Paul S. Min

Multicell CDMA network design

Traditional design rules for cellular networks are not directly applicable to code division multiple access (CDMA) networks where intercell interference is not mitigated by cell placement and careful frequency planning. For transmission quality requirements, a minimum signal-to-interference ratio (SIR) must be achieved. The base-station location, its pilot-signal power (which determines the size of the cell), and the transmission power of the mobiles all affect the received SIR. In addition, because of the need for power control in CDMA networks, large cells can cause a lot of interference to adjacent small cells, posing another constraint to design. In order to maximize the network capacity associated with a design, we develop a methodology to calculate the sensitivity of capacity to base-station location, pilot-signal power, and transmission power of each mobile. To alleviate the problem caused by different cell sizes, we introduce the power compensation factor, by which the nominal power of the mobiles in every cell is adjusted. We then use the calculated sensitivities in an iterative algorithm to determine the optimal locations of the base stations, pilot-signal powers, and power compensation factors in order to maximize the capacity. We show examples of how networks using these design techniques provide higher capacity than those designed using traditional techniques.

A nonblocking architecture for broadband multichannel switching

The paper investigates multichannel switching as a promising alternative to traditional single-channel switching where virtual paths established in a switch are between a single input channel and a single output channel. A particular non-blocking condition is derived for flip networks, which is exploited to realize a multichannel switching architecture that supports an arbitrary number of channel groups. The architecture is internally nonblocking and bufferless. Using one flip network recursively a number of times based on the number of channel groups, the resulting architecture becomes efficient in the sense that the cross point complexity is O(N log/sub 2/ N) for N inputs. Other distinguishing features are the abilities to provide multicasting, superrate switching (i.e., rates that exceed the capacity of a single channel are accommodated), multirate switching (i.e., bit pipes of different rates are supported simultaneously), multiple performance requirements (i.e., services with different performance requirements are treated accordingly), and fair access to all inputs (i.e., no input is systematically discriminated against). In multichannel switching, cells belonging to a single session can traverse multiple channels. Providing the cell sequencing integrity becomes a challenging issue. The architecture proposed in the paper accomplishes the task without employing any cell resequencing mechanism. >

Cell placement in a CDMA network

Traditional design rules, wherein cells are dimensioned in order to get an equal amount of demand in each cell are not directly applicable to CDMA networks where large cells can cause a lot of interference to adjacent small cells. In order to enable iterative cell placement we use a computationally efficient iterative process to calculate the inter-cell and intra-cell interferences as a function of pilot-signal power and base station location. These techniques enable us to improve the placement of cells in a CDMA network so as to enhance network capacity. We show examples of how networks using this design technique will provide higher capacity than ones designed using conventional techniques.

Congestion Prediction of Self-Similar Network through Parameter Estimation

In a state of emergency, in complex and dynamic situations where packet delay increases and congestion builds up, certain network nodes may not to able to handle the traffic load. To avoid the congestion build-up in advance, it is mission critical to predict the symptoms of network traffic and to preemptively alter the routing paths to allow a smooth and efficient flow of data packets for effective situation management. In this paper, we have developed a practical methodology for estimation of key parameters of self-similar network traffic using index of dispersion for counts and coefficient of determination. Self-similarity causes performance degradation in the queueing delay and buffer overflow at routers and at switches. We proved the impact of Hurst parameter and fractal onset time on the average queueing delay and the waiting-time distribution of self-similar traffic, utilizing experimental queueing analysis. Based on the understanding obtained, we can predict the congestion of data network in advance through estimation of traffic parameters. In addition, the results of this study on the delay provide a practical means of finding a lower delay path in data networks under the self-similarity

On the prediction of average queueing delay with self-similar traffic

Recent studies on a wide range of network traffic measurements including LAN and WAN have revealed the presence of self-similarity. These types of traffic hold statistical similarity across multiple time scales. Burstiness is retained even with the aggregating self-similar traffic. This property degrades the performance of a network. The queueing delay is one of the performance measures. In this study, a G/M/1 queueing model is used to model a network with self-similar traffic. The results of this study demonstrate that the delay exhibits a rise as degree of self-similarity increases. We compare an analytic average queueing delay of the self-similar traffic to the delay of simulated model to obtain a useful method for the delay prediction. By adjusting a single parameter of the truncated power-tail (TPT) distributions, we can make the analytic curve follow the simulation results. This allows us to predict the delay by computing the TPT once we measure the Hurst parameter of an input traffic and its arrival rate, and the utilization of a router. Our results can benefit control, design, and resource allocation of high-speed networks.

