Showing papers on "Telecommunications link published in 1999"

Fair scheduling in wireless packet networks

Abstract: Fair scheduling of delay and rate-sensitive packet flows over a wireless channel is not addressed effectively by most contemporary wireline fair-scheduling algorithms because of two unique characteristics of wireless media: (1) bursty channel errors and (2) location-dependent channel capacity and errors. Besides, in packet cellular networks, the base station typically performs the task of packet scheduling for both downlink and uplink flows in a cell; however, a base station has only a limited knowledge of the arrival processes of uplink flows. We propose a new model for wireless fair-scheduling based on an adaptation of fluid fair queueing (FFQ) to handle location-dependent error bursts. We describe an ideal wireless fair-scheduling algorithm which provides a packetized implementation of the fluid mode, while assuming full knowledge of the current channel conditions. For this algorithm, we derive the worst-case throughput and delay bounds. Finally, we describe a practical wireless scheduling algorithm which approximates the ideal algorithm. Through simulations, we show that the algorithm achieves the desirable properties identified in the wireless FFQ model.

Design and performance of multicarrier CDMA system in frequency-selective Rayleigh fading channels

Abstract: This paper presents the advantages and disadvantages of a multicarrier code-division multiple-access (MC-CDMA) system. The transmitter/receiver structure and, the bandwidth of transmitted signal spectrum are compared with those of a conventional direct-sequence (DS) CDMA system, and an MC-CDMA design method, how to determine the number of subcarriers and the length of guard interval is discussed. The bit error rate (BER) lower bounds for DS-CDMA and MC-CDMA systems are derived and their equivalence is theoretically demonstrated. Finally, the BER performance in downlink and uplink channels with frequency-selective Rayleigh fading is shown by computer simulation.

Random access in a mobile telecommunications system

Abstract: An uplink common physical channel (random access channel) frame structure is disclosed with a separate preamble and data portion. The preamble is used by a base station to detect that a mobile station is attempting a random access request. The data portion of the channel includes user data, and pilot symbols that provide energy for channel estimation during reception of the data portion. A guard interval (TG) is preferably inserted between the preamble and data portion of the frame, which enables data detection to occur during an idle period. As such, the frame structures for both the common physical (random access) uplink channel and dedicated physical (traffic) uplink channel are compatible.

Optimal sequences, power control, and user capacity of synchronous CDMA systems with linear MMSE multiuser receivers

Abstract: There has been intense effort in the past decade to develop multiuser receiver structures which mitigate interference between users in spread-spectrum systems. While much of this research is performed at the physical layer, the appropriate power control and choice of signature sequences in conjunction with multiuser receivers and the resulting network user capacity is not well understood. In this paper we will focus on a single cell and consider both the uplink and downlink scenarios and assume a synchronous CDMA (S-CDMA) system. We characterize the user capacity of a single cell with the optimal linear receiver (MMSE receiver). The user capacity of the system is the maximum number of users per unit processing gain admissible in the system such that each user has its quality-of-service (QoS) requirement (expressed in terms of its desired signal-to-interference ratio) met. This characterization allows one to describe the user capacity through a simple effective bandwidth characterization: users are allowed in the system if and only if the sum of their effective bandwidths is less than the processing gain of the system. The effective bandwidth of each user is a simple monotonic function of its QoS requirement. We identify the optimal signature sequences and power control strategies so that the users meet their QoS requirement. The optimality is in the sense of minimizing the sum of allocated powers. It turns out that with this optimal allocation of signature sequences and powers, the linear MMSE receiver is just the corresponding matched filter for each user. We also characterize the effect of transmit power constraints on the user capacity.

A time and frequency synchronization scheme for multiuser OFDM

Abstract: We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on the universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system.

Performance of asynchronous orthogonal multicarrier CDMA system in frequency selective fading channel

Abstract: An asynchronous multicarrier (MC) direct-sequence (DS) code-division multiple-access (CDMA) scheme for the uplink of the mobile communication system operating in a frequency selective fading channel is analyzed. The bit error rate performance of the system with either equal gain combining or maximum-ratio combining is obtained. Numerical results indicate that the system performs better than that of the conventional DS-CDMA system and another MC-DS-CDMA system.

