21-04-2011, 09:32 AM
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1. INTRODUCTION
One of the proposed 3rd generation telecom systems is Universal mobile telecommunication system (UMTS), which aims to provide a more flexible data rate, a higher capacity and more tightly integrated services than the second generation mobile. Most 3rd generation mobile phone system are using CDMA or extended TDMA by improving flexibility of service available but CDMA was found to perform poorly in single cellular system and high Inter-user interference.
Several techniques, with aim of improving cell capacity, providing multipath immunity, flexibility, high tolerance to peak power clipping and channel noise and also providing a high spectral efficiency includes Orthogonal Frequency Division Multiplexing.
OFDM allows many users to transmit in an allocated band by subdividing the available B.W into many carriers called tones. They are packed much closer together than standard FDM. This leads to OFDM providing high spectral efficiency.
2. PRESENT SYSTEM IN USE: CDMA
Code division multiple access (CDMA) is a spread spectrum technique that uses neither frequency channel as in frequency division multiple access (FDMA) nor time slots as in time division multiple access (TDMA).
CDMA technology was originally developed by military during World War II. Some properties that have made CDMA useful are:
Signal hiding and noninterference with existing systems.
Anti_jam and interference rejection.
Information security.
Accurate ranging.
Multiple user access.
Multiple tolerances.
Basic CDMA transmitter
Figure 2.1. CDMA Transmitter
With CDMA, narrow band message is multiplied by large bandwidth signal i.e. a pseudo random noise code (PN code) also called Walsh code. These PN codes are orthogonal for separating the multiple users on same channel. These are based on Walsh matrix, which is a square matrix with binary elements (1 or0 called chips) and dimensions that are power of two.
W2n =
The walsh codes are orthogonal, which means the dot product of any two rows is zero, since for any two rows exactly half the number of bits match and half do not.
At receiver, the transmitted signal is recovered by correlating the reived signal with original PNcode used by the transmitter. However, all other signals which are un-correlated to PN code spreading become more spread.
3. PROBLEMS ASSOCIATED WITH CDMA
The main weak points of CDMA are:
1. High interuser interference in reverse link.
2. Low single cell capacity.
3. Large BER for multipath immunity, peak power clipping and channel noise.
These problems can be discussed by using simulation results for CDMA technique as shown:
1] BER vs Number of users in a cell
The BER for reverse page link of a CDMA system increases as more users use the same cell.Fig.3.1 shows the BER expected based on number of users. The result is for an isolated cell with no interference from neighbouring cells, no multipath effects, no channel noise. Any of these effects would worsen the BER.
2]Multipath immunity
BER Vs Multipath delay spread for CDMA reverse link.
The noise due to multipath leads to increase in the amount of interference seen by each user, & thus increasing the received BER. From Fig.3.2 it can be seen that the BER is essentially flat for delay spread of greater than one chip time(0.8 microsecond) which to be expected as the reflected signal becomes uncorrelated. In Fig. 3.2, the multipath delay spreads leads to increase in equivalent No. of users in the cell as it increases the amount of interference seen by receiver.
3] Peak power clipping
If the transmission technique is tolerant to peak power clipping, then it allows the signal to be clipped. This clipping of signal reduces the peak to RMS signal power ratio thus allowing the signal power to be increased for the sized Transmitter. Fig.3.3 shows effect of peak power clipping on both the reverse &forward links for CDMA.
Peak power clipping for the reverse page link is also likely to be small as only ever occur due to distortion in the base station receiver .a well designed receiver is unlikely to cause significant clipping of the signal. The forward page link shows significant clipping to the transmitter. The BER is low for peak power clipping of less than 10dB, above which the orthogonal nature of walsh code used begins to collapse.
4] Channel noise
The noise performance of CDMA reverse page link is shown in Fig.3.4 due to high ;level of inter-user interference the addition of channel noise leads to only a gradual rise in the BER. The BER of the lines (10 users, 20 users and 30 users) approaches approximately. The same BER at the SNR of 0dB.the BER is very bad for more than 10 users regardless of the channel SNR, thus making 20 or 30 users unusable. For 10 users the BER becomes greater than 0.01 at approximately a SNR of 14dB, which is maximum BER that can be normally tolerated for voice communication.
