CDMA

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WELCOME

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C D M A DIVISION CODE MULTIPLE ACCESS

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LAKPA BITTU TAMANG (ECE-01/08) JITENDRA KUMAR RAI (ECE-50/08) PRESENTED BY:

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INTRODUCTION: CDMA is a new technology in wireless communication. Spread Spectrum technique is used. Mathematical codes are used to transmit & distinguish the users. The great attraction of CDMA technology has been because of the extraordinary increase in the system capacity and the service capacity. CDMA is extremely robust and provides excellent audio quality.

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HISTORY: 1 st generation cellular system introduced in 1980, uses Analog frequency modulation. Due to some drawbacks, increase in market demands led to the foundation of 2 nd generation cellular system. 2 nd generation cellular system uses Digital modulation technique which includes CDMA, GSM & TDMA. CDMA was introduced by QUALCOMM in 1994. CDMA provides 10 times more capacity than other Analog system.

BASIS OF CDMA::

BASIS OF CDMA: Based on Spread Spectrum Technology. Spread Spectrum is independent of data sequence. It provides secure communication which is not easily detectable. It rejects unwanted interference that may be intentional or unintentional. Number of independent users can share a common channel without external SYNCHRONIZING mechanism.

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COMPETITION: The Euro-Asian biggest competitive to CDMA is the GSM. CDMA is more advantageous to GSM due to less interference, low radiation level & more security. GSM uses modified version of TDMA. GSM increases the network strength using SIM card. CDMA is the fastest growing technology in wireless communication than GSM.

What is Multiple Access?:

What is Multiple Access? Each pair of users enjoys a dedicated, private circuit through the transmission medium, unaware that the other users exist. Multiple Access: Simultaneous private use of a transmission medium by multiple, independent users. Transmission Medium TYPES OF MULTIPLE ACCESS SYSTEM:

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3 MOST POPULAR MULTIPLE ACCESS TECHNOLOGIES THAT ESTABLISHED CHANNELS: FDMA (Frequency Division Multiplex Access) Each user on a different frequency A channel is a frequency TDMA (Time Division Multiplex Access) each user on a different window period in time (“time slot”) a channel is a specific time slot on a specific frequency CDMA (Code Division Multiplex Access) each user uses the same frequency all the time but mixed With different distinguishing code patterns a channel is a unique set of code patterns Frequency Time Power Frequency Time Power Frequency Time Power FDMA TDMA CDMA

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A wireless communications technology that uses the principle of spread spectrum communication( a method of wireless communications that takes a narrowband signal and spreads it over a broader portion of the available radio frequency band) 2.All users use same frequency and may transmit simultaneously 3.Narrowband message signal multiplied by wideband spreading signal, or codeword 4.Each user has its own pseudo-codeword (orthogonal to others ). 5.Receivers detect only the desired codeword. All others appear as noise . 6.Receivers must know transmitter’s codeword. CDMA FEATURES:

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GENERAL THEORY OF CDMA: Let us consider arbitrary signal with rectangular frequency spectrum. It is multiplied by the cosine function to shift the spectrum as shown in fig. below: Fig.: Frequency spectrum Fig.: Shifted frequency spectrum

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In receiver end, the signal is once again multiplied by the cosine function as shown in fig. Fig.: Frequency Spectrum of the signal before filtering LPF (Low Pass Filter) is used at receiver end to recover the signal. The bandwidth of the signal should not exceed 1/3 rd of whole frequency spectrum. Sampling should be done at 44.1kHz to prevent ALIASING. Transmitted signal: The transmitted signal is represented by squaring pulse which corresponds Sine function in the frequency. Fig.: Sine function occupying the Lowest part of Spectrum.

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GENERATING THE CDMA SIGNAL:

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Working Principle: b (t )=data signal a (t )=PN sequence s (t )=Modulated signal

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DECODING PROCESS: STEPS: Conversion of R.F signal to Digital signal. Decoding & dispreading the user’s signal. Deinterleaving & decoding. Decompression of the voice signals. Digital to Analog converter.