Performance analysis of multiple rejects ARQ at RLC (radio link control) for packet data service in W-CDMA system

We analyze the performance of multiple rejects (MR) ARQ for RLC proposed by the 3GPP. The main purpose of MR ARQ is to get efficient use of radio resources at the receiver by reducing number of acknowledgements. We analyze MR ARQ and compare its performance with selective repeat (SR) ARQ in the wireless fading channel environment. Based on analysis, it is shown that we need to increase the size of buffer for MR ARQ to hold data packets for the period of acknowledgement, and need to increase the size of buffer much more for MR ARQ in the noisier channel environment. It is shown that as the length of period for acknowledgement is increased, we need to increase the size of buffer much more for the saturated throughput. Increasing the size of buffer is needed for the efficient use of radio resources to reduce number of acknowledgements.

Effects of call arrival rate and mobility on network throughput in multi-cell CDMA

The effect of call arrival rate on the capacity of a code-division multiple-access (CDMA) cellular network is evaluated. First the inter-cell and intra-cell interferences of every cell on every other cell are calculated for a given network topology. Then the capacity region for the number of simultaneous calls in every cell is defined for specified system parameters. This region is used to evaluate the new call blocking and handoff call blocking probabilities. The network throughput is found by considering the revenue generated by accepting a new call and the cost of a forced termination due to handoff failure. Implied costs are calculated for both high and low mobility of calls between cells, and the effect of mobility on pricing is discussed. Implied costs are used to maximize the throughput with respect to the call arrival rates by applying a gradient descent algorithm.

Train queue processing for highly scalable switch fabric design

We present an approach which addresses a fundamental scaling limitation for conventional switches. Instead of processing the data packets one by one, train queue processing groups multiple data packets together and processes them as one unit. There are two basic train queuing techniques, sequential train queuing and sequential-to-parallel train queuing. The sequential train queue processing can reduce the global switch processing frequency for the switch. The sequential-to-parallel train queue processing can reduce both the global switch processing frequency and data rate by using packet grouping and slicing mechanism. By employing train queuing techniques, a high bandwidth switch can be built and feasibly implemented.

Call admission control scheme for arbitrary traffic distribution in CDMA cellular systems

Designing a call admission control (CAC) algorithm that guarantees call blocking probabilities for arbitrary traffic distribution in CDMA networks is difficult. Previous approaches have assumed a uniform traffic distribution or excluded mobility to simplify the design complexity. We define a set of feasible call configurations that results in a CAC algorithm that captures the effect of having an arbitrary traffic distribution and whose complexity scales linearly with the number of cells. To study the effect of mobility and to differentiate between the effects of blocking new calls and blocking handoff calls, we define a net revenue function. The net revenue is the sum of the revenue generated by accepting a new call and the cost of a forced termination due to a handoff failure. The net revenue depends implicitly on the CAC algorithm. We calculate the implied costs which are the derivatives of the implicitly defined net revenue function and capture the effect of increases in the number of calls admitted in one cell on the revenue of the entire network. Given a network topology with established traffic levels, the implied costs are used in the calculation of a CAC algorithm that enhances revenue and equalizes call blocking probabilities. Moreover, our algorithm provides guaranteed grade-of-service for all the cells in the network for an arbitrary traffic distribution.

Shadow prices for LLR and ALBA

Shadow prices are calculated for least loaded routing (LLR) and aggregated least busy alternative (ALBA) routing in circuit-switched networks for the blocking probability obtained from fixed point algorithms. Numerical results are presented for the calculation of these shadow prices in small networks. As an application of these shadow prices, we also formulate a constrained optimization problem to calculate the sum capacity of LLR and ALBA for a given network. Comparison of the sum capacities indicate that the optimization using shadow prices results in a significant improvement. This provides evidence that matching capacity distribution to traffic is important even when adaptive routing schemes such as LLR and ALBA are used in the network. We also calculate upper bounds on the sum capacity which serve to indicate how well the optimized LLR and ALBA perform. The numerical results also confirm that with a small number of states the capacity of ALBA approaches that of LLR.