Showing papers on "Telecommunications link published in 1995"

Information capacity and power control in single-cell multiuser communications

Abstract: We consider a power control scheme for maximizing the information capacity of the uplink in single-cell multiuser communications with frequency-flat fading, under the assumption that the users attenuations are measured perfectly. Its main characteristics are that only one user transmits over the entire bandwidth at any particular time instant and that the users are allocated more power when their channels are good, and less when they are bad. Moreover, these features are independent of the statistics of the fading. Numerical results are presented for the case of single-path Rayleigh fading. We show that an increase in capacity over a perfectly-power controlled (Gaussian) channel can be achieved, especially if the number of users is large. By examining the bit error-rate with antipodal signalling, we show the inherent diversity in multiuser communications over fading channels.

Method and system for channel allocation using power control and mobile-assisted handover measurements

Abstract: A method and system for the adaptive allocation of channels within a radio communication system, specifically a cellular network, is presented. The allocation method takes advantage of measurements made by the mobile radiotelephone and allocates channels based on the carrier to interference ratio. Using adaptive power control, consideration is given to maintaining an acceptable carrier to interference ratio while at the same time minimizing transmit power. Exemplary embodiments consider independent allocation of the uplink and downlink as well as independent determination of the uplink and downlink power levels. Other exemplary embodiments also give consideration to an efficient method for slot allocation in a TDMA communication system.

Spectrally efficient high capacity wireless communication systems

Abstract: A wireless system includes a network of base stations (1) for receiving uplink signals transmitted from a plurality of remote terminals (69) and for transmitting downlink signals to the remote terminals. Each base station (1) includes a plurality of transmitting antenna elements (18) for transmitting downlink signals and receiving antenna elements (19) for receiving uplink signals, a signal processor (13) connected to the antenna elements for determining spatial signatures and multiplexing and demultiplexing functions. A multiple base station controller (66) is used for optimizing network performance.

Multiple access cellular communication with dynamic slot allocation and reduced co-channel interference

Abstract: An apparatus and method reduces co-channel interference in multiple-access cellular communication systems in which frame time or frequency slots are allocated between uplink and downlink. An omnidirectional antenna or a set of directional antennas are used in each cell base station to communicate with users. The frame slots (710,715,720) in which the antennas communicate uplink and downlink information are arranged in accordance with a predetermined frame organization to reduce mixed co-channel interference (CCI). Mixed CCI occurs when a downlink transmission from one base station antenna in a given cell interferes with uplink reception in another base station antenna in a frequency reuse (FR) cell. A potentially-interfering antenna in a given cell is therefore directed to transmit downlink information in a different portion of the frame than that in which a potentially-interfered-with antenna in the frequency reuse cell receives uplink information. The frame slots may be allocated such that only a portion (720) of the available slots are dynamically allocated in accordance with user demand, while the remaining portions are assigned to either uplink (710) or downlink (715) communication.

Emergency locator device

Abstract: An emergency locator device which is adapted to receive information signals from global positioning satellites (GPS) and determine a position of the device in terms of latitude and longitude includes a downlink antenna to receive GPS position information, a receiver/processor for processing the GPS information, a microcontroller for controlling the operation of the device, a transceiver and an uplink antenna to establish a communications link with a telecommunications satellite and transmit a rescue signal from the emergency locator device to rescue authorities.

Variable spreading gain CDMA with adaptive control for true packet switching wireless network

Abstract: A rapidly growing interest in personal communications and mobile computing has underlined the importance of packetized wireless networks that can support mixed traffic such as voice/data/image/video in an efficient manner. In order to provide for a "true packet switching" capability, the authors propose a scheme based on variable-spreading-gain CDMA with adaptive control. This approach accommodates mixed traffic types and dynamic capacity access. In the paper, the authors outline the architecture of such wireless networks and describe the principle and the objectives of their adaptive multiple access algorithm for the uplink control. This algorithm can be implemented distributedly at the mobile stations by a simple downlink broadcast of the uplink load status. Alternatively, it can be implemented centrally at the base station that broadcasts explicitly the permission to transmit in the form of the transmission probabilities for various types of users.

