CHAPTER 1

INTERLEAVE DIVISION MULTIPLE ACCESS (IDMA)

By

SAJJAD ASHRAF

MOBILE COMMUNICATION SYSTEMS

DL STUDENT

SP2013-MSEE-011

CENTER FOR ADVANCED STUDIES IN ENGINEERING

sajjadashraf@gmail.comMob No. 0321-8503485

ABSTRACTInterleave-division multiple-access (IDMA) is a multiple access scheme that employs chip-level interleavers for user separation. The performance of code-division multiple-access (CDMA) systems is mainly limited by multiple access interference (MAI). Interleave Division Multiple Access (IDMA) is a multiuser scheme which enables user separation on the basis of interleavers. It can also be regarded as a special form of CDMA by treating interleaving index sequences as multiple access codes. It allows a very simple chip-by-chip (CBC) iterative MUD strategy with complexity (per user) independent of the user number. In this project, I have studied and discussed transmitter and receiver architecture of IDMA. TABLE OF CONTENTS41.INTRODUCTION TO CDMA

41.1 Code Division Multiple Access (CDMA)

41.1.1 Why CDMA?

41.1.2 What is CDMA?

51.1.3 Spread Spectrum Characteristics

51.2 Limitations in CDMA system

62. INTERLEAVE DIVISION MULIPLE ACCESS (IDMA)

62.1 Why IDMA?

62.2 What is IDMA?

72.3 Comparison of IDMA with CDMA

82.4 IDMA Transmitter Structure

82.4.1 Spreader

82.4.1.1 FEC Encoder

92.4.1.2 Spreader:

92.4.2 Interleaver:

92.4.3 Channel Coefficient h(k):

102.5 Iterative Chip-by-Chip Receiver

102.5.1 The First Iteration:

112.5.2 Random Deinterleaver:

112.5.3 The Decoder:

122.5.4 Random Interleaver:

122.5.5 The Second Iteration:

122.5.6 Decoder for 2nd Iteration:

132.6 Conclusion

1. INTRODUCTION TO CDMAIn this chapter, I will emphasize on the features of CDMA and then I will proceeds towards IDMA. I have discussed limiting factors in CDMA that forms the basis towards the research of a new multiple access technique. IDMA is normally defined as the special case of CDMA. Therefore after gaining basic knowledge of CDMA, I will move to the introduction of IDMA.1.1 Code Division Multiple Access (CDMA)Some of the main features of CDMA related to IDMA are high lightened as:

1.1.1 Why CDMA?The first CDMA network was commercially launched in 1995, and provided roughly ten times more capacity than analog networks, more than TDMA-based DAMPS or GSM. Since then, CDMA-based mobile cellular has become the fastest growing of all wireless technologies, with over 100 million subscribers worldwide today. In addition to supporting more traffic, CDMA-based mobile cellular systems bring many other benefits to carriers and consumers, including better voice quality, broader coverage, lower average power emission, stronger security, privacy, low probability of interception, good protection against multi-path interference.1.1.2 What is CDMA?Code division multiple accesses exploit the nature of spread spectrum transmission to enable multiple users to independently use the same bandwidth with very little interference [1]. CDMA is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space. As its name implies, CDMA (Code Division Multiple Access) assigns unique codes to each communication to differentiate it from other users in the same spectrum. Figure 1.1 of CDMA transmitter is shown here: Figure 1.1 CDMA transmittersIn a world of finite spectrum resources, CDMA enables many more people to share the airwaves at the same time than do alternative technologies. Some more basic features of CDMA are as under: 1.1.3 Spread Spectrum CharacteristicsThe basic features of Spread spectrum in CDMA are:1. The signal occupies a bandwidth much greater than that which is necessary to send the information. This results in many benefits, such as immunity to interference and jamming and multi-user access.

2. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.3. Due to spread spectrum techniques eavesdropping is avoided.4. Same frequency can be use in all cells by using CDMA.1.2 Limitations in CDMA systemSome of the main disadvantages/limitations of CDMA are:

1. With the increase of users, the performance of CDMA systems decreases.2. Orthogonality of codes use to distinguish the users in CDMA is necessary otherwise MAI and self jamming will occur in the system.3. CDMA systems require good power control; otherwise system may suffer near-far problem, interference and noise. 2. INTERLEAVE DIVISION MULIPLE ACCESS (IDMA)IDMA may be considered as a special Code-Division Multiple Access (CDMA) scheme. Some description is as under:

