New Spreading Codes for MC-CDMA and OFDM Systems (original) (raw)
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Large Code Set for Double User Capacity and Low PAPR Level in Multicarrier Systems
… on Fundamentals of Electronics, Communications and …, 2008
In this paper, a new large spreading code set with a uniform low cross-correlation is proposed. The proposed code set is capable of (1) increasing the number of assigned user (capacity) in a multicarrier code division multiple access (MC-CDMA) system and (2) reducing the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) system. In this paper, we derive a new code set and present an example to demonstrate performance improvements of OFDM and MC-CDMA systems. Our proposed code set with code length of N has K = 2N + 1 number of codes for supporting up to (2N + 1) users and exhibits lower cross correlation properties compared to the existing spreading code sets. Our results with subcarrier N = 16 confirm that the proposed code set outperforms the current pseudo-orthogonal carrier interferometry (POCI) code set with gain of 5 dB at bit-error-rate (BER) level of 10 −4 in the additive white Gaussian noise (AWGN) channel and gain of more than 3.6 dB in a multipath fading channel.
A Novel Orthogonal Minimum Correlation Spreading Code in CDMA System
International Journal of Wireless and Microwave Technologies, 2014
Code Division Multiple Access (CDMA) is a technique in which transmission of information takes place simultaneously over the same available channel bandwidth. CDMA systems make use of spread spectrum (SS) technique for transmission of information by employing spreading codes. Each user is assigned with a unique spreading code which acts as a signature code for that individual user. The CDMA system experiences Multiple Access Interference (MAI) and Inter Symbol Interference (ISI) because of the non-orthogonality of the spreading codes. Both the MAI and ISI are the functions of auto-correlation and cross-correlation values of the spreading code respectively. In this paper, a novel orthogonal spreading code called "Orthogonal Minimum Correlation Spreading Code" (OMCSC) has been proposed which can serve a large number of users and is simultaneously expected to reduce the effect of MAI and ISI. Moreover, the Bit Error Rate (BER) performance of the proposed code has been compared with the existing codes using Additive White Gaussian Noise (AWGN) channel under multiuser scenario.
IET Communications, 2013
Future 4G systems require transmission of richer multimedia services which inevitably implies an increase in data rate. Orthogonal frequency and code division multiplexing (OFCDM) technique has shown promising results in achieving a high data rate while simultaneously combating multipath fading. OFCDM is an amalgamation of orthogonal frequency division multiplexing and two-dimensional (2D) spreading. 2D spreading helps to achieve diversity gains in both time and frequency domains. The present OFCDM systems employ 1D orthogonal variable spreading factor (OVSF) codes to achieve the required 2D spreading in code multiplexed channels. However, 2D OVSF codes have better correlation properties in comparison to 1D OVSF codes. Motivated by this principle, the authors propose a spreading scheme for OFCDM systems using 2D OVSF codes. The spreading scheme is designed to increase the system throughput and reduce multi-code interference. Here, the authors study the OFCDM system performance using the proposed spreading scheme in a multipath fast fading channel with varying spreading factors in both time and frequency domains. The results are compared with the existing OFCDM systems using 1D OVSF codes.
New Multi Level Spreading Codes for DS CDMA Communication
2013
Spreading codes are used to distinguish users and spread the signal to occupy much wider band width than the minimum required band width. Spreading codes are also called as user codes. Walsh, Gold and Kasami codes are the popularly used Binary (2-level) user codes. This paper proposes new multi level spreading codes constructed using ternary and quaternary Gray and Inverse Gray codes for multiuser Direct Sequence Code Division Multiple Access systems. Multi level spreading codes discussed in this paper are non zero mean, varying power codes. An attempt is made to analyze these multi level user codes through correlation properties. The methodology explained in this paper allows to generate 3-level user codes of length-6 and 4-level user codes of length-8. It also allows generation of codes whose lengths are even multiples of the codes of length 6 and length 8. The auto and cross correlation properties and bit error rate performance of these codes and their comparison with those of Wa...
