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On our support page, you can always find Account problems, Case Tutorials, Software FAQ and many other information that may be useful to you. Professional support Our support team is ready to help you with any questions or suggests you may have. The Secret Communication System used synchronized paper tapes to perform frequency hopping to prevent guidance signals to the torpedo from being disrupted. The heart of the system was the synchronized paper tapes. These paper tapes would automatically change the frequency of the transmitter and receiver so that an enemy could not detect and lock onto the signal.

In , Lamarr and Antheil patented their idea and offered it to the Navy for free. The Navy could not comprehend the concept and declined the offer. Neither Lamarr nor Antheil pursued the idea any further and the concept of spread spectrum was lost until it appeared in equipment used during the Cuban Missile Crisis in By then, the exclusive rights to the patent had expired and neither of its inventors received money for spread spectrum.

Introduction CDMA uses a modulation technique called spread spectrum to transport a narrowband voice signal over a wide bandwidth channel. The wide bandwidth for IS is 1. Spreading In a spread spectrum system, the data information signal, b t , is multiplied by a wideband signal, c t , which is the output signal of the Direct Sequence DS generator: A pseudorandom noise PN output signal. Note that Tb is the bit interval of the information stream, and Tc is the bit interval of the DS stream.

Tc is also called a chip time. It should also be noted that the ratio of Tb to Tc is referred to as the processing gain. Spreading When two signals, b t and c t , are multiplied together, the resulting signal, b t c t , will have the same bit or chip period as the faster signal wider bandwidth ; in this case, c t.

The signal b t can be seen as altering the phase of the spreading signal c t. When c t is faster, y t contains all the information of b t , and has the faster bit rate and its correspondingly wider spectrum. In addition to being scrambled, b t is said to have had its spectrum spread. The spectrum of the signal is unchanged, and the incoming bit stream is said to be encrypted or scrambled. The terms chips and bits both refer to ones and zeroes, but they have slightly different meaning.

When talking about the digital signal that is spread over a wide bandwith signal, b t in the example, the ones and zeroes are typically called bits. The ones and zeroes of the digital signal that is being used to spread the information signal, c t in the example, are typically called chips.

To despread a received signal, b t c t , the signal is multiplied with an exact replica of the original spreading code, c t. Also, if signal propagation delays the output b t c t by some propagation time, the second occurrence of c t must be delayed by the same amount synchronization!

Why It Works! Multiplying with another code would not yield the same result 0 1 0 0 1 t. Hence, c t c t is an identity operation producing b t. Note: One c t accompanies signal transmission and sees transmission delay. The other c t is inserted at the receiver with bit boundaries aligned to the first i.

The graphic illustrates the integrate and dump process when the received chip-stream is error-free. If the received chip-stream consists of chips in error, the bit may still be detected. If a bit is received in error, higher level error-correction algorithms may detect and correct the bad bit. Linear Summation When transmitting multiple information signals at the same time, linear summation is used.

Every chip magnitude, voltage electrical field strength , is summed up. When multiple information signals, or channels, are transmitted simultaneously, their bit streams are summarized together in a linear fashion. The graphic illustrates the concept by summarizing the three signals electrical field strengths to yield a composite bit stream with varying magnitude.

When despreading the received signal the noise component will be, or continue to be, spread over the wide bandwidth spreading signal. If a low-pass filter is tuned to filter out everything except the narrowband signal, b t , the result will be a signal with a certain bit energy, Eb, for b t and a narrowband noise component, filtered N t c t , with an energy of NT or N0.

Spreading gain or processing gain is achieved when noise components, or noise-like components, remain spread when the original signal user 1 in the figure is despread. The original signal appears to have gained energy relative the noise. It can also be seen as if the noise has been suppressed. G is then called spreading gain or processing gain. Processing gain can also be seen as the number of chips per bit.

After the received CDMA signal has been despread, the resulting signal consists of a relatively narrow-band information energy and a wide-band suppressed noise energy. When passing the despread signal through a low-pass filter, the majority of the noise energy is removed, and the resulting signal consists of a narrow-band information energy Eb and a narrow-band noise energy Nt.

