In this paper, we explore the potential of developing an interweave cognitive radio (CR) system based on a hierarchical wide-band two-dimensional spread multi-carrier directed-sequenced code division multiple access (MC-DS-CDMA). First, a small CR cell is embedded in a large primary cell such that the spatial separation gives a chance to share the spreading code resources with the primary system. Second, the white, gray and black spreading codes are defined to facilitate the management of the spreading code resources. Also, a new multiple access interference (MAI) coefficient is derived to quantize the amount of intra- and inter-system interference. Third, based on the MAI coefficient, a dynamic spectrum access (DSA) algorithm, including the code assignment/reassignment, transmission power control and call admission control mechanisms, is proposed to efficiently utilize the white and gray spreading codes without damaging the primary system. The simulation results show that compared with the the case of using only the white spreading codes, some extra 53% increase in call admissions and 13% decrease in call droppings can be attained by using the gray spreading codes properly.

Conventionally, an interweave cognitive radio (CR) system can be developed based on the assumption of spectrum sharing between a wide-band CR system and some narrow-band primary systems. In this paper, we explode this assumption by constructing an interweave CR system using a hierarchical wide-band two-dimensional (2D) spread multi-carrier directed-sequenced code division multiple access (MC-DS-CDMA). In this hierarchical system, a small CR cell is embedded in a large primary cell such that the limited serving area provides one geographical protection for the primary users (PUs). To facilitate the spreading code management, we define the white, gray and black spreading codes for the CR system. Properly allocating the white and gray spreading codes but the black ones can achieve a higher spectrum efficiency without harming the PUs, which provides another dimension for the PU protection in the proposed dynamic spectrum access (DSA) algorithm. For the purpose of evaluating the candidacy of the gray spreading codes, a sophisticated interference analysis is conducted so as to define a new multiple access interference (MAI) coefficient to quantize the amount of interference imposed on a secondary user (SU). With the help of the new MAI coefficient, a less interfered white or gray spreading code can be assigned to an SU, which can bring about an extra 55% of call admissions in one of our considered cases.

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.

Maintaining the quality-of-service (QoS) requirement by transmission power adjustment while eliminating interference is an important but difficult task in the downlink multicarrier direct-sequence code division multiple access (MC-DSCDMA) systems 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 an interference avoidance (IA) code assignment strategy and then maintain the QoS via a power control mechanism. To facilitate the code assignment strategy, a new PC-based multiple access interference (MAI) coefficient is defined through performance analysis. Thanks to the help of the PC-based MAI coefficient, we can evaluate each candidate code by quantitatively predicting the amount of interference produced by a possible code assignment event. With this interference pre-evaluation, the PC mechanism can not only maintain the QoS with less transmission power but also eliminate extra MAI in the system. The simulation results show that the proposed joint scheme can maintain a predefined signal quality and significantly improve the call admission rate while largely reducing the power consumption. The contribution of this paper fills up the missing link between the power control and code assignment in the 2D-spread MC-DS-CDMA systems.

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.

Abstract We present a novel approach for real‐time rendering of translucent surfaces. The computation of subsurface scattering is performed by first converting the integration over the 3D model surface into an integration over a 2D texture space and then applying importance sampling based on the irradiance stored in the texture. Such a conversion leads to a feasible GPU implementation and makes real‐time frame rate possible. Our implementation shows that plausible images can be rendered in real time for complex translucent models with dynamic light and material properties. For objects with more apparent local effect, our approach generally requires more samples that may downgrade the frame rate. To deal with this case, we decompose the integration into two parts, one for local effect and the other for global effect, which are evaluated by the combination of available methods [DS03, MKB* 03a] and our texture space importance sampling, respectively. Such a hybrid scheme is able to steadily render the translucent effect in real time with a fixed amount of samples.

Frequency selective fading may affect the orthogonality of the spreading codes in the multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems. In this paper, we define a new performance metric called the multiple access interference (MAI) coefficient for the MC-DS-CDMA system to quantitatively predict the impact of inter-code interference with the time- and frequency-domain spreading in a frequency selective fading channel. With the help of MAI coefficient, a novel interference avoidance code assignment strategy is proposed. By jointly considering the incurred MAI effect and the blocking probability in the code tree structure, the proposed interference avoidance code assignment method can effectively reduce the MAI for the multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.

