Cdma

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  • Topic: Code division multiple access, Modulation, Signal processing
  • Pages : 12 (3296 words )
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  • Published : December 3, 2012
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rochester institute of technology|
DS-CDMA PROJECT|
Communications Systems |
|

11/4/2012|

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TABLE OF CONTENTS

1. Introduction3
Spread Spectrum4
Pseudo Noise Codes4
Properties of PN Sequence5
Walsh Code5-6
2. Main Block Diagram (Receiver Side)7
Despreader7-8
RAKE Receivers and Constellations8-10
Channel Estimator and Signal Decoder10
Multi-User Detector/Interference Canceller11
Wideband SIC11-12
Narrowband SIC12
Decoders13
Viterbi Decoder13
Turbo Decoder13
Parallel Correlator14
Demodulation Process14-16
Pulse Shape Usage17
Multi-antenna Technology17-18
Conclusion19
3. References20-21

INTRODUCTION
As the telecommunications industry faced a huge development in the last couple of decade, the demand for bandwidth has extremely grown up. The bandwidth demand pushed and stimulated a need for protocols that would increase the bandwidth efficiency. The multiple accesses combine many signals to one single communications channel. Basically, there are three types of division-based protocols which are: FDMA, TDMA and CDMA. Frequency Division Multiple Access (FDMA) – is a frequency based multiple access where the channel is the frequency. In other words, the signals in FDMA are assigned frequency channels. Time Division Multiple Access (TDMA) – divides the frequency channels into slots, so that two or more users are able to use the same frequency band. In this case, each user is assigned different time slot. Code Division Multiple Access (CDMA) – in CDMA the users are assigned different random binary sequence codes which modulates the carrier by spreading the spectrum of the waveform and assigning users with unique code sequence.

Figure 1: Difference between FDMA, TDMA and CDMA
SPREAD SPECTRUM
CDMA is the form of spread spectrum. There are three key points of spread spectrum communications which are: 1. More bandwidth is occupied by signal to be able to send information and to resist interference and multiple user access. 2. The bandwidth of the signal is dispersed by code

3. The receiver synchronizes the code to read the data from the signal. Every user is assigned different sequence code, which allows multiple users to use the same frequency.

PSEUDO NOISE CODES (PN CODES)
Pseudo Noise codes, most commonly known as PN codes, have the length of 2m – 1. The PN codes are also deterministic, meaning that it’s know both for the transmitter and the receiver side and it has the same properties as sampled white-noise. There are two types of PN codes: pseudo noise short code and pseudo noise long code. Both PN code are generated at the speed of 1.23Mbps. PN long code is 242 -1 bits long, which is approximately 4.4×1012 chips. Dividing 4.4×1012 chips by 1.23Mbps and then by (60seconds×60minutes×24hour/day) we get the PN long code period, which is 41.4 days. The PN short code is 215-1 bits long, which is 32,767 chips. The number of chips divided by 1.23Mbps and then by (60seconds×60minutes×24hour/day we get 26.67ms, meaning that the PN short code repeats every 26.67ms.

PROPERTIES OF PN SEQUENCE
Balance Property: In every period of sequence the number of binary 1’s differs from sequence of binary 0’s. Autocorrelation: When the signal compared with the same shifted signal, it’s called autocorrelation. In other words, it is the correlation of a code with a time-delayed version of itself. In order to reject multi-path interference, this function should equal 0 for any time delay other than zero. Cross-correlation: The correlation of two independent signals. In other words, signals with different frequencies and phases are compared with the input signal....
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