The difference is in what channels we are referring to, and whether the discussion is about bits or chips. When talking about the digital signal that is spread over a wide bandwith signal, the 1s and 0s are typically called bits. The 1s and 0s of the digital signal that are being used to spread the information signal are typically called chips. The pilot channel in a CDMA system is a non-spread signal bandwidth 1.

I0 normally refers to the interference level. In a practical CDMA system, the generated interference energy is much greater than the thermal noise energy; therefore, the thermal noise may be ignored.

Every user and channel in a CDMA system will have their own unique spreading code, c t. Thus, if the receiver despreads and extracts the signal for user 1, all the other users user 2, 3, , M will appear as noise or interference to user 1. In other words, the more users there are on the CDMA system, the more noise the receiver experiences. This is called noise rise and is one of the core concepts of CDMA. Reverse link loading or sector loading is a measure of the total interference from CDMA sources allowed in the system in reference to the receiver thermal noise.

As the number of users in the system increases, the noise rise increases. The noise rise increases dramatically as the loading approached the pole capacity. This noise rise is also driven by the loading of neighboring cells frequency re-use efficiency and the information data rate. Demod 1. Transmit Low bit rate speech, b t , is spread by multiplying it with a high bit rate PN pseudorandom noise code, c t.

The spread signal, b t c t , is modulated by multiplication with an RF carrier and transmitted. Receive The received signal is delayed seconds and is demodulated by multiplication with the RF carrier. The despread signal is detected by a bit detector an integrate and dump lasting Tb seconds to obtain the original digital speech. Summary When performing DS spreading, the information signal bit is multiplied with DS spreading code chips.

The DS spreading code should have pseudorandom noise characteristics, orthogonal When several information signals are transmitted the output is a linear summation of all the chip. By despreading the received signal with the same DS spreading code, the information signal can be extracted. Processing gain is the number of chips per bit.

The spread spectrum theory was developed in the s. Several spread spectrum techniques exist. The technique discussed in this course is the direct sequence DS technique, where each information signal is spread using a spreading code.

With orthogonal spreading codes with pseudo-random characteristics, several information signals can share the same spectrum. Multiple information signals are linearly summed for each chip.

At the receiving end, multiplying the transmitted signal with the exact same code used to spread an information signal will extract the original information signal. Other signals spread with other codes will appear as noise. The more noise an information signal experiences loading , the higher the noise rise.

A term often used with spread spectrum techniques is processing gain spreading gain. Processing gain is an apparent gain that is introduced when a signal is despread. During depreading, only the information signal with the exact same spreading code is extracted; all other signals will become spread with that same code.

After passing the despread signal through a lowpass filter, the noise energy level is suppressed; hence, it appears that the original information signal has gained energy. Processing gain can be expressed as the number of spreading chips per information signal bit.

Knowledge Check 3. Why is there noise rise in a CDMA system? Users are using different RF carriers and different spreading codes B. Users are using the same RF carriers and the same spreading codes C. Users are using different RF carriers but the same spreading codes D. Users are using the same RF carriers but different spreading codes.

Lesson Objectives Explain the concept of frames Describe forward error correction Explain bit interleaving. CDMA Transmitter Before the digital information signal can be transmitted in the RF environment it must undergo a number of signal processing steps. The general steps a transmitted signal undergoes is shown in the graphic. The steps are, but not limited to: Speech encoding.

This step is only used if speech information is transmitted. Data transmission omits this step. Note: The various signal processing steps do not necessarily have to be performed in the order shown. Additional signal processing steps may also be taking place.

The various signal processing step shown will be discussed in more detail throughout the course. In order to transmit speech over a digital system, it must be digitized and encoded using a vocoder. Normal speech is received as an analog signal. The analog signal is converted into a digital signal using a process called Nyquist sampling, in which the analog input is typically sampled 8, times per second.

