13: 1111100110101

BER: See Bit Error Rate.

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.

Bit Inversion Modulation: Same as code inversion modulation.

BPSK Modulation: Biphase shift keying. Modulation of an RF carrier via phase shifting, usually at 0 and 180 degrees.

CDMA: The term CDMA.refers either to the generic form of code division multiple access, or to one of the practical forms of CDMA in use today, particularly cdma2000 and CDMA One.

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.

Chip: A single bit of a pseudonoise sequence.

Chip Rate: The rate at which bits of a pseudonoise sequence are shifted, expressed in Hz. Also known as spread rate.

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.

Code Orthogonality: See Orthogonality.

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.

Dehopper: See Correlator. Often spelled with a hyphen: "de-hopper."

Dehopping: See Correlation. Often spelled with a hyphen: "de-hopping."

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.

Despreader: See Correlator. Often spelled with a hyphen: "de-spreader."

Despreading: See Correlation. Often spelled with a hyphen: "de-spreading."

Direct Sequence CDMA (DS-CDMA): The most prevalent form of code division multiple access, employing direct sequence spectrum spreading..

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.

Direct Spread Modulation: Same as direct sequence spread spectrum.

DS or DSSS: See Direct Sequence Spread Spectrum.

DS-CDMA: See Direct Sequence CDMA.

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.

Epoch: A strobe signal which indicates when a pseudonoise sequence repeats.

FCC Part 15 Rules: See Part 15 Rules.

Feedback Pattern: See Feedback Taps.

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.

FH or FHSS: See Frequency Hopping Spread Spectrum.

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.

GMSK Modulation: Gaussian minimum shift keying. A form of MSK where the shaping function is bell shaped ("normal" curve).

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.

Initial Fill: The initial content of a linear feedback shift register, or other PN sequence generating device. Also known as the preset code.

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.

Jammer-to-Signal Ratio (JSR or J/S Ratio): The dimensionless ratio of the jammer signal received to the signal-of-interest (SOI) received, over the SOI bandwidth. Usually expressed in dB.

Jamming: The typically intentional or malicious interference with another radio signal. Spread spectrum transmissions inherently attenuate jamming signals. See Antijam.

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.

JSR or J/S Ratio: See Jammer-to-Signal Ratio.

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.

LAN: See Local Area Network.

LFSR: See Linear Feedback Shift Register.

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.

LPI: See Low Probability of Intercept.

LPIR: See Low Probability of Intercept Radar.

LRS: See Linear Recursive Sequence.

M-Sequence: See Maximal Length Sequence.

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.

Maximal Sequence: Same as maximal length sequence.

MLS: See Maximal Length Sequence.

Modular Shift Register Generator (MSRG): Same as Galois form LFSR.

MRSRG: See Multiple-Return Shift Register Generator.

MSK Modulation: Minimum shift keying. A modulation technique which uses waveform shaping to significantly lower the main sidelobes. MSK can be considered a form of OQPSK.

MSRG: See Modular Shift Register Generator.

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.

Multiple-Return Shift Register Generator (MRSRG): Same as Galois form LFSR.

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.

Phase Inversion Modulation: Same as code inversion modulation.

PN Acquisition: See Acquisition.

PN Code: See Pseudonoise Code.

PN Sequence: See Pseudonoise Sequence.

PN Correlation: See Correlation.

PN Correlator: See Correlator.

PN Synchronization: See Synchronization.

PN Synchronizer: See Synchronizer.

PN Tracker: See Tracker.

PN Tracking: See Tracking.

PNG: Pseudonoise generator. Same as pseudonoise code generator.

PRBS: See Pseudorandom Bit Sequence.

PRG: Pseudorandom generator. Same as pseudonoise code generator.

PRN: Pseudorandom noise. Same as pseudonoise code.

Preset Code: See Initial Fill.

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.

Pseudonoise Sequence (PN Sequence): Same as pseudonoise code. Often spelled with a hyphen: "pseudo-noise sequence."

Pseudonoise Sequence Generator: Same as pseudonoise code generator.. Commonly spelled with a hyphen: "pseudo-noise sequence generator."

Pseudorandom Bit Sequence (PRBS): Same as pseudonoise code. Often spelled with a hyphen: "pseudo-random bit sequence." Also known as pseudorandom bit stream and pseudorandom binary sequence.

Pseudorandom Sequence: Same as pseudorandom bit sequence. Often spelled with a hyphen: "pseudo-random sequence."

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.

PSK Modulation: Phase shift keying. Modulation of an RF carrier via phase shifting. Binary PSK, quaternary PSK, and offset-quaternary PSK are three common forms of PSK.

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.

Simple Shift Register Generator (SSRG): Same as Fibonacci form LFSR.

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.

SNR or S/N Ratio: See Signal-to-Noise Ratio.

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: See Spectrum Spreading.

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.

SQPSK Modulation: Staggered quadriphase shift keying. Same as OQPSK.

SS: See Spread Spectrum.

SSRG: See Simple Shift Register Generator.

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.

TDMA: See Time Division Multiple Access.

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.

Tracker: The component of an SS radio receiver which tracks the received PN sequence. (See Tracking.) Tau-dither trackers and delay-locked loop trackers are two common types of tracker.

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.

WAN: See Wide Area Network.

WCDMA or W-CDMA: See Wideband CDMA.

Wide Area Network (WAN): Large network formed by bridging smaller LANs or using dial-up lines. WANs can span the globe.

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.

WLAN or W-LAN: See Wireless Local Area Network.