The Wireless Communication Industry

Published Date: 02 Nov 2017

Abstract

Cdma2000 belongs to the family of 3G mobile technology standards and make use of cdma channel access to be able to send voice, information and signaling data between mobile stations and base stations with a varying data rate of 9.6kbps and 2Mbps. The performance of cdma2000 with convolutional code and turbo code in fading channel is investigated in this dissertation. Only the forward link of cdma2000 is considered. Performance of turbo code in AWGN and Rayleigh fading channel without cdma2000 is also evaluated. Effects of different number of iterations, puncturing, interleaving and increasing frame length are analyzed. The simulations are performed using MATLAB software. Turbo code is known to be a better error correcting coding method and hence it has a superior performance than convolutional code. It is also more preferably used in data applications than voice communications.

Chapter 1

Introduction

Over the past decade there has been a phenomenal growth in the wireless communication industry, including both the mobile technology and their subscribers. The beginning was with the first generation cellular system which was introduced in 1980, and it used analog transmissions for traffic. The first generation had limited data rates and capacity. The need for major improvement in the communication system was vital so that the ever increasing need for newer services can be fulfilled. Therefore, in the end of 1980s, the second generation (2G) was introduced as the first digital cellular system. GSM was by far the most successful and widely used 2G system. Based on the original GSM system more advanced systems such as the 2.5G and the 2.75G have also been developed.

During the twenty first century there has been remarkable demand to enhance data rate to support a large range of multimedia services. This has resulted to the introduction of the third generation (3G) which can provide for packed switched data more effectively. Moreover, it has a higher rate of transmission and is more secure compared to the 2G systems. In this digital era, the need for future systems to be more reliable, secure, cheap and above all having the ability to deliver excellent performance at faster rates, has emerged with the fourth generation (4G) mobile communication system. Even if the fourth generation is new to the market, it is already competing with the existing systems. However, the 4G system is still undergoing through developments so that it can achieve a much higher performance.

The table below shows the evolution of the wireless communication industry. The services provided and coding techniques used by the different generations are also shown in this table.

Table 1.1: Different Generations of the Wireless Communication Industry

Standard

Technique

Used

Services Offered

Popular Systems

1G

FDMA

Voice Communications

AMPS, TACS, NMT

2G

TDMA

Or

CDMA

Voice Communications and Slow Data Transmissions

D-AMPS, CDMA, PDC, GSM

3G

CDMA

Voice Communications, Data Transmissions, Multimedia Services like video calls/ conferencing and Internet Services

W-CDMA, cdma2000

4G

OFDMA

Voice Communications, Data Transmissions, Multimedia Services, Internet Services, Online Gaming, HDTV

LTE, WiMAX

This dissertation is principally based on the third generation communication systems. In particular, the cdma2000 standard will be considered in more details. Cdma2000 standard was proposed by the Telecommunications Industry Association (TIA) of the United States and it is backward compatible to the IS-95 standard (Mandyan and Lai, 2002). It employs the spread spectrum techniques and it operates on a 5MHz bandwidth. The direct sequence CDMA scheme will be taken into account. The cdma2000 is specified for both the forward link and the reverse link, but only the performance of the forward link with Convolutional and Turbo channel coding schemes will be investigated.

The beginning of the report is by introducing the code division multiple access (CDMA) scheme in chapter 2. This chapter will also include details on the spread spectrum technique, the direct sequence CDMA and the cdma2000 system. Then chapter 3 consists of radio waves propagation and explanation on fading channel. It is followed by chapter 4 which emphasizes on the transmitter and receiver structures of cdma2000. The methodology and software implementation is presented in chapter 5. Chapter 6 deals with results and discussions. Lastly, chapter 7 is based on the conclusion and gives details about future works which can be done.

Chapter 6

Results and Discussions

The graphical representations of the simulation results are presented in this chapter since they offer a more appropriate method for the comparison purposes. Comparison of performance in the cdma2000 system is more suitably described by the bit error rate with respect to varying signal to noise ratio ().

There are several parameters that affect the performance of the cdma2000 system which include number of iterations when using turbo codes, puncturing, interleaving and input data frame length. All the parameters mentioned are being considered to analyze the performance of cdma2000.

First of all the performance of cdma2000 with convolutional code in Rayleigh fading channel is discussed. It is then followed by the performance of turbo code in AWGN and Rayleigh fading channel without cdma2000 system and the effect of puncturing in the respective channel is observed. Turbo code in cdma2000 system is then investigated with the effect of interleaver shown. The effect of input data frame length is also considered. Finally the performance of cdma2000 with convolutional and turbo code in fading channel is compared. A brief discussion of the results is given after each graph.

