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mobile computing
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INTRODUCTION: EVOLUTION OF WIRELESS SYSTEM: The first generation (1G) mobile cellular systems provided analogue voice communication to mobile users using FDMA. The 2G wireless systems provided digital voice communications using TDMA and CDMA as multiple access techniques. The 3G mobile communication systems provide wide area coverage using Wideband Code Division Multiple Access (W-CDMA) as the multiple access technique. The 4G are intended to provide high speed, high capacity, low cost per bit, IP based services. MC-CDMA is the most promising multiple access technique for 4G cellular systems. KEY PARAMETERS OF 3G AND 4G SYSTEMS Parameters 3G 4G Frequency band 1.8 to 2.5 GHz 2 to 8 GHz Bandwidth 5 to 20 MHz 5 to MHz Data rate Up to 2 mbps Up to 20 mbps Access W-CDMA MC-CDMA Switching Circuit / Packet Packet Mobile top speeds 200 Kms/h 200 hms/h MULTIPATH FADING CHANNEL: The mobile or indoor radio channel is characterized by multipath reception. The signal offered to the receiver contains not only a direct line-of-sight radio wave, but also a large number of reflected radio waves. These waves arrive at a later time than the direct signals, which causes significant degradation of the performance of the network. The time spread between the arrival of first and last multipath signal is known as ‘delay spread’ and reciprocal of delay is known as coherence bandwidth of multipath channel. In digital systems, delay spread can lead to ISI. Another critical design objective is high spectrum efficiency. The later should ensure that the network can accommodate as many users possible within a given frequency band. WHAT IS MC-CDMA? v MC-CDMA is a form of frequency diversity. Each bit is transmitted simultaneously (in parallel) on many different subcarriers. Each subcarrier has a (constant) phase offset. The set of frequency offsets form a code to distinguish different users. v MC-CDMA is a form of OFDM, but an orthogonal matrix operation to the user bits is applied first. Therefore, MC-CDMA is sometimes also called “CDMA-OFDM”. CODE DIVISION MULTIPLE ACCESS: Code Division Multiplexing Access (CDMA) is a spread spectrum technique that uses neither frequency channel nor time slots. In CDMA, all users use the same RF bandwidth and transmit simultaneously as shown in fig.1. In this multiple access technique, various users are differentiated to each other by assigning the unique codes known as ‘pseudo Random Code’. Fig.1 In DS-CDMA transmitter, the narrowband signal is spreaded by the larger bandwidth signal in time domain, which is known as pseudo random code (PN code) as shown in fig .2a. The spreaded signal is modulated using BPSK with fo as the carrier frequency. Fig.2 Where cj(t)=cj1,cj2, … a NDS k is the PN Code of kth user ,NDS is the procession gain. Fig.2b shows the power spectrum of transmitted signal. The spreading codes assigned to each user must be orthogonal to each other in order to suppress the interference from other user. In case of high data rate applications, the bandwidth of transmitted signal becomes greater than the coherence bandwidth of the channel, due to which the channel becomes frequency selective to these signals and hence transmitted signal experiences frequency selective fading. A Rake receiver can be used to enhance the system performance in frequency selective fading channel. The bit error rate performance depends on how many fingers the rake receiver employs. In a Direct Sequence Code Division Multiple Access (DS-CDMA) system based on the Rake receiver structure, the system is limited by Self- Interference (SI) and Multiple Access Interference (MAI), which results from the imperfect auto-correlation characteristics and imperfect cross-correlation characteristics of the spreading codes respectively. Although DS-CDMA has some advantages than TDMA and FDMA it has certain limitations for the following two reasons.
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