Without transmit power control (TPC) and Rake combining, the uplink capacity of a direct sequence code division multiple access (DS-CDMA) packet mobile communication system significantly degrades due to the near-far problem and multipath fading. In this letter, assuming a single cell system with an interference-limited channel, the impact of the joint use of Rake combining and TPC on the uplink capacity is evaluated by computer simulation. Slow TPC is found to give a link capacity larger than fast TPC. This is because, with slow TPC, the received signal power variations due to fading remain intact and this results in a larger capture effect.

The forward link capacity plane of a hierarchically structured cellular CDMA system, in which a single frequency band is used for both macrocell and microcell layers, is obtained for isolated microcells (hotspots). The impact of each neighbour microcell and macrocell on the capacity plane, for a reference mobile station as the worst case, is also investigated. The results for the case of three microcells in each macrocell show that 69% of macrocell interference to microcell mobile stations comes from the closest macrocell. It is also found that 80% of macrocell interference to the reference macrocell mobile station comes from the central cell and the first cell tier around it.

DS-CDMA provides a flexible support for the low-to-high bit rate of multimedia services upon a specific user's request. A simple capacity expression is derived for a power-controlled reverse link of orthogonal multi-code DS-CDMA with multiple connections. It is found that an orthogonal multi-code user having multiple connections is equivalent to a single connection user, but with a spreading factor reduced by a factor of the total number of parallel codes and a required signal energy per symbol-to-interference plus noise power spectrum density ratio which is the average taken over multiple connections. Furthermore, the use of antenna diversity is found equivalent to the use of higher spreading factor increased by a factor of the number of antennas.

The soft handoff is widely adopted in code division multiple access (CDMA) systems for its many advantages mainly resulting from site diversity. However, in the forward link, other cell interference can be increased by soft handoff, decreasing system capacity. In future mobile systems, provision for the sufficient forward link capacity is very important since the forward link load is much higher than the reverse link load in mobile multimedia services such as Internet access. In this paper, we consider a combined handoff strategy in which voice services are provided with soft handoff whereas data services are supported with hard handoff. We analyze the effect of handoff method on the forward link performance. The performance measures we use are the outage probability of the bit energy to noise density ratio and the capacity based on the outage probability. As a result, we show that the combined handoff is very useful in CDMA cellular networks supporting both voice and data services simultaneously.

Both macrodiversity and microdiversity can effectively overcome the harmful effect of fading. Much of previous work focused on their benefits to the reverse link in CDMA systems. However, their effects on the forward link are less well understood. In this paper, we analyze the CDMA forward-link capacity with macrodiversity and microdiversity. It is shown that macrodiversity causes forward-link capacity loss since the extra forward-link channels supported by the involved base stations enhance not only the received signal power, but also the total interference. Unfortunately the latter gains more whatever power allocation scheme is adopted. Based on the analysis of the cause of capacity loss, we further present a new transmission scheme, in which some joint control among the involved base stations is made to assure that the signals arrived at the desired mobile in phase and simultaneously. The simulation results show that in the new transmission scheme much higher capacity can be achieved with macrodiversity and the capacity increases rapidly with the number of involved base stations. A comparison of the forward-link capacity with microdiversity and macrodiversity indicates that both types of diversity can bring benefits to the forward-link capacity. However, with macrodiversity higher capacity can be obtained at the cost of complexity.

Reverse link performance analyses of single-code (SC) and multi-code (MC) CDMA systems in multipath fading environments are presented. The degree of orthogonality loss among multiple spreading code channels is characterized by introducing the orthogonality factor. This factor depends on the multipath delay power profiles of the propagation channel and the number of paths resolved at a Rake receiver. The link capacity and the signal power of both CDMA systems are then analyzed according to varying system parameters, including spreading bandwidth, traffic load, the orthogonality factor, and the number of spreading codes assigned to a user. Analytical results show that the SC-CDMA system provides a larger link capacity and MC users require more power than SC users. The dominant parameters affecting the performance difference are the spreading bandwidth and multipath delay power profiles.

The reverse-link of the DS-CDMA cellular system requires transmit power control (TPC) and diversity reception. This paper develops the expression of the received signal-to-interference ratio (SIR), and evaluates the outage probability using the Monte Carlo simulation to obtain the link capacity. The link capacities with received signal strength (SS)-based TPC and SIR-based TPC are compared. This paper investigates the required maximum and minimum transmit powers and the capacity gain of the SIR-based TPC over SS-based TPC as well as the effect of the diversity reception on the link capacity and transmit power. The reverse-link capacity is compared with the forward-link capacity to check the balance of capacities between both links.

