In this contribution we examine the joint optimization of the outer-loop power control (OLPC) and spreading factor in order to improve the spectral efficiency in the uplink of wideband code-division multiple-access (WCDMA) systems, in the presence of additive white Gaussian noise (AWGN) and multiple access interference (MAI), and while maintaining an average transmit power constraint. The variable spreading factors (VSF) and the target signal-to-noise ratio (SNR) -set by the OLPC – are adapted according to the interference-to-noise ratio (INR) and the target instantaneous bit error rate (BER). We use the standardized orthogonal VSF (OVSF) codes used for the uplink dedicated physical data channel (UL-DPDCH) and evaluate the performance over Nakagami flat fading channels. It is shown that the proposed scheme improves the spectral efficiency compared to systems that use constant spreading factor and OLPC, for a wide range of practical SNRs and signal-to-interference ratios (SIRs)
Published in: 2006 IEEE 17th International Symposium on Personal, Indoor and Mobile Radio Communications
Date of Conference: 11-14 September 2006
Date Added to IEEE Xplore: 11 December 2006
Conference Location: Helsinki, Finland
Introduction
The third generation (3G) wireless communication systems focus on different service classes, notably conversational, streaming, interactive, and background, which are characterized by different quality of service (QoS) requirements such as delay, transmission-and error rates [1] that make the radio resource management challenging. On the other hand, the bandwidth in today’s wireless systems is scarce. So it is imperative to devise methods to maximize the spectral efficiency in order to achieve an adequate and reliable QoS for voice and data traffic over the limited bandwidth. Since achievable performance is tightly coupled to the time varying radio channel conditions, including path gains and interference powers, adaptive resource allocation is an important means for improving the spectral efficiency [2]. Hence, it has been researched ardently in the last few years for implementation in the 3G systems and also instigated in some of the 3G Partnership Project (3GPP) spec-ifications. Adaptation of power and rate comprises a range of techniques developed in order to improve the spectral efficiency in wireless time-varying channels [2]–[8]. It was shown in [3] that the Shannon capacity of a fiat-fading channel can be theoretically achieved by varying both transmission rate and power. In [4] it was shown that this capacity can also be achieved by varying the transmit power alone. However, these schemes do not provide insight into the best adaptive methods that need to be used under practical constraints. In [5] the impact on spectral efficiency of adapting various modulation parameters under different constellation restrictions and bit error rate (BER) constraints was investigated for different modulation techniques and fading distributions. It was shown that an increase in the spectral efficiency is achievable by optimally varying combinations of parameters such as transmission rate, power, and instantaneous BER1
The instantaneous BER constraint implies that the system must maintain a constant probability of bit error for each fading value [5]. The joint adaptation of transmission rate and outer-loop target signal-to-noise ratio (SNR), to the BER was considered in [6]. It was verified that the spectral efficiency can be improved without increasing the average transmit power. The OLPC helps to keep the quality of the communication at the required level by setting the SNR-target for the fast inner-loop power control, which attempts to maintain this target by adjusting the transmit power2
In the UMTS the OLPC is typically executed at a rate of 10-100 Hz, and the frequency of the fast power control is 1.5 kHz [[9], p. 239].. The use of variable spreading factors (VSF) to adapt the transmit rate to the level of MAI was studied in [7], and it was verified that a VSF-assisted direct-sequence CDMA is capable of increasing the system throughput while maintaining a given target BER.
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