A real-time sub-carrier allocation scheme for multiple access downlink OFDM transmission

Abstract: In this paper, we propose sub-carrier allocation algorithms for a multiple access scheme in downlink OFDM transmission. Knowing the channel characteristics of all the users at the base station, the sub-carrier allocation algorithm assigns sub-carriers to the users in a way that the total transmit power is minimized. An optimal algorithm is presented to provide the best sub-carrier assignment. The complexity of the optimal solution renders it impractical for real-time application. To deal with the frequently changing channel characteristic of a fast time-varying multipath fading channel, a heuristic algorithm based on constructive assignment and iterative improvement is proposed which can give out a valid solution in real time. Experimental results show that the performance of this real-time algorithm is close to that of the optimal allocation.

CDMA/TDD system for wireless multimedia services with traffic unbalance between uplink and downlink

Abstract: This paper proposes a code division multiple access (CDMA) time division duplex (TDD) system for wireless multimedia services with traffic unbalance between uplink and downlink. In the proposed system, the number of uplink time slots in a TDD frame differs from that of downlink. Moreover, the difference can be reset by the network operator according to the traffic pattern. We evaluate the performance of the proposed system under multimedia environment using Markov analysis and computer simulation. The results show that the frequency utilization is maximized even when the uplink and downlink traffic volumes are unbalanced. This, in turn, reduces drastically the blocking rate of multimedia calls (connections) in the proposed system compared with that in the traditional CDMA systems where the uplink and downlink use equal bandwidth.

Method and system for facilitating timing of base stations in an asynchronous CDMA mobile communications system

Abstract: A method for estimating relative timing of base stations in an asynchronous mobile communications system comprises receiving at a first mobile station first and second downlink signals transmitted by first and second base stations; transmitting an uplink signal from the first mobile station to the first and second base stations; and calculating estimated relative time difference between the time bases of the first and second base stations using receive times at the first and second base stations of said uplink signal, transmit times of the first and second downlink signals, and a time difference at the first mobile station between a receive time of the second downlink signal and the transmit time of said uplink signal to account for propagation delays between first and second mobile stations.

Downlink scheduling in CDMA data networks

Abstract: We identify properties of optimal scheduling schemes for downlink traffic in a code division multiple access (CDMA) data-only network. Under idealised assumptions, we show that it is optimal for each base station to transmit to at most one delay-tolerant user at a time. Moreover we prove that a base station, when on, should transmit at maximum power for optimality. For a linear network, we characterise the optimal schedule as the solution to a linear program. As a by-product, our analysis yields bounds on throughput gains obtainable from downlink scheduling.

Group-blind multiuser detection for uplink CDMA

Abstract: Previously developed blind techniques for multiuser detection in code division multiple access (CDMA) systems lead to several near-far resistant adaptive receivers for demodulating a given user's data with the prior knowledge of only the spreading sequence of that user. In the CDMA uplink, however, typically the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. In this paper, group-blind techniques are developed for multiuser detection in such scenarios. These new techniques make use of the spreading sequences and the estimated multipath channels of all known users to suppress the intracell interference, while blindly suppressing the intercell interference. Several forms of group-blind linear detectors are developed based on different criteria. Moreover, group-blind multiuser detection in the presence of correlated noise is also considered. In this case, two receiving antennas are needed for channel estimation and signal separation. Simulation results demonstrate that the proposed group-blind linear multiuser detection techniques offer substantial performance gains over the blind linear multiuser detection methods in a CDMA uplink environment.

Method and apparatus for determining signatures for calibrating a communication station having an antenna array

Abstract: A method and apparatus for estimating the downlink signature for a remote transceiver which is part of a wireless communication system that includes a main transceiver for communicating with the remote transceiver. The main transceiver includes an array of transmit antenna elements. The method uses the remote transceiver for receiving signals when the main transceiver transmits downlink calibration signals. When the main transceiver also has a receive antenna array, the remote transceiver can transmit uplink calibration signals to the main transceiver for determining an uplink signature. The downlink and uplink signatures are used to determine a calibration function to account for differences in the apparatus chains that include the antenna elements of the arrays, and that enable downlink smart antenna processing weights to be determined from uplink smart antenna processing weights when the main transceiver includes means for smart antenna processing according to weights.

