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CDMA in Satellite System:

1 CDMA in Satellite System

Outline :

Slide No. 2 Outline Basic Concepts Cellular System Frequency reuse Hand Off Satellite Communications System Why Satellite Communications System Multiple Access Schemes CDMA – Core Concepts Advantages of CDMA Challenges of CDMA Conclusion

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Slide No. 3 Basic Concepts – Cellular System

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Slide No. 4 Basic Concepts - Frequency Reuse f 7 f 7 f 2 f 2 7 cell re-use pattern f 6 f 6 f 1 f 5 f 3 f 4 f 1 f 5 f 7 f 6 f 3 f 4 f 2

Basic Concepts – Hand Off:

Slide No. 5 Basic Concepts – Hand Off f 2 f 1 MSC

Satellite System:

Slide No. 6 Satellite System Traditionally weather satellites satellites for navigation Telecommunication Broadcast service over a wide area global telephone connections connections for communication in remote places or underdeveloped areas  Satellite mobile communication for 3 rd generation systems

Why Satellite Communications System ?:

Slide No. 7 Why Satellite Communications System ? Main Advantages Extend capability of existing terrestrial cellular system Coverage in remote locations Large coverage area Seamless service to subscriber in any part of the world Challenges Propagation delay Attenuation Low Spectral efficiency

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Slide No. 8 base station or gateway Typical satellite systems Inter Satellite Link (ISL) Mobile User Link (MUL) Gateway Link (GWL) footprint small cells (spotbeams) User data PSTN ISDN GSM GWL MUL PSTN: Public Switched Telephone Network

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Slide No. 9 Multiple Access Schemes for Satellite Communications A multiple access scheme is expected to provide users to simultaneously access a satellite terminal efficiently In a satellite communications system, designing a multiple access scheme is one of the most challenging issues

Frequency Division Multiple Access (FDMA) assigns individual channels (i.e frequencies) to individual users :

Slide No. 10 Multiple Access Schemes Frequency Division Multiple Access (FDMA) assigns individual channels (i.e frequencies) to individual users

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Slide No. 11 Multiple Access Schemes Time Division Multiple Access (TDMA): The spectrum is divided into time slots, in each slot only one user can transmit or receive TDMA shares a single carrier with several users

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Slide No. 12 Multiple Access Schemes Code Division Multiple Access (CDMA): All the users in this system, use the same carrier frequency and may transmit simultaneously Lets take a closer look at it!

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Slide No. 13 Multiple Access Schemes

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Slide No. 14 CDMA Spread Spectrum Systems Wireless Communication technique that uses transmission bandwidth substantially larger than the signal bandwidth Many users can simultaneously use the same bandwidth without significantly interfering with one another There are many applications including Global Positioning System, Wireless LAN e.g. Bluetooth CDMA In CDMA user data is multiplied with a pseudo-noise (PN) sequence called the spreading signal This causes the spectrum to be spread to several orders of magnitude greater than the minimum signal bandwidth

PN Sequence:

Slide No. 15 PN Sequence PN sequence is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receiver The rate of the PN code called the chip rate, must be much higher than the rate of the information signal

How it works !:

Slide No. 16 How it works ! The information signal is demodulated at the receiver by cross-correlation with a locally generated replica of the user’s PN sequence. The signal is de-spread and is restored to the original signal. Cross-correlation with PN sequence of other users results in a very small noise at the receiver.

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Slide No. 17 Why CDMA (I) ? Exploitation of multipath – RAKE Receiver Satellite path diversity exploitation for: Improved quality of service Fading effects mitigation Immunity from jamming signals, external interference and multipath distortion Full frequency reuse easing resource allocation

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Slide No. 18 Why CDMA (II) ? f 1 f 1 Soft handoff – Improved quality of Service BS

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Slide No. 19 Challenges Nonlinearity in satellite communications causes degradation in performance Power Control - Near / Far Effect Open loop power control Closed loop power control Synchronization


Slide No. 20 Conclusion Satellite Communications is required to extend the capabilities of cellular systems in 3 rd Generation Systems CDMA is preferred multiple access scheme because of its advantages like diversity and multipath exploitation, soft handoff characteristics CDMA however, faces challenges like performance degradation due to nonlinearity in satellite system and handling of power control and synchronization

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Slide No. 21 Back up slide

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