Conference Paper

A Low Interference Time-Slicing Code Assignment for the 2D-Spread MC-DS-CDMA Systems

Inst. of Comput. & Commun. Eng., Nat. Cheng Kung Univ., Tainan
DOI: 10.1109/VETECS.2009.5073304 Conference: Vehicular Technology Conference, 2009. VTC Spring 2009. IEEE 69th
Source: IEEE Xplore

ABSTRACT In addition to some design flexibilities, the low interference property of code division multiple access (CDMA) makes multi-carrier direct sequence CDMA (MC-DS-CDMA) suitable for possible unlicensed transmissions in the licensed bands. Introducing the concept of time division multiple access (TDMA) into MC- DS-CDMA may further create a new dimension dealing with the interference problem. In this paper, we proposed a time slicing code assignment for the two-dimensional (2D) spread MC-DS- CDMA. The time slicing technique distributes interference equally over transmission time intervals (TTIs) by assigning users different frequency-domain spreading code groups and transmitting in their corresponding TTIs. Combing with the previous interference avoidance code assignment and power control mechanism for the 2D-spread MC-DS-CDMA, the proposed time slicing scheme is proved to be capable of eliminating interference more efficiently. In terms of the call admission rate, the improvement can be as large as 10% in one of the considered cases. A good operating point of time slicing can also maintain a high throughput, while achieving lower call dropping rate and lower power consumption. The results of this paper can provide some important and interesting hints for the system designers.

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    ABSTRACT: Maintaining the quality-of-service (QoS) requirement while eliminating interference is an important but difficult task in the downlink MC-DS-CDMA with two-dimensional (2D) spreading. To achieve this goal, we integrate the power control (PC) mechanism and the interference avoidance code assignment strategy. This joint policy is to firstly eliminate interference via the interference avoidance (IA) code assignment strategy and then maintain the QoS via power control. To facilitate the code assignment strategy, by performance analysis, we define a new PC-based multiple access interference (MAI) coefficient. Thanks to the help of the MAI coefficient, we can evaluate each candidate code by quantitatively predicting the possible interference it may produce. With this interference pre-evaluation, the extra MAI in the system can be avoided. Subsequently, the power control mechanism can maintain the QoS using less power to further eliminate MAI. The simulation results show that the proposed joint scheme can maintain a predefined signal quality and achieve a high code admission rate while reducing the power consumption to 68% in the considered case.
    IEEE Transactions on Wireless Communications 01/2009; 8:5582-5591. · 2.76 Impact Factor
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    ABSTRACT: In a multicarrier system, transmit power allocation over different subchannels is an effective means of improving the performance. We develop the optimal transmit power allocation scheme to improve bit-error rate (BER) performance in a multicarrier system with diversity reception. A simple suboptimal scheme is also derived from the optimal one, and an asymptotic case referred to as the equal-signal-to-noise ratio scheme is discussed. Numerical results show that the optimal and suboptimal power allocation schemes significantly outperform the equal power allocation scheme. The effects of the modulation level, the number of receiving antennas, and the number of subchannels on the BER performance are also investigated.
    IEEE Transactions on Communications 11/2004; · 1.98 Impact Factor
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    ABSTRACT: We propose a subcarrier allocation scheme for multiple access MC-DS-CDMA systems, where random signature sequences are used to identify the users. The optimal solution to such a subcarrier allocation problem has its complexity grows exponentially as the number of users and subcarriers grow. We proposed a sub-optimal method. In our proposal, each user performs channel measurement, orders the subcarrier channel gains and feedback the information in the order of the subcarriers channel gains to the base station. The base station uses these information from all users, together with the previous subcarrier assignment matrix, adaptively generates the next subcarrier assignment matrix for forward link transmission, with the objective to minimize the overall system BER performance. While selecting the subcarriers, we take into consideration the multiple access interference.
    Communications, 2004 IEEE International Conference on; 07/2004

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