It was previously shown (under simplifying assumptions) that the proportional fair algorithm gives equal power and time to users who only differ in their distance from the BS, their fading characteristics being the same (see Holtzman, J.M., Proc. VTC2000-Spring, p.1663-7, 2000). We give here a more general result for two classes of users with different fading characteristics. All other things being equal, the user class with more fading variability gets more throughput with a lower (but not much lower) fraction of time transmitting. We also demonstrate a separability property-the effects of variability and distance from the base station are separable.

This work focuses on the problem of efficient exploitation of the available bandwidth in order to provide high bit rates on the wireless link, as will be required in future wireless systems interfacing to broadband fixed networks. In particular, the uplink of a CDMA system with two user classes is considered. One of the classes consists of delay-intolerant users requiring support for a constant information bit rate, while the other consists of delay-tolerant users needing support for an information bit rate larger than a given value. It is assumed that when not transmitting information, synchronization contact is maintained with the base station at a given rate. The objective is to maximize the throughput of the delay tolerant users, while ensuring that the interference to other cells is as low as possible by minimizing the sum of all of the transmit powers used by the mobiles. Two transmission modes for the delay-tolerant users are considered. In the first mode, all of the users are allowed to transmit information when they wish. In the second mode, the transmissions of the delay-tolerant users are scheduled, so that only a limited number of them are transmitting information at any given time instant. It is shown that the second transmission mode, which is a time-scheduled scheme for the delay-tolerant users (with hybrid CDMA/TDMA as a special case), affords a better throughput while imposing the same average power requirements as conventional transmission. The results in this paper can be interpreted using results from previous work based on information theory.

In previous work, access control for data was proposed as a method to enhance capacity while ensuring adequate quality of service (QoS) in an integrated voice/data CDMA system. The motivation behind access control is to schedule data packet transmissions when voice activity is low and curtail data transmissions when the voice load is heavy. In this work, a simple and flexible real-time access control scheme proposed in previous work, is extended. The performance measures used are the probability of outage and delay for data. It is shown how the system can support more users with access control than without, for a given probability of outage. The penalty lies in introducing delay for the data users. Performance of the control schemes is analyzed for different parameters and tradeoffs involved in their choice are discussed. It is demonstrated that with appropriate choice of parameters, the proposed real-time control scheme approaches the performance of optimum prediction schemes, in terms of the tradeoff between capacity and delay.

This paper presents the wireless intelligent ATM (WIATM) network, designed to provide wireless broadband integrated services. The WIATM network takes advantage of the ATM-cell relay paradigm for integrated services through a radio link with quality of service (QoS) guarantee. The design of the WIATM network architecture is an independent wireless network, which is consistent with the inherent cellular/PCS network architecture, as a wireless customer premises equipment/network (CPE/CPN) to access the ATM transport network in the B-ISDN infrastructure. An independent network architecture design separates the wireless access network from the ATM backbone network; this provides flexibility for wireless resource management with low rate source codecs with minimal tolerable QoS considered to increase the spectral efficiency, and mobility support by taking advantage of the functionalities of the IS-41 circuit-switching handoff procedures. The protocol design of the air interface is to meet the QoS requirements of wireless B-ISDN services and to be compatible with that of B-ISDN UNI. A hybrid concatenated error control scheme distributed through the protocol layers is used to target individual QoS requirements of different services. The convolutional coding and interleaving in the wireless physical layer protocol are used to guarantee QoS of voice services. A concatenated coding with additional 36 bit BCH code in the wireless ATM layer, which replaces the VCI/VPI of the ATM header field, improves the QoS up to the requirement of video services. The VCI/VPI field in WIATM is an overlapped routing information routing with the address control by radio port controller, and is thus not needed in the wireless ATM layer protocol. The retransmission scheme for data service only is added in the wireless data link layer, which is on top of wireless AAL, to meet its QoS requirement. Examples of signaling flows for call registration, call setup, and supporting handoff are shown in the design of the wireless network layer protocol. The AIN (advance intelligent network) signaling functionalities are considered for multimedia service control in the access network and interconnection to the ATM network. A parent-child creative basic call state model (BCSM) for wireless integrated services is introduced in both call origination and termination.

In previous work, access control for data has been proposed as a method to ensure adequate quality of service (QoS) in an integrated voice/data CDMA system. The motivation behind access control is to schedule data packet transmissions in slots when voice activity is low and to curtail data transmissions when the voice load is heavy. In this work, the class of probabilistic access control schemes, wherein data transmissions are controlled by dynamically changing the permission probability, are considered. The trigger for changing the permission probability is a measure of the current uplink load. Perfect power control is assumed first, and the trigger for access control is the power control feasibility condition, Schemes based on prediction are analyzed. While prediction schemes are complex to implement, they do provide an upper bound for performance of access control schemes. A simple and practical access control scheme, proposed earlier in the literature, is then extended. It controls the permission probability for data based on uplink load and a 1-bit broadcast feedback to all of the mobiles. The performance of this scheme depends on the choice of three parameters. It is demonstrated that, through a combined choice of these parameters, access control can be tuned to perform as desired and to yield significant capacity gains over not using access control. Results are then extended to the case of imperfect power control, where the outage criterion is based on limiting the total received power at the base station. In this case, too, the simple control scheme is shown to work well.