The product of Nyquist sampling is a digital waveform called PCM pulse code modulation. The PCM output is transferred to a vocoder voice coder , which compresses the digitized voice signal into either Rate Set 1 RS1 with an output of 8 kbps, or Rate Set 2 RS2 with an output of 13 kbps, depending on the type of vocoder. In CDMA, variable rate vocoders are used. VCELP produces high quality speech at lower bits rates, less than 16 kbps.

The VCELP speech coder takes samples of quantized speech and produces a variable number of bits in a 20 ms speech frame. The speech vector to be coded is matched against a codeword codeword index, gain, pitch lag, and pitch gain that minimizes the error between digitized and synthesized speech. The vector codebook index, gain and pitch parameters , and the LPF linear predictive filter coefficients are multiplexed and transmitted to the receiver.

These parameters are used by the decoder in the receiver to reproduce the synthesized speech of the transmitter. Speech Activity Natural speech includes active periods and quiet periods Spurts and pauses average talk cycle 3. Natural speech includes active periods and quiet periods called spurts and pauses. Spurts are generally syllables and words, while pauses include the times in a conversation when the party is listening. The average speech time and non-speech time can be modeled as shown in the figure.

By taking advantage of the variations in speech that occur during a normal conversation, the variable rate vocoder can dynamically change its rate. During normal speech, speakers take pauses and breaths, events in which no speech is transmitted. Since the transmitter only transmits the lowest bit rate required, the required transmit power is minimized, and the channel interference is reduced.

Number of bits per frame depends on data rate and additional information added to the frame A frame quality indicator can be added to each frame: CRC. Frame Information bits are grouped into frames. A frame is the basic timing interval in the system. The length of a frame depends on what channel on which it is transmitted e. For a traffic channel transmitting traffic information, the frame length will be 20 ms for IS and IS, and The number of bits per frame depends on the current data rate and if any additional information is added to the frame.

The purpose of the frame offset is to spread out the exact transmission time for the channel so that the processing delay at the base station can be minimized, e. Frame Quality Indicator A frame may include a frame quality indicator, depending on what channel the frame is transmitted, and the data rate of the frame. The frame quality indicator can support two functions at the receiver.

The first function is to determine whether the frame is in error. The second function may be to assist in the determination of the data rate of the received frame. Other parameters may be needed for rate determination in addition to the frame quality indicator, such as symbol error rate evaluated at different data rates. A CRC is a class of linear error detecting codes which generate parity check bits by finding the remainder of a polynomial division.

The CRC is calculated on all bits within the frame, except the frame quality indicator itself and the encoder tail bits. Viterbi decoder. Forward error correction FEC encoding provides channel bit error detection and correction capability at the receiver.

FEC enables noise- and interference-free communication over a wide range of input signal-to-impairment conditions by adding redundancy to the bit-stream. Encoding Process Example Lets say that the encoder receives a number of bits and multiplies them by three. If the input to the encoder is , the encoder reproduces each bit by a factor of three. The resulting output is Multiplying the input data frame provides a measure of protection against loss of data caused by interference. Assume that a given frame is damaged during transmission, it is possible that the receiver would not be able to reconstruct the frame without having access to the additional bits.

Using the example of , if we did not encode the frame and it was damaged by interference, the received frame may be 1XXX1. The additional bits generated by the encoding process provide the receiver with a backup source that may allow it to reconstruct the original frame.

Encoders, Decoders Two types of encoders are used in the technologies discussed in this course, convolutional encoder and turbo encoder. The decoder used is often the Viterbi decoder. The encoders and decoder will be discussed in more detail. Convolutional Encoder Output depends on current and previous bits. Constraint length, K, e. The convolutional encoder and symbol repetition take advantage of the bandwidth in CDMA spread spectrum systems to introduce redundancy into the original data stream.

The receiver uses the redundancy as an opportunity for error correction. Through the use of convolutional encoding, symbol energy and transmit power can be reduced, and the system will still achieve the same FER frame error rate. Convolutional Encoder Characteristics A convolutional encoder is primarily characterized by two parameters: The coding coefficient, R, and the constraint length, K.