Downlink power control algorithms for cellular radio systems

Abstract: Power control is an effective technique to reduce cochannel interference and increase capacity for cellular radio systems. Optimum centralized power control can minimize the outage probability, but requires the information of all link gains in real time, which is very difficult to successfully implement for a large system; besides, the computational complexity of an optimum power control algorithm makes it impractical for real implementations. In this paper, we propose some centralized power control algorithms with reasonable computational com- plexity. One of the algorithms, called the SMIRA algorithm, has an outage probability that is very close to the minimum. We also study a class of distributed power control algorithms that can achieve a balanced carrier-to-interference ratio with probability one. Among the class of algorithms, we found that the one proposed in (6) gives the minimum outage probability.

Open-loop power control error in a land mobile satellite system

Abstract: In order to combat large scale shadowing and distance losses in a land mobile satellite system, an adaptive power control (APC) scheme is essential. Such a scheme, implemented on the uplink ensures that all users' signals arrive at the base station with equal average power as they move within the satellite spot beam-an important requirement in a CDMA system. Because of the lengthy round-trip delay on a satellite link, closed-loop power control systems are only of marginal benefit. Therefore, an open-loop APC scheme is proposed to counteract the effects of shadowing and distance loss. A fairly general channel model, consisting of log-normal shadowing and Rician fading, is assumed. This can be applied to a specific two-state land mobile satellite channel model, involving shadowed intervals with Rayleigh fading and unshadowed intervals with Rician fading. It is found that the power control error can be approximated by a log-normally distributed random variable. To quantify the performance of the APC, the standard deviation of the power control error in decibels is analyzed as a function of the specular power-to-scatter power ratio, the measurement time and the vehicle velocity. To illustrate the usefulness of the results, we analyze the effect of the power control error on the system capacity of a CDMA mobile satellite link. >

Transceiver algorithms of antenna arrays

Abstract: A transceiver for improving the spectral efficiency and the capacity of cellular communication systems is disclosed. The transceiver uses an antenna array for communicating in a cellular communication system with a plurality of mobile stations. In addition, the transceiver contains a spatial filter connected to an antenna array wherein the spatial filter has as many outputs as there are array elements and as many inputs as there are spatial channels. A splitter then splits data to be transmitted to each mobile station into a number of parallel data streams, the number of parallel data streams corresponds with the number of spatial channels in the system, wherein the data streams, are delayed based upon uplink measurements. Finally, power allocators are provided for allocating transmission power to each possible channel based upon long-term SNR measurements on the uplink.

A flexible and high performance cellular mobile communications system based on Orthogonal Multi-Carrier SSMA

Abstract: The conventional Direct-Sequence Spread-Spectrum-Multiple-Access (DS-SSMA) system with RAKE-receiver is an interesting approach in mobile communications in order to combat the frequency selectivity of the channel. The main advantages of DS-SSMA are well known, but its capacity is limited by other-user interference. Another interesting SSMA technique that combats the frequency selectivity of the channel and achieves high spectral efficiency is based on Orthogonal-Frequency-Division-Multiplexing (OFDM). This new SSMA technique combines the principles of DS-SSMA with OFDM and is called Multi-Carrier Spread-Spectrum-Multiple-Access (MC-SSMA). By providing high frequency diversity, this combination enables the possibility to perform a maximum-likelihood detection (MLD) resulting in high performance/capacity, to use the spectrum in an efficient way and to retain many advantages of the conventional DS-SSMA system. In addition, it allows both simple cell-separation by using frequency hopping and simple hardware realization. An example for the downlink of a mobile communications system, i.e. the transmission from the base station to a mobile station, using MC-SSMA with Walsh-Hadamard code spreading is studied. Different coherent detection algorithms such as conventional detection, MLD, and iterative detection are analyzed. The analytical performance of MLD is evaluated. It is also shown analytically, and by simulations that the MC-SSMA system with MLD outperforms the DS-SSMA system with RAKE-receiver. Up to 64 active users can be transmitted in a 1.25 MHz bandwidth. Each user has a rate of 16.0 kbit/s by using BPSK modulation. At a signal to noise ratio (SNR) of 13 dB a bit error probability ofP b =10−3 can be guaranteed without channel coding. This results in a spectral efficiency of about 0.8 bit/s/Hz. Under the same conditions the conventional DS-SSMA system results in a spectral efficiency of about 0.15 bit/s/Hz. Hence, using MC-SSMA for the downlink of a cellular mobile communications system is a promising approach.