Reverse link performance analysis in single-code and multi-code CDMA systems is presented. Results show that the single-code system yields better performance than does the multi-code system in terms of link capacity and signal power. This improvement increases as spreading bandwidth is reduced and the number of spreading codes assigned to a user is increased.

This paper proposes applying fast transmit power control (TPC) to the forward link of a direct sequence-code division multi-access (DS-CDMA) cellular system. Orthogonal spreading is assumed at a base station transmitter and coherent RAKE combining is assumed at a mobile station receiver. In DS-CDMA cellular mobile radio, the multiple access interference (MAI) from other cells and background noise limit the forward link capacity. Therefore, to increase the link capacity, fast transmit power control (TPC) can be introduced, which is similar to that developed for the reverse link, i. e. , the transmit powers of forward link channels are independently raised or lowered according to the instantaneous signal-to-background noise plus interference ratios (SIR's) measured at mobile stations. Fast TPC is fast enough to track the multipath fading as well as slow variations in the distance-dependence path loss and shadowing. On the average, the transmit power is increased to a user closer to the cell edge so that the effects of both other-cell MAI and background noise can be reduced while it is decreased to a user closer to the cell center. The effect of the TPC parameters (TPC interval, TPC target value, TPC step size, etc) on the forward link capacity in single- and multi-cell environments is evaluated by computer simulation. It is shown that fast TPC can almost double the forward link capacity in a multi-cell environment.

The outage probability of a forward link DS-CDMA cellular system with fast transmit power control (TPC) based on signal-to-interference ratio (SIR) is investigated. The expression for SIR at the output of RAKE receiver is developed, and the outage probability is evaluated by using Monte Carlo simulation. We consider two kinds of channel models: random delay resolvable path model and tapped delay line model which are suitable models for a few distinct paths channel and highly frequency-selective-channel model, respectively. The outage probability of a system with fast TPC based on SIR is compared to that without fast TPC. The use of orthogonal spreading codes is compared to that of the random spreading codes in terms of outage probability. The effects of the maximum and minimum transmit powers and the dispersive loss of signal power on the outage probability are also investigated.

In this paper, we study the link capacity assignment problem in packet-switched networks (CA problem) focusing on the case where link cost function is a piecewise linear concave function. This type of cost function arises in many communication network design problems such as those arising from developments in communication transmission technologies. It is already known that the method of link set assignment is applicable for solving the CA problem with piecewise linear convex cost function. That is, each link in the network is assigned to one of a group of specific sets, and checked for link set contradiction. By extending the method of link set assignment to the case of piecewise linear concave cost function, an important characteristic of the optimal solution of the CA problem is derived. Based on this characteristic, the non-differentiable link cost function can be treated as a differentiable function, and a heuristic algorithm derived from the Lagrange multiplier method is then proposed. Although it is difficult to determine the global optimum of the CA problem due to its non-convexity, it is shown by numerical results that the solution obtained from the proposed algorithm is very close to the global optimum. Moreover, the computation time is linearly dependent on the number of links in the problem. These performances show that the proposed algorithm is very efficient in solving the CA problem, even in the case of large-scale networks.

We propose an approach to obtain the relation between the number of voice call users and the number of data call users in the reverse link of a DS-CDMA cellular system with mixed rate traffic. The analyzed results show that as the number of data call users with high bit rate increases, the number of allowable voice call users decreases rapidly and linearly.

A simplified analysis is presented for the reverse link capacity of DS-CDMA mobile radio with transmit power control (TPC) based on measurement of signal-to-interference plus background noise (SIR) when users require different levels of quality. The link capacity is defined as the maximum achievable sum of the required SIRs, and the increase in transmit power due to SIR-based TPC is discussed. Also analyzed is the total link capacity when narrowband DS-CDMA systems share the radio spectrum of a wideband system. The capacity loss due to non-uniform use of the spectrum is discussed.

When communication network planning-design is performed, especially in a short-term case, it is important to utilize existing facilities in the construction of the new network. In this paper, link capacity assignment problem (CA problem) for packet-switched networks is investigated with the consideration of the existing network. To deal with this, per-unit cost of existing link capacity is thought to be less than that of newly installed capacity and a link cost function is modeled by a non-linear, non-differentiable one which is composed of two portions of capacity cost. After formulating the CA problem, two optimum algorithms derived from Lagrange multiplier method are presented and a modified algorithm is used for solving the CA problem in order to reduce the computation time. Some numerical results show that according to the values of link traffic flows, there will be links whose capacities must be set equally to the existing values. Moreover, when link cost difference is introduced in the CA problem, the number of links that the capacities of which have to be changed from existing values is less than that of linear cost function case, i.e., the case without consideration of the cost difference in link capacity.