The coding coefficient, R, determines the amount of redundancy to be generated in the bit stream. The constraint length, K, determines the memory of the convolutional encoder, or the number of shift-registers K The output from the encoder depends not only on the bit currently going into the encoder but also on the previous bit that has passed through the encoder.

A long memory creates a more robust bit stream but it also creates more delay in the transmission. Also, the benefit of the convolutional encoder versus the complexity is diminishing as K becomes greater than nine. Modulo-2 addition can be realized using XOR gates. The previous frame transmitted has filled the encoders shift registers r0, r1, r2, with zeroes using the encoder tail bits to clear the encoder.

When each bit is fed into the encoder, the output depends on the input an each of the shift registers values. When the input bit stream is 1 0 1 1, the output will be 1 1 1 0 1 0 1 1.

More robust than convolutional codes Can increase throughput Adds additional delay to the traffic data. The turbo encoder can be seen as two convolutional encoders operating in parallel. The convolutional encoders are also called constituent encoders. A turbo interleaver selects the input to each convolutional encoder. The output of the two convolutional encoders are concatenated with the appropriate symbol repetition and puncturing to achieve the correct symbol rate.

Turbo codes are more robust than convolutional codes but add additional delay to the traffic data. Therefore, turbo codes are not suitable for voice traffic, but function well for data traffic. Putting it in simple words, turbo codes do a lot of processing to encode relatively large chunks frames of information before transmission and to extract it upon reception. Viterbi Decoder Developed and analyzed in by A. Viterbi Efficient in determining the most likely bit sequence based on symbol organization Decoding algorithm is proprietary to Qualcomm.

Reference: Viterbi, A. Theory, col IT13, April , pp. Decoding an encoded signal is much more complex than encoding the signal. The Viterbi decorder is often used as the decoder. The Viterbi decoder receives the frame from the deinterleaver and, based upon the organization of the symbols in the frame, determines the most likely sequence of bits in the frame maximum likelihood decoding.

Given the encoder bit redundancy coding coefficient and memory constraint length , the decoder can detect and correct corrupt encoder symbols. The algorithm used to perform Viterbi decoding is proprietary to Qualcomm, and is incorporated in chip sets purchased or licensed from Qualcomm.

Ensure constant symbol rate for interleaver Depends on channel and data rate. The output from the encoder is called encoder symbols. These symbols are repeated and punctured as necessary before entering the bit interleaver. The purpose is to ensure a constant symbol rate for the interleaver. Also, when a symbol is repeated N times, its transmit power can be reduced by a factor of N and still provide the same energy for the receiver. Repeating the symbols will generate even more redundancy, whereas puncturing of symbols will reduce the redundancy.

How often to repeat and puncture the symbols depends on the channel used and the data rate transmitted. Enables the channel decoder process to work under fading conditions Receiver deinterleaves the bits back into correct order. Receiver Enter bits row-wise Recover bits column-wise. The bit interleaver works closely with the encoder to provide additional communication reliability by interleaving the encoded bits so that the transmitted frame is, essentially, transmitted multiple times.

The decoder is very efficient in detecting and correcting nonconsecutive corrupted bits, but not as efficient for consecutive corrupted bits. With interleaving, the decoder is provided with additional opportunities to reconstruct frames damaged during transmission. When the frame is deinterleaved, the bits are restored to their encoded position. The decoder in the receiver is able to compare the received bits with those immediately adjacent, and decode the speech frame into a duplicate of the original.

The block interleaver randomizes the bits to further reduce the effects of interference. Through the process of encoding, code repetition and block interleaving, the potential for one or more frames to be completely undecipherable by the receiver is substantially reduced.

These processes are crucial to maintaining a low FER. The coded signal is interleaved by writing a block of coded bits into an array, a matrix, according to a bit pattern and then reading from that array according to another bit pattern.