A Promising New Wideband Multiple-Access Scheme for Future Mobile Communications Systems

Abstract: Recently, “Interleaved Frequency-Division Multiple-Access” (IFDMA) has been introduced as a new spread-spectrum multiple-access scheme for mobile communications which is suitable for both downlink and uplink applications. IFDMA combines spread-spectrum multicarrier transmission with frequency-division multiple-access and, therefore, is capable of achieving frequency diversity while multiple-access interference is avoided. Moreover, compared to other wideband multiple-access schemes IFDMA shows several additional advantages. The cost to achieve these advantages is to allow for intersymbol interference. However, the effects of intersymbol interference can be reduced significantly by applying an equalizer at the IFDMA receiver. In this paper, the principle of IFDMA is reviewed and it is shown that IFDMA is a promising new wideband multiple-access scheme suitable for future mobile communications systems. The frequency diversity achievable with IFDMA is determined as well as performance results for uncoded and coded transmission taking into account different equalization techniques. As a result, uncoded IFDMA applying optimum equalization performs even better than uncoded multicarrier code-division multiple-access applying maximum-likelihood multi-user detection. In the case of coded IFDMA transmission coding gains up to 6.8 dB at a bit-error-rate of 10−-4 are achieved using a standard convolutional code. An additional performance gain of up to 2.2 dB is possible if equalization and decoding are considered jointly.

Satellite emergency voice/data downlink

Abstract: A satellite emergency voice/data downlink method and apparatus that sends cockpit audio and flight data to a satellite communications system upon detection of a serious event or a pilot generated signal.

Downlink scheduling in CDMA data networks

Abstract: We identify optimality properties for scheduling downlink transmissions to data users in CDMA networks. For arbitrary-topology networks, we show that under certain idealizing assumptions it is optimal for a base station to transmit to only one data user at a time. Moreover, for data-only networks, we prove that a base station, when on, should transmit at maximum power for optimality. We use these two properties to obtain a mathematical programming formulation for determining the optimal transmission schedule in linear data-only networks, with time allocations playing the role of decision variables. The optimality conditions imply that there exist (i) subsets of outer users on either side of the cell that should be served when only the neighboring base station on the opposite side is on; (ii) a subset of inner users in the center of the cell that should be served when both neighbors are on; (iii) a subset of users in the intermediate regions that should receive transmissions when both neighbors are off. Exploiting these structural properties, we derive a simple search algorithm for finding the optimal transmission schedule in symmetric scenarios. Numerical experiments illustrate that scheduling achieves significant capacity gains over conventional CDMA.

Uplink admission control and soft capacity with MUD in CDMA

Abstract: This paper presents an analysis of the WCDMA uplink capacity with admission control and multiuser detection (MUD). An uplink admission control strategy based on the received wideband interference is introduced in this paper. The CDMA uplink capacity formula is used, and the uplink capacity is calculated in terms of the average number of channels per cell assuming the presented admission control strategy. The amount of supported traffic with a given blocking probability is calculated with Erlang B formula. In this paper the Erlang B formula is extended to cover the soft blocking case in WCDMA uplink when an interference based admission control is used. The difference in the capacity between soft and hard blocking is shown with a few examples, and it is referred here as soft capacity. Also, the effect of the base station multiuser detection to the soft capacity is evaluated. It is shown that with an interference based admission control algorithm up to 50% more users with 144 kbps real time data can be supported than with a non-interference based admission control approach.

Multiple access interference suppression with linear chip equalizers in WCDMA downlink receivers

Abstract: The 3rd generation cellular communications standard will be based on wideband CDMA (WCDMA). To obtain adequate performance multiuser receivers, such as LMMSE receivers, can be used. Unfortunately, previously suggested LMMSE receivers cannot be applied in WCDMA systems employing long scrambling codes. In this paper, linear receivers suitable also for downlink with long scrambling codes are studied. These receivers equalize the channel prior to the despreading, thus restoring the orthogonality of users and suppressing multiple access interference (MAI). The numerical results show that chip equalizer based receivers can offer significant performance improvement in comparison to the conventional RAKE receiver.

