Thit Minn

Dynamic assignment of orthogonal variable-spreading-factor codes in W-CDMA

This paper presents an optimal dynamic code assignment (DCA) scheme using orthogonal variable-spreading-factor (OVSF) codes. The objective of dynamic code assignment is to enhance statistical multiplexing and spectral efficiency of W-CDMA systems supporting variable user data rates. Our scheme is optimal in the sense that it minimizes the number of OVSF codes that must be reassigned to support a new call. By admitting calls that would normally be blocked without code reassignments, the spectral efficiency of the system is also maximized. Simulation results are presented to show the performance gain of dynamic code assignment compared to a static assignment scheme in terms of call blocking rate and spectral efficiency. We also discuss various signaling techniques of implementing our proposed DCA scheme in third-generation wideband CDMA systems.

Supporting rate guarantee and fair access for bursty data traffic in W-CDMA

This paper presents a new protocol for statistical multiplexing of bursty data traffic in the forward (base-to-mobile) link of a wireless wideband code division multiple access (W-CDMA) system using orthogonal variable spreading factor (OVSF) codes. At the heart of the protocol is an efficient scheduling algorithm that dynamically assigns an OVSF code to a mobile user on a timeslot-by-timeslot basis and allows many users with bursty traffic to share a limited set of OVSF codes. An important feature of our protocol is that it can provide a heterogeneous data rate guarantee to each mobile user and fully utilize the system capacity. Moreover, the unreserved bandwidth of the network can be shared fairly among competing mobile users.

Exact bit-error rate analysis of multirate DS-CDMA schemes

The exact error probability of multirate schemes in DS-CDMA is computed. In particular, the performance of variable spreading-gain and multi-code transmission methods in both additive Gaussian noise and multipath fading channel models is studied. Conventional matched filtering and a diversity combining RAKE receiver are used at the receiver. Based on our results, we deduce conditions in which the bit-error rate (BER) measurement based on the classical Gaussian approximation is valid. We show that both transmission schemes are comparable under various channel models and signal-to-noise ratio levels.

Variable chip rate CDMA

A direct-sequence spread spectrum multiple access scheme called variable chip rate code-division multiple access (VCR-CDMA) is presented. The scheme supports different information bearing data rates for users or devices accessing various multimedia applications by varying the transmission (chip) rates and overlaying the frequency spectra of channels with different data rates. This scheme is applicable to home and office wireless environments that operate in unlicensed shared radio frequency bands. It is suitable for applications where various types of electronic devices and appliances of low duty cycle require tetherless connections to a central controller through a shared wireless medium. By assigning code channels of different chip rates, the total number of simultaneously active channels is increased. By using multiple orthogonal codes and/or codes of a different spreading factor, the multiple data rate capability of this scheme can be enhanced.

Performance of multirate techniques in W-CDMA

We analyze and compare the error performances of two multirate transmission schemes in W-CDMA: multicode (MCD) and variable spreading gain (VSG). First, we show that in an AWGN channel, the VSG data user performs better than the MCD user if the number of in-cell voice users are low and the background noise is weak. However, as the noise power or the number of voice subscribers increases, the performances of the two schemes become similar. We then show that in a multipath fading channel, the two schemes achieve very similar error performances as long as their receivers are able to track all resolvable signal paths. However, when the receiver can only track multipath components within the current symbol interval, the VSG user yields a much worse error probability than the MCD user. This is because the VSG data user has a much shorter symbol interval. Simulation results are illustrated to support the analysis.

Linear transformation of multi-level signal set in multi-code CDMA

We propose a technique that linearly transforms a multi-level multi-code CDMA signal into an antipodal set. The transformation is, equivalent to a linear block code with elements from the real space. Unlike precoding (which also results in constant envelope signals) there is no loss in data rate due to redundancy since the code rate is 1. Using geometrical arguments, we show that our transformation is equivalent to a rotation or reflection of signal space. We also show that precoding can be classified as a special case of our proposed scheme. In fact, we show that precoding is equivalent to a parity-check linear block code with a modified Hadamard matrix. We also show a close relation between two seemingly different multi-rate schemes of OVSF-CDMA and multi-code CDMA.

A simple constant envelope modulation scheme for orthogonal multicode CDMA system

We propose a simple linear transformation technique to alleviate the signal envelope fluctuation in an orthogonal multicode CDMA system. This method is capable of converting the multilevel transmitted signals of multicode CDMA into a constant envelope signal, which then allows the use of efficient nonlinear power amplifiers. This feature leads to significant cost-reduction in the implementation of multicode CDMA systems in the uplink. The error performance and the overall complexity of this technique is shown to be very similar to that of a conventional orthogonal multicode CDMA system.

Improving the performance of linear MMSE detectors in multi-cell DS-CDMA system

In a single-cell DS-CDMA system, linear MMSE detectors has been shown to significantly outperform conventional detectors in terms of spectral efficiency. Furthermore, simulation shows that the spectral efficiency of asynchronous CDMA systems using MMSE detection and random spreading codes can reach four times that of orthogonal CDMA. In the case of universal frequency reuse, however, this is not necessarily true because of the presence of strong inter-cell interference (ICI). This paper analyzes the reason for the superior performance of MMSE detection in single-cell asynchronous CDMA systems and investigates the potential for performance enhancement using MMSE detection in multi-cell environment with sectorization and a higher frequency reuse factor. Our results indicate that, with sectorization and reduced frequency reuse efficiency, using the MMSE detector can still significantly improve the overall system throughput surpassing that of CDMA systems using conventional detection with universal frequency reuse.

A throughput enhancement scheme for DS-CDMA systems using linear multiuser detectors

It is well-known that the performance of linear multiuser detectors such as decorrelators and MMSE deteriorates as number of users exceeds the available code dimension. This paper shows that these detectors are much less sensitive to this problem in asynchronous CDMA systems than in synchronous case. We further show that with optimal spreading factors, single-cell asynchronous CDMA systems with linear multiuser detectors are able to maintain a good error performance even if number of users grows exceedingly beyond the spreading factor. We proceed to suggest a scheme to enhance the throughput of a single-cell DS-CDMA system by employing multiuser detection and controlled random user transmission delays. This method has the potential of exceeding the single-cell capacity of orthogonal CDMA by a factor of 4 or more. Further, we extend this mechanism to a multicell environment.