Bit Interleaving - Example Interleave and deinterleave the bit stream b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12 b13 b14 b15 b16 Interleaver matrix. Step 1: Enter bits column-wise Step 2: Transmit bits row-wise b1 b5 b9 b13 b2 b6 b10 b14 b3 b7 b11 b15 b4 b8 b12 b Summary Transmitted speech needs to be encoded by a vocoder. Forward error correction FEC encoding is used to make signal more robust.

Convolutional encoder Turbo encoder. FEC Decoder can detect and correct corrupt bits. Bit interleaving spreads out corrupt bits. Works with FEC decoder. Before a digital information signal is transmitted, it is encoded to make it more suitable for transmission. If speech is transmitted, it has to be encoded using a vocoder. The vocoder transforms the speech information into information symbols that can be more efficiently transmitted.

An additional benefit of the vocoder is that the data rate of the vocoder can vary based on the activity amount of information during a time period of speech. Every information signal undergoes forward error correction FEC. FEC is a process with which the information signal is transformed into a bitstream with more bits than the original information signal.

When the encoded bitstream arrives at the decoder, the decoder can detect and correct corrupt bits in the original information signal. To maximize the efficiency of the FEC decoder, the bits are interleaved before transmission. When interleaved, the bits are not transmitted in their natural order. If an RF fade generates consecutive corrupt bits, the deinterleaver will spread out those bits and make it easier for the FEC decoder to correct them.

Step 2: Interleave 4x4. Enter bits column-wise, transmit row-wise Interleaver matrix. Step 4: Deinterleave. Enter bits row-wise, recover column-wise Deinterleaver matrix.

Why are the information bits grouped into frames? To increase the data rate of the transmission To accommodate FEC and bit interleaving To reduce facility cost by packetizing the information Any of the above. CDMA Transmitter Before the digital information signal can be transmitted in the RF environment, it must undergo a number of signal processing steps.

The steps are, but not limited to: Speech encoding: This step is only used if speech information is transmitted. In this lesson, we will take look at the use of codes in CDMA, and what code correlation means. Multiply regenerated PN code sequence with incoming sequence. Average the result and look for maximum. If not maximum, shift regenerated sequence and repeat. To maintain close chip- and code-synchronization, GPS time is used as reference.

The average of the product of the 15 bit PN code with a shifted version of itself is shown for various bit shifts. The plot of the average value calculated over the code period for all bit shifts is called the autocorrelation function. In order for CDMA to maximize performance, the codes must be aligned.

GPS time is used as the time reference in a CDMA network to aid the transmitter and receiver when they are synchronizing their codes. If the signal is different from either signal, then the correlation is call a crosscorrelation.

Introduction A direct sequence DS or pseudo-noise PN stream is generated in an m-stage maximal-length shift register. The sequence, or PN code, that is produced at the output repeats itself after a maximum period of time corresponding to 2m -1 shifts. The smallest time increment in the output sequence is of duration Tc, which is called the time chip. For the resulting output waveform to have the desired pseudo-noise property, the outputs of certain stages must have feedback connections that are made in a certain fashion.

Example In the illustration shown, we are feeding back the modulo-2 sum XOR of stage m and m-1 to the input to obtain the output. Different feedback connections result in distinct coded outputs. The initial states of the stages in the register is 1 0 0 0. A 0 0 1 1 CLSG. The function depends on: Forward or reverse link channel Technology used.

PN Long Code The long code gets its name from the fact that it takes about Information about the long code is broadcast to the mobile station by the Sync Channel or Control Channel to help the mobile lock onto the base station, and helps provide separation from other base stations.

The PN short code on the forward link is used to provide the base station with a unique identification that the mobile station uses to identify the serving base station. Because CDMA communication is conducted on a common frequency with several calls in progress simultaneously, identifying the serving base station and sector is an important issue for the mobile station. Walsh Function The user signal or control channel is multiplied by the Walsh code.

The Walsh code provides each user or channel with a unique identifier and, in DS spreading, may spread the frame across the bandwidth. Typically used for scrambling and identification Forward link: Scrambles information Reverse link: Identifies mobile. The long code is generated using a PN code-sequence generator with 42 stages, or shift-registers. The length of the long code is bits long.