6.1 Performance of cdma2000 with Convolutional Code in Rayleigh Fading Channel

Table 6.1: Parameters for Convolutional Coding In cdma2000

Bit Rate

9.6 kbps

Chip Rate

3.6864 Mcps

Channel

Rayleigh Fading

Parameters for Convolutional Code:

Code Rate of 1/3

Generator Code: and constraint length

Code Rate of 1/4

Generator Code: and constraint length

Decoder

Soft Decision Viterbi Decoding

Figure 6.1: Performance of cdma2000 with Convolutional Coding in Rayleigh Fading Channel

In the above figure, two different code rates together with the uncoded BER are compared. As seen, the use of interleavers realizes a significant coding gain since they are known to reduce effect of burst errors (as described in section 4.2.4).

Initially the performance of the uncoded BER outperforms that of the coded BER. However, as the SNR increases, there is a dramatic improvement in the performance of the coded BER. It is also noted that that the performance in terms of the bit error rate for the code rate of 1/4 is better than that of code rate 1/3. The coding gain for these two code rates is about 1.2 dB.

6.2 Performance of Turbo Code in AWGN Without cdma2000

Table 6.2: Parameters of Turbo Coding in AWGN without cdma2000

Channel

AWGN

Modulation

BPSK

Recursive Systematic Convolutional (RSC) parameters

n = 2, k = 1 and K = 3

Polynomial Code:

Interleaver

Pseudorandom Interleaver

Puncturing

Code Rate of 1/2

Decoder

Log-MAP Algorithm

6.2.1 Performance of Turbo Code in AWGN for Different Number of Iterations

Figure 6.2: Performance of Turbo Code in AWGN with Different Number of Iterations

From this figure, it can be observed that at low SNR the performance of convolutional code is better than the performance of turbo code. As the value of increases the performance of turbo codes recovers rapidly. Hence, at high values turbo code provides a more superior performance than convolutional code. In addition, it is seen that turbo code performs much better as the number of iteration increases but there is only a slight improvement in the performance with 16 iterations compared to 8 iterations since there is enough information in the decoders. The coding gain between 8 iterations and 16 iterations is only about 0.2 dB.

6.2.2 Effect of Puncturing of Turbo Code in AWGN

Figure 6.3: Effect of Puncturing of Turbo Code in AWGN

In figure 6.3, the performance of turbo code in AWGN with a code rate of 1/3 is compared to a code rate of 1/2. A code rate of 1/3 signifies that no puncturing has been done while code rate 1/2 implies that half parity bits from each encoder are punctured.

The performance when using a code rate of 1/3 is much better than when using a code rate of 1/2. The coding gain is approximately 0.3 dB. If puncturing is carried out the performance degrades. However if all the parity bits are transmitted, i.e., a code rate of 1/3 is used, it will require a larger bandwidth. Hence, there is a trade-off between the performance and the required bandwidth.

6.3 Performance of Turbo Code in Rayleigh Fading Channel without cdma2000

Table 6.3: Parameters of Turbo Coding in Rayleigh Fading Channel without cdma2000

Channel

Rayleigh Fading

Modulation

BPSK

Recursive Systematic Convolutional (RSC) parameters

n = 2, k = 1 and K = 3

Polynomial Code:

Interleaver

Pseudorandom Interleaver

Puncturing

None

Decoder

Log-MAP Algorithm

6.3.1 Performance of Turbo Code in Rayleigh Fading Channel for Different Number of Iterations

Figure 6.4: Performance of Turbo Code in Rayleigh Fading Channel with Different Number of Iterations

As the number of iteration increases, the performance of turbo code in Rayleigh fading channel improves significantly. Here, the assumption made is that the fading amplitude and phase difference caused by the channel are estimated accurately by the receiver.