Variable quality of service in CDMA systems by statistical power control

Abstract: The low bandwidth and high error rates of the wireless channel make joint source-channel coding desirable for optimizing resource usage. The authors argue that a mechanism for providing variable quality of service (QOS) is essential for joint source-channel coding, and show that its implementation by way of power control is a natural choice for CDMA. Present-day power control techniques focus on controlling interference. The authors present a power control algorithm which simultaneously minimizes interference and provides variable QOS contracts for different traffic types in a CDMA system. The algorithm accommodates different QOS requirements by assigning different power levels to each traffic type, and can add or drop connections dynamically while ensuring that QOS specifications are satisfied. Both the uplink and downlink are analyzed, and the scheme is statistically formulated to handle variable-rate traffic.

An effective transmission beamforming scheme for frequency-division-duplex digital wireless communication systems

Abstract: Most wireless communication systems use different carriers for uplinks and downlinks, hence the downlink beamforming can only be performed based on the directions-of-arrival (DOA) information of the uplink signals. In this paper, we propose an effective downlink transmission scheme for TDMA mobile communication systems via the integration of direction finding and blind signal estimation techniques. With an M-element antenna array, our new scheme can estimate up to 2M/sup 2//3 DOAs of direct path and multipath signals, while a conventional DOA estimation algorithm can resolve no more than M DOAs. RF experiments show that by incorporating these additional DOA estimates, much improved interference suppression was obtained.

Variable spreading gain CDMA with adaptive control for integrated traffic in wireless networks

Abstract: The growth in personal communications and mobile computing has motivated investigation into packetized wireless networks for integrated services such as voice/data/image/video. Variable spreading gain CDMA (VSG-CDMA) is a candidate system that accommodates mixed traffic types and dynamic capacity access. The adaptive multiple access algorithm provides for a graceful integration of various services and can give priority to ongoing services over new access attempts. This algorithm can be implemented distributedly at the mobile stations by a simple downlink broadcast of the uplink load status. Previous work described VSG-CDMA and the dynamic access algorithm. It was shown how the priority feature works for a single service class. In the present paper, the authors extend the analytic study to the case for integrated service types and give numerical results for the integration of voice and data.

Method and device for efficient error correction in a packet-switched communication system

Abstract: The method and device of the present invention provide for efficient error correction in a packet radio system by 1) eliminating wasted bandwidth due to the unnecessary retransmissions that occur as a result of lost positive acknowledgments, 2) occupying less downlink bandwidth than a scheme requiring positive acknowledgments, and 3) minimizing packet delay by providing two levels of error correction at the datalink layer.

Channel allocation and downlink beamforming in an SDMA mobile radio system

Abstract: On the downlink of an SDMA mobile radio system, co-channel interference has to be kept down by beamforming. In this paper two DOA-based beamforming approaches are presented minimizing array signal power while maintaining given signal-to-noise-and-interference ratios for all users. The linear approach is computationally cheap, so that it is suited for channel allocation as well, quickly evaluating the spatial separability of a specific mobile radio scenario. The nonlinear algorithm yields optimum results but only converges quickly, if provided with a good starting point. Therefore, its basic application is burst-to-burst updating of the beamformer.

Multiuser blind channel estimation and spatial channel pre-equalization

Abstract: In this paper, we study two of the fundamental operations of a TDD smart antenna system, namely, the uplink channel and sequence identification and downlink selective transmission. For uplink, our focus is on the development of a blind estimation algorithm which is capable of resolving a multiuser system without the use of training sequence or any input statistics. For downlink, we propose a spatial channel pre-equalization scheme which simultaneously eliminates the intersymbol interference (ISI) and the co-channel interference (CCI) for all users using FIR filters. Both algorithms were validated by RF experiments using the smart antenna testbed developed in the University of Texas at Austin.