With a rate of 1. The effective long code is typically used to scramble the bit-stream transmitted, or to assign a unique identification to a user. Traffic Channel When the long code mask is used for traffic channel, public or private versions are possible. The public mask uses the mobile stations electronic serial number ESN in a permutation specified in the standard. The permutation prevents high correlation between long codes corresponding to consecutive ESNs.

Note that this is not encryption, since the ESN is known, then the mask is known. The private long code mask is determined by referring to a controlled appendix of the standard.

The distribution of the appendix is controlled by TIA. Access Channel The access channel number is the channel number being used by the mobile to initiate communication with, or respond to a message from, the base station with a specific identification number.

Note: The masks shown are to illustrate the concept. The masks used for IS channels are defined in a slightly different way. Typically used for spreading and identification Forward link: Quadrature spreading with specific offset identifying antenna face Reverse link: Quadrature spreading with zero-offset. The short codes, or PN short codes, are bits long and are used for both spreading and identification. Signals transmitted from a single antenna in a particular CDMA radio channel share a common PN code phase or time offset.

Including the zero offset sequences, PN-I-0 t and PN-Q-0 t , there are possible time offset indices to identify cells. The time offsets used for the PN code is based on orthogonal coding in which the spread signal is split and sent to a quadrature spreader whose output is offset by 90 degrees. On the reverse link, the quadrature spreader is using the zero-offset PN codes. The forward link uses a pilot signal to allow the mobiles in the cell to synchronize to the base station.

The pilot is to be used by the mobile demodulator to provide a coherent reference which is effective even in a fading environment, because the desired signal and the pilot fade together. All users in the same cell or sector share the same quadrature pair of modified PN codes, often referred to as pilot PN offset.

Each base station will use the same PN short codes with a different offset, PN offset. Since these codes have different offsets, they will have very low correlation with each other.

Mobiles identify their serving base station by looking for the appropriate offset assigned to the given base station. Shown in the figure are two PN sequences which differ only in their offset.

Typically used for identification Identifying channels IS reverse link uses Walsh codes for digital modulation. Must be time-aligned to have zero correlation. Not always zero cross-correlation at other alignments. One of the main functions of the Walsh codes is to identify channels that are being transmitted.

In order to efficiently identify the channels, Walsh codes must be orthogonal correlation between two different codes equal 0 and orthonormal correlation between the same code equal 1. Note: Walsh codes must be time-aligned to have zero correlation. They do not always have a zero cross-correlation at other alignments.

Hadamard, Rsolution d'une question relative aux dterminants, Bull. The Hadamard matrix generates codes that are orthogonal. The process to generate the codes can be seen in the slide. In other words, there are as many orthogonal Walsh codes as there are bits in a Walsh code.

For example, Walsh code W78 consists of 8 bits. Shown is an example where the correlation cross-correlation is calculated for two different Walsh codes, W28 and W When integrating over the code length 8 chips , the value is zero, or close to zero. Please note that when calculating the correlation for the same code, e. Shown is an example where the correlation cross-correlation is calculated for two different Walsh codes, W24 and W The code length used for integration is now four chips instead of eight as in previous example.

Even though the code length differs between the two codes, when integrating over the code length, the value is still zero or close to zero, indicating that the two codes are orthogonal. In this example two different Walsh codes, W24 and W68 are compared. The code length used for integration is four chips.

When integrating over the code length, the value is not zero, indicating that the two codes are not orthogonal. W24 from H4 is repeated in H8, both in original and inverted form.

Therefore, the codes are not orthogonal. If Walsh codes of variable length are used together, then a shorter code precludes using all longer codes derived from the shorter code. The tables show the orthogonality relationships between Walsh codes of variable length.