IJERT-Dynamic Spreading Code Allocation Strategy for A Downlink MC-CDMA System
International Journal of Engineering Research and Technology (IJERT), 2012
https://www.ijert.org/dynamic-spreading-code-allocation-strategy-for-a-downlink-mc-cdma-system https://www.ijert.org/research/dynamic-spreading-code-allocation-strategy-for-a-downlink-mc-cdma-system-IJERTV1IS5304.pdf The MC-CDMA (Multi-Carrier Code Division Multiple Access) transmission is a promising solution for the physical layer of future broadband wireless communication systems which will have to support multimedia services. By combining OFDM (Orthogonal Frequency Division Multiplexing) and CDMA, we obtain a high speed transmission capability in multipath environments and large multiple access capacity. Unlike CDMA, MCCDMA performs the spreading operation in the frequency domain, mapping each chip of the user spreading code on one subcarrier, and thus introduces frequency diversity. However when the MC-CDMA signal propagates through a frequency selective fading channel, the code orthogonality is destroyed and the resulting MAI limits the system performance. Several approaches have been proposed to mitigate MAI and to improve signal detection. The conventional single-user detection techniques, applying per subcarrier equalization as in OFDM systems and then correlation with the code of the desired user, offer poor performance. Indeed, by channel inversion, ZF (Zero Forcing) can eliminate MAI but in return-noise amplified on deeply faded subcarriers. The other techniques including MRC (Maximum Ratio Combining), EGC (Equal Gain Combining) and MMSE (Minimum Mean Square Error) cannot restore the orthogonality of codes and lead to residual MAI. Therefore more advanced methods such as MUD (Multi-User Detection) have been developed. In this paper, we investigate the impact of Walsh-Hadamard spreading code allocation on the performance of a downlink MC-CDMA system in a time varying frequency selective channel. The analysis shows that this impact is important on the multiple access interference and the inter-carrier interference power levels. We propose a code allocation strategy that minimizes the global interference power and significantly improves the performance of the MC-CDMA system.
Low Density Spreading for next generation multicarrier cellular systems
2012
Multicarrier-Low Density Spreading Multiple Access (MC-LDSMA) is a promising technique for high data rate mobile communications. In this paper, the suitability of using MC-LDSMA in the uplink for next generation cellular systems is investigated. The performance of MC-LDSMA is evaluated and compared with current multiple access techniques, OFDMA and SC-FDMA. Specifically, Peak to Average Power Ratio (PAPR), Bit Error Rate (BER), spectral efficiency and fairness are considered as performance metrics. The link and system-level simulation results show that MC-LDSMA has significant performance improvements over SC-FDMA and OFDMA. It is shown that using MC-LDSMA can significantly improve the system performance in terms of required transmission power, spectral efficiency and fairness among the users.
Code-spread CDMA using maximum free distance low-rate convolutional codes
IEEE Transactions on Communications, 2000
maximum theoretical performance can be obtained by spreading with low-rate error control codes. Previously, orthogonal and super-orthogonal codes have been proposed for this purpose. We present in this paper a family of rate-compatible low-rate convolutional codes with maximum free distance. The performance of these codes for spectrum spreading in a CDMA system is evaluated and shown to outperform that of orthogonal and super-orthogonal codes as well as conventionally coded and spread systems. We also show that our low-rate codes will give simple encoder and decoder implementations. With these codes any 1=n rate can be obtained for constraint lengths up to 11, resulting in a more exible and powerful scheme than those previously proposed. Keywords| CDMA, low-rate convolutional codes, orthogonal codes, super-orthogonal codes, rate-compatible, maximum free distance.
Spreading Code Design for Downlink Space-Time-Frequency Spreading CDMA
IEEE Transactions on Vehicular Technology, 2008
In this paper, we analyze the recently proposed downlink space-time-frequency spreading code-division multiple-access (STFS-CDMA) scheme. A spreading code design criterion is first derived for STFS-CDMA. From the spreading code design criterion, we can see that the two original spreading codes adopted in STFS-CDMA, i.e., the Walsh-Hadamard code (WHC) and the double-orthogonal code (DOC), both cannot achieve full space and frequency diversity, no matter how many users exist in the system. Then, a novel spreading code, i.e., permutated DOC (PDOC), is proposed. PDOC-coded STFS-CDMA (PDOC-STFS-CDMA) can obtain full space and frequency diversity when the number of users in the system is only one, but it cannot obtain full space and frequency diversity when the number of users is larger than one.
IEEE Communications Letters, 2000
We present a new dynamic spreading code selection technique to obtain low peak-to-average-power ratio (PAPR) of an orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) system with 4-QAM modulation for the down-link in mobile communication. In this method, initially, each user is assigned a low and a high PAPR spreading code, which allows selecting one when the system is operating. The spreading code of each user is dynamically selected so that total PAPR level of the whole user group present in the system is minimized. With the proposed technique, the PAPR level of a system with 10 users and 64 sub-carriers using 64 chip Walsh-Hadamard (WH) codes, can be limited to 15 dB while the worst case theoretical maximum could go up to 28.1 dB. Although 64 chip WH codes are employed to evaluate the performance it can be generalized to all other sets of spreading codes.
MIMO-OFDM scheme based on Permutation spreading
2012
In this paper, a novel transmission scheme is developed to effectively combine permutation spreading technique with MIMO-OFDM to obtain improved bit error rate performance in the presence of frequency selective fading channels with low system complexity. Unlike conventional MIMO-OFDMA, where users are separated in different frequency bands (subchannels), and each user is coded separately using STBC or SFBC, the proposed