Multiple-access capacity in mobile user satellite systems

Abstract: The channel capacity of a satellite direct sequence CDMA system is analyzed including the effects of faded user interference, overlapping antenna beams, imperfect equalization of the antenna pattern across an antenna cell, and diversity reception. Simplified models are used to describe the impact of these effects on the channel capacity of single and multiple cofrequency CDMA systems. In a comparison of the uplink and downlink paths, the uplink of the CDMA system is shown to limit the channel capacity because the downlink can utilize code-orthogonality and coherent demodulation. In a multiple system comparison between band-shared CDMA and band-segmented FDMA/TDMA technologies, FDMA/TDMA is shown to provide about the same capacity for uniformly distributed traffic conditions over many cells and dramatically better capacity when traffic is concentrated in one cell. Due to the peak nature of telephony, this result supports the use of band-segmented systems in mobile user satellite systems. >

Point-to-multipoint broadband services drop with multiple time slot return channel for customer premises equipment served by fiber optic telecommunication system employing STS-based transmission format containing asynchronous transfer mode cells

Abstract: A broadband fiber optic communication system conveys telecommunication messages over a fiber optic link between a master site and one or more remote sites. The remote sites are coupled over an unshielded twisted pair-configured, communication link to an optical network unit, which is ported to the fiber optic link. In order to convey broadband information signals that have been downlinked, from the master site to the optical network unit, to the remote sites, and to provide for return messages from the remote sites, a point-to-multi-point communication scheme is provided. Pursuant to this scheme, an upstream transceiver in the optical interface unit transmits STS-1 frames, which contain broadband information signals, such as asynchronous transfer mode (ATM) data cells, intended for one or more remote sites, and a return time slot-representative control code associated with each destination remote site, over the communication link to the remote sites. At a respective remote site, a downstream transceiver uses the return time slot-representative control code to control its time of transmission over an uplink slotted bus return channel. The remote site also transmits communication signals that encapsulate asynchronous transfer mode data cells. The data in these cells is extracted by the optical network unit and forwarded over the fiber optic link to the master site.

Power control method and arrangement for handover in a mobile communication system

Abstract: The invention relates to a power control method and arrangement for a handover in a mobile communication system. Each cell is assigned a maximum transmitting power level (MS_TXPWR_MAX(n)) at which the mobile stations (MS) are allowed to transmit in the cell, and an optimum level for the uplink signal (MsOptLevel(n)) which the mobile station is to achieve following the handover. The transmitting power level (MS_TXPWR(n)) of the mobile station following the handover is lower than the maximum transmitting power level of the target cell (BTS2) by an amount equal to the difference between the receive level of the optimum level of the uplink signal of the target cell and the receive level of the downlink signal (RXLEV_NCELL(n)) of the target cell as measured prior to the handover, if the measured receive level of the target cell is higher than the optimum level of the uplink signal of the target cell.

Electronic equipment audio system

Abstract: A modular telecommunications system for an electronic audio system being specially configured for wireless telecommunication in accordance with a preselected standard. Modular units are adapted to be secured within AM/FM radios, tape players, automobiles, and the like, for establishing a telecommunications link with a wireless network. The modular unit comprises a housing for mounting a telecommunications transceiver in the particular item of electronic audio systems, and a microphone or keyboard is provided for generating user communication signals. In this manner, electronic equipment may be purchased for its conventional, non-telecommunications use such as entertainment or commercial monitoring with the added flexibility of wireless telecommunications. An electronic audio system having conventional audio components is modified for utilizing a telecommunications module containing a transceiver. A microphone and keypad are added to the electronic audio equipment for utilizing the functions of the telecommunications module.