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System Capacity: The capacity of a system is approximated by : is the maximum number of simultaneous calls is the processing gain is the total signal to noise ratio per bit is the inter-cell interference factor. f c is Chipping Frequency (the bit rate of the PN code). f i is Information Frequency (the bit rate of the digital data).

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CDMA BELONGS TO TWO BASIC CATEGORIES: SYNCHRONOUS (ORTHOGONAL CODES) 2. ASYNCHRONOUS (PSEUDORANDOM CODES)

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1. SYNCHRONOUS (ORTHOGONAL CODES) CDMA: Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. Fig : An example of four mutually orthogonal digital signals.

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2. ASYNCHRONOUS (PSEUDORANDOM CODES) CDMA: Each user is assigned a pseudo-noise code (PN code) instead of a vector code. Occupy a very large bandwidth. Generated at random at the transmitter side. Appears as a noise into the background. Very low cross-correlation with other sequences. Spreads the narrow-band message signal into a wide-band signal.

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MULTI-CARRIER SYSTEM IN CDMA: Is a Data transmission technique where several sub-carriers are used to transmit data signal. Modulation & Demodulation is done by IDFT and DFT. It increases the spectral density and reduces frequency selective fading. All sub-carriers uses the same information offering better frequency diversity. FEATURES: Low interchannel interference. Sensitivity to non-linear amplification. Robust against multi-path fading. Robust against narrow band interference or jamming signal.

CDMA IN MOBILE COMMUNICATION::

CDMA IN MOBILE COMMUNICATION: CDMA was used earlier in military application like anti-jamming and secure communication. CDMA in mobile communication have made evolutionary development. Availability of low cost, high dense digital integrated circuit. Reduced size, weight and cost of subscriber station. Lower transmission power.

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CDMA IN CELLULAR RADIO STATION: Frequency hopping technique is used. Exact synchronization should be done at the receiver end. Switching time or guard time between the frequency hopping pulses must be larger enough to reduce the synchronized error and multiple fading. Its range of bit rate required for the video transmission is 1.2 bps to Mbps.

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MULTI-INTERFERENCE IN CDMA SYSTEM: In CDMA system, all users transmit signal at same frequency band and distinguished at receiver end by unique spreading code assigned to the user. Due to the zero correlation values between the spreading code assigned to the user, there is some interference at the receiver end, which is called MULTI-USER INTERFERENCE . METHOD OF REDUCTION OF MULTI-INTERFERENCE: By reducing cross correlation in spread spectrum system. Power control. Capacity improvement with CDMA antenna array.

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MULTI-PATH FADING IN CDMA SYSTEM: Fading in CDMA system is mostly due to the random variation in received signal or undesired variation in intensity and loudness of radio waves. It can be eliminated using dispreading correlator at receiver end. Multi-path fading cannot be eliminated if multi-path components are separated by less than the correlated time producing flat fading. A B

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ADVANTAGES: DISADVANTAGES: Capacity increase. No frequency management or assignment. Reduce average transmitted power. Improves telephone traffic capacity. Reduce the effects of Multi-path fading Improves voice quality. Provides reliable transport mechanism for data communication. Multi-path fading. Multi-user interference or Multiple access interference (MAI).

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CONCLUSION: CDMA is radically new concept in wireless communication. It has gained wide spread acceptance in cellular system and has dominated worldwide wireless market, increase in system capacity and service quality. Though there is some limitation in CDMA like multiple access interference, multi-path fading which makes CDMA underdog of 4G but still it may win.

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BIBLIOGRAPHY: http://www.cellular.co.za/cdma.htm http://www.authorstream.com http://www.altera.com http://www.wikipedia.org http://images.google.co.in http://www.wisegeek.com

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THANK YOU