Rule Walsh codes appearing on the same row are not orthogonal with each other. Walsh codes that do not share one or more rows with another Walsh code in use are orthogonal. However, it is orthogonal with, for example, W and W W4 W4 Walsh Code Lengths Walsh Code Lengths W8 W8 W16 W32 W64 W W16 W32 W64 W 11 11 11 65 65 33 33 33 33 11 97 97 17 17 17 17 17 81 17 81 49 49 49 49 11 99 99 99 73 73 41 41 41 41 99 25 25 25 25 25 89 25 89 57 57 57 57 55 55 55 69 69 37 37 37 37 55 21 21 21 21 21 85 21 85 53 53 53 53 55 13 13 13 13 13 77 13 77 45 45 45 45 13 13 29 29 29 29 29 93 29 93 61 61 61 61 W4 W4 Walsh Code Lengths Walsh Code Lengths W8 W8 W16 W32 W64 W W16 W32 W64 W 33 33 33 67 67 35 35 35 35 33 99 99 19 19 19 19 19 83 19 83 51 51 51 51 33 11 11 11 11 11 75 11 75 43 43 43 43 11 11 27 27 27 27 27 91 27 91 59 59 59 59 77 77 77 71 71 39 39 39 39 77 23 23 23 23 23 87 23 87 55 55 55 55 77 15 15 15 15 15 79 15 79 47 47 47 47 15 15 31 31 31 31 31 95 31 95 63 63 63 63 Quasi-Orthogonal Codes To gain more Walsh codes, quasi-orthogonal codes may be used.

IS provides the functionality to expand the Walsh code space by using so called quasiorthogonal Walsh codes or Walsh functions. While this may seem like a desirable feat, it is important to keep in mind that the quasi-orthogonal codes are not fully orthogonal. This means that two different codes are interfering with each other to some degree. The sequence is also multiplied by the repeated sequence of masks with symbols 1 and j j is the complex number representing a 90o phase shift , which correspond to the rotate enable Walsh function values of 0 and 1, respectively.

They work together. Forward link: Short code PN offset identifies antenna face Walsh code identifies channels within the antenna face.

Scrambling: If b t and c t have the same rate then y t has the same rate, and the spectrum of the signal is unchanged b t is said to be encrypted or scrambled. Spreading: If c t has a higher rate than b t , y t has the faster rate and its correspondingly wider spectrum In addition to being scrambled, b t is said to have had its spectrum spread.

CDMA codes are used to perform scrambling and spreading. See the technology specific lessons for more details. When c t is faster, y t contains all the information of b t , and it has the faster bit rate and its correspondingly wider spectrum. Multiply with PN short codes Quadrature spreading. Digital Modulation With digital modulation the bit or bit pattern generates certain energies in the quadrature-phase Q-phase and in-phase I-phase components of a signal.

Quadrature spreading ensures that other-user interference appears as though it has both random phase and amplitude information i. RF amplification: Amplifies the RF signal after modulation Digital channel have their individual gain. RF Modulation The orthogonality of cosines and sines makes it possible to transmit and receive two independent signals simultaneously on the same carrier frequency. This is known as quadrature modulation.

In the quadrature mixer, the Q-phase and I-phase are multiplied with sin 2fct and cos 2fct , respectively, making the signal a RF signal. Quadrature modulation is an efficient method of transmitting two message signals within the same bandwidth.

It requires precise phase synchronization of transmitter and receiver. RF Amplification Once the signal has been modulated in the RF domain, it passes through an RF amplification process to generate the signal strength needed for RF transmission. Multi-user environment: Combining the CDMA codes makes each user and channel unique in the area Receiver knows what codes to look for All other codes appear as noise. Receiver Processes Just as the transmitter is wrapping the information signal in a number of signal processing layers, the receiver must unwrap the signal by performing the signal processing steps in the reverse order: De-modulation De-spreading De-scrambling De-interleaving De-coding FEC De-coding speech , if speech information is transmitted.

The combination of a set of codes long code, short code, Walsh code makes that information signal appear unique; all other code combinations appear as noise.

End-To-End Overview Transmit coded digital information b t 1. Provides straightforward setup, intuitive administration and comprehensive report distribution options. Explore Our Newest Features.

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