In the multi-rate multi-carrier direct sequence code division multiple access (MC-DS-CDMA) system, how to allocate subcarrier power to improve the received signal quality, control the multiple access interference (MAI) at an acceptable level, and simultaneously maintain high call admission is an interesting and challenging issue. In this paper, we propose a joint subcarrier power allocation and code assignment scheme for the synchronous multi-rate MC-DS-CDMA system with time- and frequency-domain spreading. The proposed scheme consists of a subcarrier power allocation method to optimize the received signal power as well as an interference avoidance code assignment strategy to eliminate MAI. For this purpose, we define a performance metric called the MAI coefficient to quantitatively predict the incurred MAI before assigning a spreading code. The simulation results show that the proposed joint subcarrier power allocation and code assignment strategy not only can reach a better received signal quality but also achieve high call admission rate.

Multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems can support multi-rate services with a great deal of flexibility by dynamically changing time-and frequency-domain spreading gains. However, in a frequency-selective fading channel, the multi-rate users using the time spreading codes with an ancestor-to-child relation, even with orthogonal frequency spreading codes, may still be affected by different quantities of multiple access interference (MAI). How to design an effective radio resource management strategy for the MC-DS-CDMA systems subject to different levels of MAIs becomes a new and important issue. In this paper, based on the error rate analysis in the physical layer, we observe and define a new performance metric, called the MAI coefficient, which can quantitatively predict the MAI effect imposed on a particular code in a multi-rate MC-DS-CDMA system. Furthermore, from the radio resource management perspective, we develop an interference avoidance code assignment scheme based on the MAI coefficient. By simulations and analysis, we demonstrate that the proposed interference avoidance code assignment method can effectively reduce the MAI in a multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.

Multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) becomes an attractive technique for the future fourth-generation (4G) wireless system because it can flexibly adapt transmission rates by changing both time and frequency spreading factors and possesses many physical-layer advantages in dispersive fading channels. However, power control errors (PCE) and the complete multiple access interference (MAI) from all the intersubcarriers may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an analytical method to evaluate the joint effects of the PCE and the complete MAI on the multirate MC-DS-CDMA system. From analysis and simulation, we obtain some important insights into the performance issues of the MC-DS-CDMA system. First, the effect of PCE can exacerbate the impact of the complete MAI on the MC-DS-CDMA system, or vice versa . For BER=10/sup -3/ in a considered case, the joint effect of the complete MAI and PCE further degrades the performance by 2.1 dB compared with the sum of the degradation from the complete MAI and the PCE individually. Second, increasing frequency or time-domain spreading gain can improve the performance of the MC-DS-CDMA system, but the system also becomes more sensitive to power control errors. Third, a larger PCE can possibly make the frequency-domain diversity diminish faster than the gain obtained from the time-domain spreading although an MC-DS-CDMA system with a larger frequency-domain spreading gain (M) is usually better than that with a larger time-domain spreading gain (G/sub o/). In our example, for the standard deviation of PCE (/spl sigma//sub e/) equal to 0 dB, the BERs with (M,G/sub o/)= (4, 16) and (16, 4) are 9.3/spl times/10/sup -4/ and 3.7/spl times/10/sup -5/, respectively, while for /spl sigma//sub e/=4 dB, the BER performances of the two cases are all in the order of 10/sup -3/.