Future wireless systems will have to support multimedia services such as voice, data and fax. Issues in providing multimedia services on wireless include multiple access, bandwidth rationing and power control. We address the latter two issues. CDMA is assumed to be the access method. Different services have different quality (QoS) requirements, maximum power and/or minimum rate constraints. In order to achieve the required QoS they can alter their power and/or rate of transmission. Different rates are represented by varying the processing gain. Since users interfere with each other, achieving each user's QoS requirement is completely coupled with the powers. We formulate this as a constrained optimization problem. The objective functions used are minimum power and maximum rates. The former reduces interference seen by other cells due to the current cell, the latter attempts to achieve the best possible throughput for users in the current cell. We develop bounds on the total number of users of each class that can be supported simultaneously while maintaining adequate QoS and meeting resource constraints.

This paper analyzes the performance of a soft handoff algorithm of the type that has been proposed in IS-95 for a cellular CDMA system. While the analysis is stimulated by IS-95, the focus is more on developing applicable analysis methodology rather than on specifically modeling the present IS-95 algorithm. In the soft handoff scheme, multiple base stations are involved in the communication to and from one mobile station to improve the handoff performance on the boundaries between two base stations by providing channel diversity. On the other hand, there are additional resources used during soft handoff. There is, thus, a tradeoff between diversity advantage and resource utilization. The analysis provides quantification for this tradeoff. The analysis is validated with simulation. The analytical results can be used to gain insight and help select the appropriate handoff thresholds.

An analytic model has been previously developed for evaluating the performance of handoff algorithms based on relative signal strength measurements, i.e., the difference of signal strength from two base stations (BS). In this paper we have extended this analysis by including an additional criterion based on absolute signal strength in the handoff decision algorithm. The absolute signal strength is the averaged value of the received signal level from current serving BS measured by the mobile unit. Recommended, e.g., in the GSM system, this value has to be below a fixed threshold to initiate a handoff (a necessary, not sufficient condition). In this way the mobile unit should be assigned to a new BS if the absolute signal strength from the old BS drops below the threshold and the relative signal strength between the new and old BS reaches the hysteresis level. The model is proved to be sufficiently accurate by comparison with simulation. The effect of adding absolute signal strength is illustrated.>

The performance of a DS/CDMA system using M-ary orthogonal modulation with noncoherent demodulation is evaluated. The system operates in a multipath fading channel. A RAKE receiver structure with equal gain combining is used for demodulation. An approximation to the bit error probability is given which depends only on the first- and second-order moments of the multipath energies. The analysis results are compared with the results from computer simulations. It is seen that the approximation is accurate for multipath energies with realistic coefficient of variation. The system performance is also evaluated in terms of the capacity, which is defined as the number of users that can be supported at a given bit error probability. The approximation is used to evaluate the capacity reductions due to power variations caused by multipath fading.>

Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied.>

Mobile radio fading analyses typically assume a single frequency carrier transmitted through the propagation medium. The results are then used for the case of narrowband transmission. New phenomena occur with wideband transmission. This paper presents an analysis of the mitigation of the Rayleigh fading effect for wideband direct sequence code division multiple access (DS/CDMA) signaling. There are measurements that show this phenomenon. The reduction of the Rayleigh fading effect is an advantage of CDMA over narrowband transmission systems and it eases the burden of CDMA power control. A general expression for the coefficient of variation of the received wideband signal power is derived. It is shown how the coefficient of variation of the received power decreases as the bandwidth spreading increases. This paper also analyzes the time correlation needed for other analyses (e.g., CDMA power control).>

The authors develop and analyze models of power control that consider other aspects of code-division multiple access (CDMA) systems, such as interleaving and coding on the land mobile radio channel. The orientation is that a power control scheme keeps the received powers at the base station almost equal, and the performance degradation incurred if the powers are not exactly equal will be quantified. In doing so, the authors consider the performance implications of control latency and a maximum speech delay constraint. Because of positive correlations between the fading channel amplitudes, the effectiveness of the combination of interleaving and coding in combating the effects of power variations due to slow Rayleigh fading is reduced. It is shown that power control and interleaving/coding are most effective in complementary parameter regions, thus providing a degree of robustness for both fast and slow Rayleigh fading.>

For narrowband signals the envelope of the received signal is well characterised by a Rayleigh random process. Rayleigh fading causes large variations in the signal envelope. For wideband signals, the effect of Rayleigh fading is mitigated. This paper considers the effect of receiver processing on the mitigation effect for wideband DS/CDMA signals. The mitigation effect is shown here by a reduction of the coefficient of variation of the received power as a function of bandwidth spreading. The parameters chip rate, processing gain and the number of multipath components tracked are investigated for their effect upon the mitigation-the mitigation effect is sensitive to combinations of these parameters.>

The exact calculation of error probabilities for direct-sequence spread-spectrum multiple-access (DS/SSMA) systems has been addressed in the literature. The exact calculation is computationally difficult, so emphasis has been on approximations and bounds. One particularly attractive approximation is to just use a signal-to-noise ratio in a Gaussian approximation, the 'standard approximation'. Unfortunately, that approximation is not generally accurate enough. An improved Gaussian approximation with good accuracy has recently been presented. The author derives an accurate Gaussian approximation which is also computationally very simple.>