Dynamic allocation of channels in a cellular telephone system

Abstract: A cellular communication system has a plurality of cells and a plurality of channels. The plurality of channels includes a plurality of measurable channels for which interference can be measured in both uplink and downlink directions and a plurality of unmeasured channels for which interference are not made in both uplink and downlink directions. The measurable channels, designated probe channels, may be channels for use in a digital system. The unmeasured channels may be channels for use in an analog system, or broadcast channels. A method and apparatus for allocating the plurality of channels comprises initially organizing the plurality of channels into a plurality of channel groups, wherein each channel group includes at least one probe channel, and wherein at least one of the channel groups further includes at least one unmeasured channel. Then, each one of the cells performs an allocation routine comprising the steps of monitoring at least one of the channel groups by making and collecting uplink and downlink interference measurements only on the probe channel of each monitored channel group over a predetermined time period. Then, the collected uplink and downlink interference measurements are analyzed in accordance with an adaptive channel allocation strategy. Finally, a selected one of the monitored channel groups is allocated to the cell in dependence on the analyzed uplink and downlink interference measurements. In this manner, the unmeasured channels are also subjected to an adaptive channel allocation strategy.

Transmission link supervision in radiocommunication systems

Abstract: A digital cellular communications system has a bidirectional transmission link for conveying encoded speech and control data in the form of blocks, a switching unit coupled to a first end of the transmission link, and a base station coupled to a second end of the transmission link. The switching unit includes a speech encoder for encoding speech signals, an error detection code generator for generating an error detection code from the encoded speech signal, and a formatter, coupled to the error detection code generator, for producing a formatted block to be transmitted on the transmission link, the formatted block having a header, the encoded speech data, and the error detection code. The base station has a transmission link error detector for signalling the existence of an error in a received block in response to the received block containing an error. Similarly, the switching unit has a transmission link error detector for signalling the existence of an erroneously received block which has been transmitted in the uplink direction. The number of errors is monitored by a transmission link supervisor which reports if the errors indicate that the quality of the transmission link has degraded beyond a predetermined threshold.

Total capacity in a shared CDMA LEOS environment

Abstract: We are interested in determining the feasibility of having multiple service providers share the same spectrum in transponder-type. Satellite-based, code-division multiple-access mobile communications. To accomplish this, we compare the total capacity when n systems are operational to that when only a single system uses the frequency band. This comparison is made for both the uplink and the downlink under one of two possible constraints, either a maximum power flux density (PFD) limit per system, or an aggregate PFD limitation. While there are circumstances when the total capacity of multiple systems can achieve a greater value than that of a single service provider, conditions depend on the degree of shadowing, and whether or not the PFD limit is on a per system basis or on an aggregate basis from all systems. >

Adaptive channel allocation in a mobile communications system

Abstract: In a cellular communication system, a method and apparatus are provided for allocating a channel for use on a connection between a base station and a mobile station in a cell, without making measurements of downlink interference. Throughout the system, uplink and downlink power levels are regulated so that, within each cell, their sum changes by an amount whose magnitude is substantially equal to the magnitude of change in path gain between the mobile station and the base station in that cell, and which sum is opposite in sign to the amount of change in path gain. This power regulation scheme assures that there will be a strong correlation between system-wide measurements of uplink and downlink interference. Therefore, with the power regulation scheme in place, each cell determines uplink carrier to interference (C/I) ratios for the free channels available for use in the cell and selects an acceptable free channel having a C/I value that is within a predetermined acceptable range. If uplink C/I on the acceptable free channel is greater than the uplink C/I value of a channel currently assigned to a connection, then an intra-cell handover is performed so that the connection will be reassigned to the acceptable free channel. This technique is useful for providing an adaptive channel allocation scheme to systems, such as analog cellular communications systems, in which downlink interference measurements cannot be made.

Method and means for frequency hopping in a radio communication system

Abstract: A radio communications systems which utilize channel hopping such that those channels which have high channel quality for a given connection are used more often than channels having lower channel quality for the same connection. Channel quality, for instance interference, is measured with respect to uplink connections and with respect to downlink connections. The measured interference values are then stored in an interference list for each of the connections in the radio communications system. The interference lists are converted to corresponding weight lists for each of the connections in both uplink and downlink in the base station. A channel which has a high weight value for a given connection will appear more often in corresponding hopsequence lists than a channel which has a lower weight value.