6.3.2 Effect of Puncturing of Turbo Code in Rayleigh Fading Channel

Figure 6.5: Effect of Puncturing of Turbo Code in Rayleigh Fading Channel

The code rate of 1/3 (unpunctured) offers a better performance than the code rate of 1/2 at the expense of the required bandwidth. If all parity bits are transmitted (code rate 1/2) there is a significant degradation in the performance. The coding gain is about 1 dB which is quite large if we compared it to the AWGN channel (Figure 6.3)

6.4 Performance of cdma2000 With Turbo Code in Rayleigh Fading Channel

Table 6.4: Parameters for Turbo Coding in cdma2000

Channel

Rayleigh Fading

Modulation

BPSK

Recursive Systematic Convolutional (RSC) parameters

n = 2, k = 1 and K = 3

Polynomial Code:

Interleaver

Pseudorandom Interleaver

Puncturing

None

Decoder

Log-MAP Algorithm

Frame Length

1024

6.4.1 Effect of Interleaver in the Performance of cdma2000 with Turbo Code

Figure 6.6: Effect of Interleaver in Performance of cdma2000 with Turbo Code

In this figure, turbo code with 5 iterations has been used to show the effect of interleaver. There is a considerable improvement in the performance of turbo code if interleavers are employed. Here the coding gain is about 2 dB. Interleavers reduce the effect of burst errors and hence fading effects also are reduced.

6.4.2 Effect of Frame Length in the Performance of cdma2000 with Turbo Code

Figure 6.7: Effect of Frame Length in the Performance of cdma2000 with Turbo Code

The performance of turbo code enhances as the frame length increases. However with increasing frame length there is a considerable time delay being introduced. Hence large frame lengths cannot be used when transmitting voice, but they can be acceptable if only data is being transmitted.

6.5 Comparison of the Performance of cdma2000 with Convolutional Code and Turbo Code in Fading Channel

Figure 6.8: Comparison of the Performance of cdma2000 with Convolutional Code and Turbo Code in Fading Channel

The uncoded BER, convolutional code with a rate of 1/4 and turbo code with 5 iterations are compared in the above figure. Turbo code has a higher coding gain than convolutional code. It outperforms the convolutional code by approximately 4 dB. Hence, the performance of cdma2000 with turbo code is much better than that of convolutional code.

Chapter 7

Conclusion

7.1 Conclusion

The main objective of this project was the comparative analysis of cdma2000 with convolutional and turbo coding in fading channel. The cdma2000 system was constructed using the software and its performance was evaluated for both coding schemes. For the turbo code, its performance was first investigated in AWGN and Rayleigh fading channel. Then it was subjected to the cdma2000 system where the effect of interleaver and frame length were considered.

From the simulation results obtained for the AWGN channel, it is seen that turbo code has a much better performance than convolutional code (Figure 6.2). From the first iteration itself, it is observed that there is a coding gain of about 1 dB compared to convolutional code. As the number of iterations increases, the performance of turbo code tends to approach the Shannon limit. The same result can be observed for turbo code in Rayleigh fading channel with different number of iterations.

After 8 iterations, the improvement in performance is nearly negligible. Higher number of iterations causes a delay in the decoding process. Hence in practical situations a trade-off must be made between the BER and the number of iterations. From figure 6.3 and figure 6.5, it is also seen that puncturing degrades the performance in both the AWGN and Rayleigh fading channel. Here also, a trade-off between the required bandwidth and performance must be done. However, it is noted that the performance of turbo code in AWGN exceeds the performance in the Rayleigh fading channel.

In cdma2000 system, the use of channel interleavers increases the performance significantly. As seen in figure 6.6, a coding gain of about 2dB is achieved. Hence channel interleavers provide a diversity technique which can lessen the effect of fading drastically. The effect of frame length was also investigated and it was found that larger frame lengths provide a more outstanding performance. Nonetheless this improvement in performance is at the expense of considerable time delays. These latencies are not tolerable in voice communications.

The aim of the simulations was to determine which coding scheme provides a better performance in cdma2000. Figure 6.8 clearly shows that turbo code has a superior performance than convolutional code. Nevertheless, in practical situations there are some limitations as turbo codes are not appropriate for voice communication but they do work efficiently for data applications.

7.2 Possible Areas for Future Investigations

The following future works can be implemented and hence improving the results obtained in this project:

In this project only the forward dedicated physical channel, which is meant for a single user, has been considered. The performance of cdma2000 in the forward common physical channel can be evaluated for multiple users. Therefore to have a better idea how the performance is affected in real systems, the effects of multi-user interference can be taken into account.

The multi-carrier technique is able to deal with multi-user interference more effectively and the 5MHz bandwidth is shared between three single mode carriers. Hence instead of Direct Sequence cdma2000, the Multi-Carrier cdma2000 can be considered.

To obtain a better idea about the decoding process of turbo codes, other decoding algorithms such as the Max Log-MAP algorithm and the SOVA algorithm can be used. The performance of these two algorithms is slightly worse than the Log-MAP algorithm but they provide less complexity.