The parallel multichannel stop-and-wait (SAW) hybrid automatic repeat request (HARQ) mechanism is one of key technologies for high-speed downlink packet access in the wideband code division multiple access system. However, this parallel HARQ mechanism may encounter a serious stall problem, resulting from the error of the negative acknowledgement (NACK) changing to the acknowledgement (ACK) in the control channel. In the stall situation, the receiver waits for a packet that will be no longer be sent by the transmitter and stops delivering the medium access control (MAC) layer packets to the upper layer. The stall issue seriously degrades the quality of service for the high-speed mobile terminal owing to the high probability of NACK-to-ACK errors. In this paper, we present an analytical approach to compare three stall avoidance schemes: the timer-based, the window-based, and the indicator-based schemes. To this end, we first propose a new performance metric-gap processing time, which is defined as the duration for a nonrecoverable gap appearing in the MAC layer reordering buffer until it is recognized. Second, we derive the probability mass functions and the closed-form expressions for the average gap processing time of these three stall avoidance schemes. It will be shown that our analytical results match the simulations well. Further, by analysis, we demonstrate that the indicator-based stall avoidance scheme outperforms the timer-based and the window-based schemes. The developed analytical approaches can help determine a proper number of processes for the parallel SAW HARQ mechanisms. We also show that the analytical formulas can be used to design the number of acceptable fully loaded users for an admission control policy subject to the gap processing time constraint. In the future, our analysis can facilitate the MAC/radio link control (RLC) cross-layer design because the gap processing time in the MAC layer is closely related to the RLC timeout mechanism and the window size in the RLC retransmission mechanism.

The MC-DS-CDMA possesses the advantages of the immunity against multipath dispersion of orthogonal frequency division multiple access (OFDM) and the flexibility of the time-domain and frequency-domain spreading technique. However, power control errors (PCE) may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an easily computed analytical method to evaluate the bit error rate performance of the multi-rate MC-DS-CDMA system with the time/frequency 2-domain spreading subject to PCE. Furthermore, the proposed analysis also considers the effect of multiple access interference (MAI) from all the subcarriers in the multi-rate MC-DS-CDMA system. We find that power control errors will offset the gain of frequency domain spreading. More importantly, the larger the value of frequency domain spreading, the more the sensitivity to PCE.

In this paper, we investigate stall avoidance mechanisms for the W-CDMA system with high speed downlink packet access (HSDPA). The stall avoidance mechanisms are aimed to reduce transmission delays and keep in-sequence delivery of MAC layer data to the upper layer. By analysis and simulation, we compare three stall avoidance mechanisms, including the timer-based, the window-based, and the indicator-based methods. For comparison, we introduce a new performance metric called the gap processing time. Our results show that for E/sub b//N/sub 0/ equal to 7 dB in the AWGN channel, the average gap processing time for the indicator-based method is 8.49 transmission time intervals (TTIs) and those for the window-based method and the timer-based method are 36.38 TTIs and 175.5 TTIs, respectively. In a Rayleigh fading channel, we find that the gap processing time of the window-based and the indicator-based methods are higher than in the AWGN channel, while the timer-based method has the same gap processing time in both the AWGN channel and the Rayleigh fading channel.

In this paper we present a simple analytical model to evaluate the impact of measurement errors on the closed-loop power control for code division multiple access (CDMA) systems. By introducing a new performance measurement, the probability of false command in power control, P/sub FC/, a closed form formula for calculating P/sub FC/ with consideration of measurement errors is presented. Furthermore, we derive a bit error rate (BER) performance bound in terms of P/sub FC/ for the CDMA system with closed-loop power control (CLPC). The proposed analytical approach can quantitatively evaluate the performance of the CLPC taking into account of the effects of measurement errors and Doppler frequency under a Rayleigh fading channel. Through simulation and analysis, we show that the proposed analytical BER bound can accurately estimate the BER performance of the CLPC under the impact of measurement errors. Interestingly, we find that the CLPC is less sensitive to measurement errors due to the non-linear operation in the one step up/down power control scheme compared with the variable-step size power control.

We investigate the error statistics of the close-loop power control scheme with variable spreading factors for the multi-rate services in the third generation wideband CDMA (WCDMA) system. We demonstrate that the long scrambling pseudo-noise code, besides its well known feature in differentiating users and base stations, can improve power control false command over a frequency-selective fading channel as well. Our computer simulation results indicate that the SIR measurement error can be reduced by 3 dB in a typical case, and the power control false command can be reduced from 27 % to 10 % in comparison to using pure short Walsh codes. It is shown that the close-loop power control error is a composite function of the spreading factor, target E/sub b/ / N/sub o/ and Doppler frequency. The statistics of closed loop power control false command can be easily characterized by a Bernoulli distribution with a parameter provided by our results.