Method and apparatus for mitigating interference in a communication system

Abstract: A communication system (100) that includes a base unit (101) and a subscriber unit (e.g., 103) employs a method (400) and apparatus (101) for mitigating interference (135) therein. The base unit (101) receives an uplink communication signal from the subscriber unit (103) at an uplink frequency and conveys a downlink communication signal to the subscriber unit (103) at a downlink frequency. Upon receiving (403) an uplink communication signal from the subscriber unit (103), the base unit (101) determines (405) a quality metric for the uplink frequency. When the quality metric is below a quality threshold (407), the base unit (101) and the subscriber unit (103) transfer (415) the communication signal to an alternate uplink frequency, while the downlink frequency remains unchanged.

Method and apparatus for echo reduction in a hands-free cellular radio using added noise frames

Abstract: A method and apparatus for reducing the echo in a hands-free digital cellular communications system is presented. Acting upon speech signals being present in the downlink, uplink speech frames generated by the hands-free communication device are attenuated to prevent the downlink speech signals from being transmitted at full volume back on the uplink as an echo. Added to the attenutated uplink speech frame is a noise frame generated by the hands-free communication device, which provides a consistent level of background noise to the person at the other end of the call. The noise codeword is further randomly ordered to prevent any modulative effects from the process.

Uplink-noise limited satellite channels

Abstract: Systems which transmit data through a nonlinear relay, such as a satellite, must deal with a composite channel that can be separated into two distinct channels-the uplink channel between the user and the relay, and the downlink channel between the relay and the final destination. If the system has a strict power limitation and high data rate demands, such as a small satellite transmitting through NASA's TDRSS Network, the dominant noise is present on the uplink rather than the downlink channel. Such a system is deemed to be uplink-noise limited and presents the designer with a number of problems not encountered in a more typical downlink-noise limited channel. Whereas the transmitted signal constellation can be pre-distorted to take into account the effect of the nonlinearity in the down-link limited channel, no amount of pre-distortion will solve the problems encountered when the majority of the noise is present before the nonlinearity. Instead, the receiver must be modified to reflect the non-Gaussian noise due to the operation of the nonlinearity on Gaussian noise. Under three assumptions-there is no downlink-noise present, the downlink channel is wideband relative to the data, and the filter proceeding the nonlinearity meets both matched filter and Nyquist requirements-such modifications can be made based on the nature of the nonlinearity. By mapping the ideal decision region through the nonlinearity, performance almost identical to that of a linear-wideband AWGN channel can be achieved. This paper develops the theoretical performance of the receiver described for a nonlinearity typical of a satellite channel. Performance curves are presented for QPSK, 8PSK, 16PSK and 16QAM modulation schemes.

Bus arbitration infrastructure for deployment of wireless networks

Abstract: A method and apparatus for expanding the physical coverage area of a wireless communications network. More specifically, a number of transceivers are connected in a series configuration to a single base station, and each transceiver transmits signals to selected portions (subcells) of the physical coverage area. Each transceiver is connected to a delay unit where all of the delay units are adjusted so that downlink signals transmitted from the single base station will be transmitted from the transceivers to their respective subcells in phase with each other. Since all of the downlink signals from each of the transceivers result from the same downlink signal from the base station and since they are in phase with each other, a wireless unit anywhere in the physical coverage area will receive a downlink signal which is error free. Further, with this invention, when uplink signals are transmitted simultaneously from a number of wireless units in different subcells of the physical coverage area to the transceivers, the selection units will instantaneously select one and only one uplink signal to be received by the base station. Any one of the other simultaneously transmitted signals will be rejected throughout the duration of their signal.

Asymmetric speech coding for a digital cellular communications system

Abstract: A digital cellular communication system provides an asymmetric speech coder for improving audio quality in the uplink channel. A high rate speech coder is provided in the uplink of the digital cellular communications system to improve the audio quality. To compensate for the higher band width consumed by this high rate speech coder, the uplink uses a channel code, such as a convolutional code, that generates fewer symbols per each input bit, thereby consuming less bandwidth. The loss of coding gain as a result of the change in the convolutional code is compensated for by applying a special power control strategy. The power control strategy increases the transmit power for transceivers using the asymmetric speech coder.