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In general, link-adaptive schemes, such as adaptive modulation and coding (AMC) and hybrid automatic repeat request (HARQ), enhance system capacity in time-varying wireless channels. For these link-adaptive schemes to be applied, resources have to be adaptively and dynamically allocated every frame. Thus, the system needs control messages to send information to users about dynamic resource allocation. Such information includes user ID, resource position, level of modulation, and coding and automatic repeat request (ARQ) information. However, the transmission of these resource-allocation messages causes control overhead. In this paper, we introduce a scheme that applies AMC to the transmission of resource-allocation messages and analyze its performance in systems that support truncated ARQ, such as link-layer ARQ and HARQ. In addition, we show that using AMC to transmit control messages is a good way to reduce control overhead; in particular, it is very effective when the number of users per frame is large, as in voice over Internet Protocol (VoIP) services. Index Terms--Adaptive modulation and coding (AMC), control messages, control overhead, resource allocation.

We propose a new location tracking scheme for subscribers on transportation systems (TSs) in which a large number of fast-moving users have the same location as the TS. The group location tracking (GLT) scheme in which a group location update is performed instead of the IS-41's individual location update has been proposed. We propose a distributed GLT (DGLT) scheme based on several virtual visitor location registers (VVLRs) and representative identities. We analyze the GLT and DGLT schemes in view of the database (DB) access cost as well as the signaling cost under one-dimensional network architecture. Compared with the IS-41 scheme, the schemes based on group management generally have a lower signaling cost and a lower DB access cost due to a significant reduction in the number of location updates. user mobility increases or the number of users on a TS increases the performance of the group management schemes improves. The DGLT scheme, in which a corresponding home location register selects a closest VVLR among several possible VVLRs, generally has a lower signaling cost although a DB access cost is increased slightly compared with the GLT scheme. We also obtain the regions in which the DGLT is more efficient than the IS-41 and the GLT scheme. The DGLT scheme is an appropriate solution for high signaling cost systems and the GLT scheme is useful for high DB access cost systems. Index Terms--Location management, location update, mobility.

Two distributed heuristic clustering schemes are proposed that will minimize the required transmission power in two-tiered mobile ad hoc networks. Both schemes can be implemented and executed in real time and can be adopted for periodic or event-driven cluster reconfiguration. Scheme performance is simulated and compared with optimum configurations based on the mean transmission power and the call drop rate as performance measures. Numerical results show that the proposed schemes deliver performance similar to optimum results. Index Terms--Ad hoc networks, clustering scheme, energy conservation.

A new data transmission scheduling scheme considering short-term fading conditions in the downlink of code-division multiple access (CDMA) systems is proposed. In this scheduling method, a mobile station (MS) connected to a base station (BS) estimates the fading condition of the next frame based on the received signal power variation and feeds back this information to the corresponding BS. The BS determines the MSs to which it transmits data based on the fading information from the MSs. Analytical and simulation results show that the proposed scheme saves the transmit power greatly when the speed of an MS is relatively low and the Rician factor is small. The maximum amount of the power reduction is about 9 dB. This reduced transmit power results in the data throughput increase of CDMA systems. Index Terms--Code division multiple access (CDMA), data transmission, power reduction, scheduling.

In this paper, we propose a direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system with two user classes and analyze the throughput of the proposed system. Mobile stations (MSs) are divided into two classes according to its priority or traffic characteristics such as delay-intolerant and delay-tolerant. Different permission probabilities are assigned to each class so that the appropriate quality of service can be provided. We assume that the generation of class 1 and 2 messages are Poisson distributed and the message is divided into several packets before transmission. The system is modeled as a two-dimensional Markov chain under the assumption that the number of packets transmitted immediately by both user classes is geometrically distributed and the packet length is constant. We calculate the packet success probability and the throughput as a function of the signal-to-noise ratio (SNR) during packet transmission, considering the number of overlapped class 1 and 2 messages and the amount of their time overlap. Moreover, we show that the proposed system differentiates user messages according to class and maintains a high throughput even under heavy traffic conditions using access control based on the channel load. Index Terms--Access control, service class, throughput, unslotted ALOHA.

The Intermational Mobile Telecommunications IMT-2000 system uses a microcell concept to provide multimedia services and to support increasing number of users. However, in a microcell system, the number of handoffs is greatly increased. To solve this problem, a multitier cellular structure is proposed, in which high-speed mobile terminals (MTs) are serviced in macrocell and low-speed MTs are serviced in microcell to minimize the number of handoffs. In this system, it is important to precisely estimate the speed of the MT for the correct selection of the macrocell/microcell. We propose two macrocell/microcell selection schemes based on a new velocity estimation method in a multitier cellular system which uses the sojourn time in a microcell overlapping region. The proposed schemes have various advantages such as good performance when MT direction is varying, efficient user allocation to cells, quick velocity estimation capability, easy implementation, and low power consumption. We analyze and simulate the conventional and our proposed schemes in the Manhattan cell model, showing the proposed schemes have better performance than the conventional schemes. Index Terms--Cell selection, multitier cell, sojourn time, velocity estimation.

Code-division multiple-access (CDMA) cellular systems use soft handoff. Although the capacity of CDMA systems is interference-limited in nature, channel shortages may occur because soft handoff uses several channels simultaneously. To cope with this problem, we propose an improved handoff method that borrows channels from stationary calls participating in soft handoff and allocates the borrowed channels to handoff requests by moving calls when a channel shortage occurs. Borrowing from stationary calls is possible because these calls do not undergo fast-fading and do not require receiver diversity. The proposed method is designed to avoid increased interference resulting from channel borrowing. The proposed channel-borrowing handoff scheme is analyzed in a situation involving both moving and stationary calls. A comparison is made between the performances of the typical IS-95-based handoff scheme and the proposed scheme. Numerical results show that the proposed scheme is better than the IS-95 scheme in view of the handoff refused probability, the handoff queuing delay, and total interference. Index Terms--Code-division multiple-access (CDMA) systems, channel borrowing, soft handoff, speed estimation.

As mobile telephone traffic demands increase in the future, interest in cellular systems with a hierarchical structure will grow. Hierarchical cellular system can provide high system capacity, efficient channel utilization, and an inherent load-balancing capability. In this type of system, calls blocked in a microcell flow over to a macrocell which is superimposed on a number of microcells. When overflow calls which are accomodated in a macrocell cannot be returned to a microcell, the macrocell channels become saturated. Thus, we propose a hierarchical cellular system with an underflow scheme which permits overflow calls to return to microcells. We model this underflow scheme and analyze system performance. Index Terms--Handoff, hierarchical, overflow, underflow.