1. INTRODUCTION
One of the proposed 3rd generation telecom systems is Universal mobile telecommunication system (UMTS), which aims to provide a more flexible data rate, a higher capacity and more tightly integrated services than the second generation mobile. Most 3rd generation mobile phone system are using CDMA or extended TDMA by improving flexibility of service available but CDMA was found to perform poorly in single cellular system and high Inter-user interference.
Several techniques, with aim of improving cell capacity, providing multipath immunity, flexibility, high tolerance to peak power clipping and channel noise and also providing a high spectral efficiency includes Orthogonal Frequency Division Multiplexing.
OFDM allows many users to transmit in an allocated band by subdividing the available B.W into many carriers called tones. They are packed much closer together than standard FDM. This leads to OFDM providing high spectral efficiency.
2. PRESENT SYSTEM IN USE: CDMA
Code division multiple access (CDMA) is a spread spectrum technique that uses neither frequency channel as in frequency division multiple access (FDMA) nor time slots as in time division multiple access (TDMA).
CDMA technology was originally developed by military during World War II. Some properties that have made CDMA useful are:
Signal hiding and noninterference with existing systems.
Anti_jam and interference rejection.
Information security.
Accurate ranging.
Multiple user access.
Multiple tolerances.
Basic CDMA transmitter
Figure 2.1. CDMA Transmitter
With CDMA, narrow band message is multiplied by large bandwidth signal i.e. a pseudo random noise code (PN code) also called Walsh code. These PN codes are orthogonal for separating the multiple users on same channel. These are based on Walsh matrix, which is a square matrix with binary elements (1 or0 called chips) and dimensions that are power of two.
W2n =
The walsh codes are orthogonal, which means the dot product of any two rows is zero, since for any two rows exactly half the number of bits match and half do not.
At receiver, the transmitted signal is recovered by correlating the reived signal with original PNcode used by the transmitter. However, all other signals which are un-correlated to PN code spreading become more spread.
3. PROBLEMS ASSOCIATED WITH CDMA
The main weak points of CDMA are:
1. High interuser interference in reverse link.
2. Low single cell capacity.
3. Large BER for multipath immunity, peak power clipping and channel noise.
These problems can be discussed by using simulation results for CDMA technique as shown:
1] BER vs Number of users in a cell
The BER for reverse page link of a CDMA system increases as more users use the same cell.Fig.3.1 shows the BER expected based on number of users. The result is for an isolated cell with no interference from neighbouring cells, no multipath effects, no channel noise. Any of these effects would worsen the BER.
2]Multipath immunity
BER Vs Multipath delay spread for CDMA reverse link.
The noise due to multipath leads to increase in the amount of interference seen by each user, & thus increasing the received BER. From Fig.3.2 it can be seen that the BER is essentially flat for delay spread of greater than one chip time(0.8 microsecond) which to be expected as the reflected signal becomes uncorrelated. In Fig. 3.2, the multipath delay spreads leads to increase in equivalent No. of users in the cell as it increases the amount of interference seen by receiver.
3] Peak power clipping
If the transmission technique is tolerant to peak power clipping, then it allows the signal to be clipped. This clipping of signal reduces the peak to RMS signal power ratio thus allowing the signal power to be increased for the sized Transmitter. Fig.3.3 shows effect of peak power clipping on both the reverse &forward links for CDMA.
Peak power clipping for the reverse page link is also likely to be small as only ever occur due to distortion in the base station receiver .a well designed receiver is unlikely to cause significant clipping of the signal. The forward page link shows significant clipping to the transmitter. The BER is low for peak power clipping of less than 10dB, above which the orthogonal nature of walsh code used begins to collapse.
4] Channel noise
The noise performance of CDMA reverse page link is shown in Fig.3.4 due to high ;level of inter-user interference the addition of channel noise leads to only a gradual rise in the BER. The BER of the lines (10 users, 20 users and 30 users) approaches approximately. The same BER at the SNR of 0dB.the BER is very bad for more than 10 users regardless of the channel SNR, thus making 20 or 30 users unusable. For 10 users the BER becomes greater than 0.01 at approximately a SNR of 14dB, which is maximum BER that can be normally tolerated for voice communication.