Medium access control layer for packetized wireless systems

Abstract: An arrangement where, in addition to the control channels that are used to set up sessions, there is a logically and physically separate and independent set of control channels that can be used as session control channels. Illustratively, the physical GSM frame arrangement that employs 8 logical slots is used, with one slot both in the uplink and downlink dedicated to these separate control channels. In the downlink, such a control channel provides channel grant information, requests to make and transmit power measurements, poll mobile units, etc. In the uplink, such a control channel can be used to request access to a traffic channel, to report on power measurements, to request special hand-off handling, to respond to polling, etc. Because the uplink session control channel does not need to carry a significant amount of information, the GSM slot can be divided into minislots, with each minislot constituting a session control channel for a group of mobile units.

Channel estimation for DS-CDMA downlink with aperiodic spreading codes

Abstract: Synchronous code-division multiple-access (CDMA) techniques possess intrinsic protection against cochannel interference when orthogonal spreading codes are used. However, in the presence of multipath propagation, the signals lose their orthogonality property, leading to increased cross correlation. In these cases, channel estimation may be needed in order to improve detection. We propose and compare several algorithms for channel estimation of a synchronous CDMA point-to-multipoint link (downlink) that uses aperiodic spreading waveforms. We compare by simulation and analysis a subspace approach, a pilot-aided approach, and a decision-based approach.

Outage evaluation for slow frequency-hopping mobile radio systems

Abstract: In a shadowing-free environment, the improvement introduced by slow frequency-hopping (SFH) on a time-division multiple-access based mobile radio system can be taken into account by redefining the minimum carrier-to-interference ratio. This protection ratio, with SFH, is dependent on the transmission system, channel model, traffic, and frequency reuse parameters. In this paper, the above-mentioned analysis is used in order to investigate the capacity of a SFH mobile radio system, with reference to both the uplink and downlink, by taking into account a complete scenario, i.e., shadowing, fast fading, power control, antenna diversity, discontinuous transmission, and forward error correction with nonideal interleaving and sectorization. Outage probability is evaluated by a completely analytical methodology for the uplink, whereas the downlink requires a semianalytical approach to take users' positions into account. Comparison with a pure simulative approach is used to validate the results.

Channel allocation in a telecommunications system with asymmetric uplink and downlink traffic

Abstract: In a synchronous cellular radio telecommunications network that employs a time division multiple access (TDMA) channel allocation scheme, an effective approach to allocating channels or time slots in a TDMA time frame containing asymmetric downlink and uplink traffic channel fields can be achieved by dividing the downlink and uplink traffic channel fields into a number of regions, based on an expected level of intercell interference associated with each region. In addition, the level of susceptibility to intercell interference for each mobile unit operating in a given cell is continuously established. The allocation of channels or time slots is then carried out for each of the mobile units based on, among other things, the level of susceptibility associated with each of the mobile units, the level of expected intercell interference associated with each of the regions, and whether the channel or time slot is being allocated for the purpose of conveying information in the uplink or downlink traffic channel field.

Communication satellite system with dynamic downlink resource allocation

Abstract: A communication satellite system (58) dynamically allocates additional downlink resources on the satellite (11) to selected downlink signals to compensate for signal degradation. A typical cause of such degradation is rain where the downlink station is located. Another cause of such degradation is increased communication traffic or bandwidth demand within a particular region.

Downlink beamforming approach for frequency division duplex cellular systems

Abstract: The present invention relates to radio communication antennas and in particular relates to multi-user detection for CDMA antenna array receivers Here is provided a method of operating a FDD radio communications system wherein a base station has a constrained beamformer comprising at least two antennas with L channel taps and is operable to maximize diversity gain at the mobile The total SNR of all the multipath components received at the mobile will be less than for the maximum SNR algorithm, but the diversity gain should outweigh this loss for (nearly) orthogonal channel taps The performance of an uplink fixed weight beamformer is determined from the correlation matrix of the fadings of the uplink sampled impulse response Computation techniques are simplified and processing time can be considerably reduced

Adaptive interference suppression in multiuser wireless OFDM systems using antenna arrays

Abstract: This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly time-varying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin.