2.1 Why IDMA?The performance of code-division multiple-access (CDMA) systems is mainly limited by multiple access interference (MAI). In the wake of the success of turbo codes, turbo-type iterative multi-user detection (MUD) has been extensively studied to mitigate MAI, and significant progress has been made. However, complexity and cost has always been a stringent concern for MUD. Iterative multi-user detection has been widely investigated as a potential approach to enhance the performance of CDMA systems and significant progress has been made recently. The complexity of CDMA multi-user detection has always been a serious concern for practical systems, which increases rapidly with the number of users. An interleave-based multiple access scheme has been studied in for high spectral efficiency, improved performance and low receiver complexity. IDMA inherits many advantages from CDMA, in particular, diversity against fading and mitigation of the worst-case other-cell user interference problem. Furthermore, it allows a very simple chip-by-chip iterative MUD strategy.2.2 What is IDMA?Interleave-division multiple-access (IDMA) [2] is proposed scheme that employs chip-level interleavers for user separation. It is a special case of CDMA in which bandwidth expansion is entirely performed by low-rate coding. It can also be regarded as a special form of CDMA by treating interleaving index sequences as multiple access codes. The performance of an IDMA system is strongly dependent on interleavers. Different users are separated only by different interleavers as opposed to different frequency carriers as in FDMA or different time slots as in TDMA or different signature sequences as in CDMA. All of the users employ a common spreading sequence. This scheme relies on interleaving as the only means to distinguish the signals from different users and hence it has been called interleave-division multiple-access (IDMA).

Figure 2.1 IDMA transmitters2.3 Comparison of IDMA with CDMAIDMA may be considered as a special Code-Division Multiple Access (CDMA) scheme. The differences and commonalities of conventional CDMA in conjunction with channel coding (top part) and IDMA (bottom part).

Figure 2.2 Comparison of IDMA with CDMAIDMA may be interpreted as follows:1. The ordering of interleaving and spreading is reversed. In IDMA, forward error correction (FEC) and spreading can be combined in a single encoder (ENC), which is the same for all users. As a consequence, very low-rate encoding is used. The spreader has no fundamental relevance any more. The spreader (i.e., a repetition code) may be used to simplify the overall encoder, however.2. The interleaver may be interpreted as a key word. Only authorized receivers are able to decode the message. 2.4 IDMA Transmitter StructureThe Figure 2.3 shows the transmitter structure of the multiple access scheme under consideration with K simultaneous users.

SHAPE \* MERGEFORMAT

Figure 2.3 Transmitter structure of an IDMA scheme with K simultaneous users

2.4.1 SpreaderIn CDMA the spreader comes after the Interleaver, which is key principle for user separation, shown in fig 2.1. While in IDMA, spreader consists of two parts, FEC encoder and Spreader.

2.4.1.1 FEC EncoderBoth CDMA and IDMA uses Low rate Convolution Encoder. Convolution codes can be devised for correcting random errors, burst errors or both [3]. It consists of buffer which takes 1 bit and transforms it into 2 bit encoded stream. Basic Convolution encoder is shown in figure below,

Figure 2.4 Convolution Encoder of rate 1/2 In this model, the input data sequence dk of user-k is encoded based on a low-rate code C, generating a coded sequence ck= [ck (1),, ck (j), ..., ck (J)]T, where J is the frame length. Both of Convolution encoded streams are concatenated and then treated as independent channel for every user. 2.4.1.2 Spreader:Referring to figure 2.3, the spreading operation in CDMA is done after interleaving and is used for user separation. Each user is assigned orthogonal code and Number of user is dependant on length of spreading code.

While in IDMA, spreading is performed before Interleaving and is not used for user separation. There are basic two types of spreading codes, Orthogonal Codes and Repetitive codes.Orthogonal codes consist of equal number of 1 and -1. Data is balanced i.e., consist of equal number of 1s and -1, after spreaded by Orthogonal codes. CDMA uses orthogonal codes.While in Repetitive code, each bit is repeated into number of times, equal to the length of spreaded code. An example of repetitive code is [1 1 1 1 1 1 1 1]. In repetitive code, data after spreading is biased towards 1 or -1 depend upon the value of spreading code.2.4.2 Interleaver:As discussed earlier, the main difference between CDMA and IDMA is interleaving. The basic purpose of Interleaver in CDMA is to combat against burst errors. While the only mean for user separation in IDMA system is unique Interleaver, which is user specified. There are two basic conditions for designing of Interleavers.

Firstly, Data within User should be UNCORELATED. That is, there should not more than one bit in each spreaded sequence. Further, any bit at any location in a byte should be uncorrelated. Secondly, Data of all Users should be UNCORELATED. This can be maintained by giving different interleaving pattern for different users.

Referring figure 2.3, ck is permutated by an interleaver (k, producing xk = [xk (1), , xk(j), , xk(J)]T. Following the CDMA convention, the elements in xk are called chips. 2.4.3 Channel Coefficient h(k):Channel coefficient is used for following purposes,

a. Used for unequal power distribution for all the users. This is basic requirement for approaching Shannons limit.b. Used to combat Fading and Multipath effects.

2.5 Iterative Chip-by-Chip Receiver

Receiver structure of an IDMA scheme with K simultaneous users are shown in figure 2.5. The iterative chip-by-chip receiver consists of an Elementary Signal Estimator (ESE) and a bank of K single-user a posteriori probability decoders DES. We first assume that the channel has no memory. Then the ESE starts a chip by chip estimation called the Maximum Likelihood estimation. The received signal at time instant j can be written as

j=1,2,..J (2.1)where denotes the transmitted chip from user-k at time instant j, the channel coefficient for user k, and nj zero-mean additive white Gaussian noise (AWGN) with variance 2 = N0/2.