Downlink channel decorrelation in CDMA systems with long codes

Abstract: We develop linear detectors suitable for a code division multiple access (CDMA) mobile receiver using long codes. The special signal structure in the downlink transmission is exploited to obtain a simple detection rule. A least-squares (LS) detector, a best linear unbiased estimator (BLUE) detector, and a linear minimum mean-square error (LMMSE) detector are derived. For the LMMSE detector we consider an adaptive implementation. The results show that improvement can be achieved using the proposed detectors compared with that of the conventional RAKE receiver.

An uplink CDMA system architecture with diverse QoS guarantees for heterogeneous traffic

Abstract: The uplink access control problems for cellular code-division multiple-access (CDMA) systems that service heterogeneous traffic with various types of quality-of-service (QoS) and use multicode CDMA to support variable bit rates are addressed. Considering its distinct QoS requirements, class-I real-time traffic (e.g., voice and video) is differentiated from class-II non-real-time traffic (e.g., data). Connection-oriented transmission is achieved by assigning mobile-oriented code channels for class-I traffic, where each corresponding mobile needs to pass an admission test. Class-II traffic is transmitted in a best-effort manner through a transmission-rate request access scheme which utilizes the bandwidth left unused by class-I traffic. Whenever a mobile has class-II messages to transmit, the mobile requests code channels via a base station-oriented transmission-request code channel, then, according to the base station scheduling, the transmission is scheduled and permitted. Addressed are the admission test for class-I connections, transmission power allocation, and how to maximize the aggregate throughput for class-II traffic. The admission region of voice and video connections and the optimum target signal-to-interference ratio of class-II traffic are derived numerically. The performance of class-II traffic transmissions in terms of average delay is also evaluated and discussed.

System and method for variable reassignment of transmission channels

Abstract: A method of assigning a channel to a service in a wireless communications network based on interferences levels and channel quality. Channels are preassigned and then reassigned using an algorithm that assigns a channel to a service based on uplink interference and channel quality measurements. Reassignment can be achieved in one processing pass, rather than requiring multiple iterations.

Interference suppression in CDMA downlink through adaptive channel equalization

Abstract: In the synchronous CDMA downlink using orthogonal spreading codes, the multiple access interference (MAI) is essentially caused by the multipath channel. This is why the MAI can be suppressed by channel equalization. In this paper we present a receiver algorithm which performs adaptive channel equalization by estimating the transmitted chip sequence. The adaptation uses Griffiths' algorithm and converges to the LMMSE solution. The receiver is applicable in systems using long code scrambling such as in the 3/sup rd/ generation wideband CDMA proposals, and it is shown to offer significant performance gains when compared to conventional RAKE receivers.

Methods and apparatus for wireless communication using code division duplex time-slotted cdma

Abstract: Methods and apparatus for wireless communication in systems such as omni-beam and narrow-beam fixed wireless loop (FWL) systems. In a first technique in accordance with the invention, referred to as code division duplex (CDD) time-slotted CDMA, uplink and downlink portions of the system are separated using code division duplexing, while the users within a given cell are also separated using codes, e.g., using time-slotted CDMA. In a second technique in accordance with the invention, referred to as time division duplex (TDD) time-slotted CDMA, uplink and downlink portions of the system are separated using time division duplexing, e.g., time slots, while the users in a given cell are separated using codes, e.g., time-slotted CDMA. Both the CDD and TDD techniques may make use of an electronically-steered beam which is designed to provide simultaneous coverage within a given cell for two or more users separated by codes. In a third technique in accordance with the invention, referred to as orthogonal frequency division multiplexing (OFDM), uplink and downlink portions of the system are separated in frequency, while the users are, e.g., also separated in frequency.

Group-blind multiuser detection for uplink CDMA

Abstract: The blind techniques for multiuser detection in CDMA systems lead to several near-far resistant adaptive receivers for demodulating a given user's data with the prior knowledge of only the spreading sequence of that user. In the CDMA uplink, however, typically the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. In this paper group-blind techniques are developed for multiuser detection in such scenarios. These new techniques make use of the spreading sequences and the estimated multipath channels of all known users to suppress the intra-cell interference, while blindly suppressing the inter-cell interference. Simulation results demonstrate that the proposed group-blind linear multiuser detection techniques offer substantial performance gains over the blind linear multiuser detection methods in a CDMA uplink environment.