Figure 2.5 Receiver structure of an IDMA scheme with K simultaneous users2.5.1 The First Iteration:

The iterative chip-by-chip receiver in Fig. 2.5 consists of an elementary signal estimator (ESE) and a bank of K single-user a posteriori probability decoders for the despreading operation (DES) working in a turbo-type manner, as shown in Fig. 2.5. The ESE performs chip-by chip estimation.

(2.2)where represents a distortion term with respect to . During the first iteration ESE allocates equal energy to every bit of every user which is the mean energy obtained by dividing the received bit to number of users (8,16 and 32) by using the following equation.

(2.3)

Each as a random variable with mean E () and variance Var () (initialized to 0 and 1 respectively). During first iteration the mean of random variable and the mean of received bit would be same because every user has the same amount of distributed energy. This information is then sent to Random Deinterleaver.2.5.2 Random Deinterleaver:

For user-k, the corresponding ESE outputs are de-interleaved to form and delivered to the DES for user-k. Every user deinterleaver is distinguished by different and unique initial seed provided to it. 2.5.3 The Decoder:

The IDMA decoder has two distinct parts; The Despreader (DES) and, The Decoder.

Output of every Deinterleaver works as input to the decoder. The DES performs a soft-in/soft-out chip-by-chip de-spreading operation as detailed below. For simplicity, we focus on the chips related to the first bit of user-k. The treatment for other chips is similar. Recall that is spread into a chip sequence by using spreading code.

The in fig 2.5 is uncorrelated which is approximately true due to interleaving. Using the A posteriori probability (APP) each received bit is despreaded by multiplying it with the spreading code in bitwise manner and then divides it by the length of spreading code. This despreaded data is then respreaded in a similar way as did in the transmitter. The a posteriori LLR for can be computed using as

(2.4)The extrinsic LLR for a chip within for only first iteration is defined by

Ext {()}=

(2.5)2.5.4 Random Interleaver:

Let the interleaving for user-k be expressed as i.e =. The output from Decoder (extrinsic LLRs) is now fed back to ESE for the second iteration. This output for every user passes through unique Random interleaver (which is same as used in transmitter for every user) before entering ESE.

2.5.5 The Second Iteration:

For this and onward iteration the previously initialized mean and variance (as 0 and 1 respectively) are updated to new values calculated as follows

(2.6a)

2

(2.6b)

Using the central limit theorem, in (2) can be approximated by a Gaussian random variable with

(2.7a)

(2.7b)

The ESE outputs are the logarithm likelihood ratios (LLRs) about {} computed based on (2.7) as

(2.8)2.5.6 Decoder for 2nd Iteration:

The decoder implements the following equations to calculate for next iterations.

(2.9a)

(2.9b)

This is the decision making point and =+1 if = and =-1 otherwise Therefore,

(2.9c)Difference between 1st and 2nd Iteration Decoder:

In the 1st iteration take the mean of individual bit i.e E(xj) and respread it. While in the 2nd iteration we take the difference (respreaded data) and (input to the decoder). This iterative process is repeated a preset number of times. In the final iteration the DES produces hard decisions on information bits based on (2.5).The detection algorithm does not rely on coding unlike other methods used in CDMA, but introducing coding can further enhance performance (details omitted here). The principle can be generalized to situations with multipath fading. For complexity, notice that (2.3) involves summations over all of the users, but the results (and so the cost) are shared by all of the users.

2.6 Conclusion

A new multi user scheme named IDMA has been studied. The basic principle is to use interleavers for user separation. It allows very low-cost multiuser detection. The related cost is only two additions and two multiplications per chip per user per iteration. A very large number of users can be processed with modest computing power. Complexity is not a serious concern for multiuser detection. Near Shannon capacity performance is observed for multiple access channels.

BIBLIOGRAPHY

[1] William Stallings., Data and Computer Communications, 7ed, Prentice Hall,Inc., February 8, 2006.[2] Li Ping, Lihai Liu, K. Y. Wu, and W. K. Leung, Interleave-Division Multiple-Access, IEEE Trans. Wireless Commun., Vol. 5, No. 4, Apr. 2006, pp. 938-947.

[3] B.P Lathi., Modern Digital and Analog Communication System, 3rd ed., Oxford University Press., March 1998.[4] Bernard Sklar., Digital Communications-Fundamentals and applications,2nd ed, Pearson Education,Inc., New Delhi, 2001 [5] Simon Haykin, Michael Moher., Modern Wireless Communications, Pearson Education , Inc., India, 2005.

[5] Behrouz A.Forouzan., Data Communications and Networking,3rd ed, McGraw-Hill,Inc., New York,2003.[6] Theodore S. Rappaport., Wireless Communications- Principles and Practice, 2nd ed, Prentice Hall, Inc., December 2001. X2

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