Showing papers on "Spectral efficiency published in 1997"

Wavelet packet modulation for orthogonally multiplexed communication

Abstract: Utilizing multidimensional signaling techniques, a generalized multirate wavelet-based modulation format for orthogonally multiplexed communication systems is presented. Wavelet packet modulation (WPM) employs the basis functions from an arbitrary pruning of a dyadic tree structured filter bank as orthogonal pulse shapes for conventional quadrature amplitude modulation (QAM) symbols. This generalized framework affords an entire library of basis sets with increased flexibility in time-frequency (T-F) partitioning. The bandwidth efficiency and power spectral density figures of merit for the general signal are derived and shown to be that of standard QAM.

Area spectral efficiency of cellular mobile radio systems

Abstract: We consider the average area spectral efficiency (ASE) of variable-rate transmission cellular mobile systems. This efficiency is defined as the sum of the maximum average data rates/Hz/unit area supported by a cell's base station. We study this efficiency as a function of the reuse distance for the uplink of FDMA and TDMA systems under different interference configurations. Results indicate that, based on the worst-case interference configuration, the optimal reuse distance is approximately four. However, this optimal reuse distance is two for the best-case and the average interference configurations (i.e. frequencies should be reused every cell). In addition, the ASE decreases as an exponential of a 4th order polynomial relative to the cell size. This result quantities exactly how much cellular system capacity increases with decreased cell size. We also quantify the increase in ASE due to antenna sectorization. We conclude by analyzing the effect of traffic loading on the ASE when a fixed channel assignment is employed.

The capacity of downlink fading channels with variable rate and power

Abstract: We obtain the Shannon capacity region of the down-link (broadcast) channel in fading and additive white Gaussian noise (AWGN) for time-division, frequency-division, and code-division. For all of these techniques, the maximum capacity is achieved when the transmitter varies the data rate sent to each user as their channels vary. This optimal scheme requires channel estimates at the transmitter; dynamic allocation of timeslots, bandwidth, or codes; and variable-rate and power transmission. For both AWGN and fading channels, nonorthogonal code-division with successive decoding has the largest capacity region, while time-division, frequency-division, and orthogonal code-division have the same smaller region. However, when all users have the same average received power, the capacity region for all these techniques is the same. In addition, the optimal nonorthogonal code is a multiresolution code which does not increase the signal bandwidth. Spread-spectrum code-division with successive interference cancellation has a similar rate region as this optimal technique, however, the region is reduced due to bandwidth expansion. We also examine the capacity region of nonorthogonal code-division without interference cancellation and of orthogonal code-division when multipath corrupts the code orthogonality. Our results can be used to bound the spectral efficiency of the downlink channel using time-division, frequency-division, and code-division, both with and without multiuser detection.

Multi-carrier spread-spectrum

Abstract: I. General Issues of Multi-Carrier Spread-Spectrum. Multi-Carrier Spread-Spectrum: An Attractive Special Case of General Multiuser/Multisubchannel Transmission Methods J. Lindner Overview of the Results About Multi-Tone CDMA Detection L. Vandendorpe. Comparison of Multiple Access Schemes for an OFDM Downlink System H. Rohling, et al. Link/System Level Performance of an OFDM/CDMA Downlink Configuration J.P. Castro, et al. II. Applications of Multi-Carrier Spread-Spectrum. Performance Analysis of a New Multi-Code and Multi-Carrier Hybrid Transmission Scheme for Future Broadband Mobile Communication Systems H. Harada, R. Prasad. A Spread-Spectrum Multi-Carrier Multiple-Access System for Mobile Communications S. Kaiser, K. Fazel. Orthogonal Frequency-Division Multiple-Access with Frequency Hopping and Diversity H. Sari. M-ary Orthogonal Modulation for MC-CDMA Systems in Indoor Wireless Radio Networks A. Dekorsy, K.-D. Kammeyer. Comparison of WLAN Multi-Carrier DS-SS Physical Layer Configurations Measured Indoor Environment P. Banelli, et al. III. Coding and Detection for MC-SS. Multi-Carrier CDMA Using Convolutional Coding and Interference Cancellation over Fading Channels J.J. Maxey, R.F. Ormondroyd. An Approach for a Multi-Carrier Spread-Spectrum System with RAKE Receiver A. Nahler, G.P. Fettweis. Multi-Carrier Modulated Orthogonal Code-Division Multiple-Access: D.Th. Magill. Interleaved FDMA - A New Spread-Spectrum Multiple-Access Scheme U. Sorger, et al. Aspects on Wideband Multi-Carrier Communication J.E.M. Nilsson. Equalization and Coding for Extended MC-CDMA over Time and Frequency Selective Channels J. Egle, et al. Detection Method for MC-CDMA Based on a Recurrent Neural Network Structure W.G. Teich, et al. IV. Synchronisation and Channel Estimation. Sensitivity of OFDM/CDMA to Carrier Phase Jitter H. Steendam, M. Moeneclaey. Time Domain Uplink Channel Estimation in Multi-Carrier CDMA Mobile Radio System Concepts B. Steiner. Subspace-Based Joint Time-Delay and Frequency-Shift Estimation in Multi-Tone Code Division Multiple Access (MT-CDMA) Systems M. Eric, et al. Pilot-Symbol-Aided Channel Estimation in Time and Frequency P. Hoeher, et al. A Family of Extended Gaussian Functions with a Nearly Optimal Localization Property Ch. Roche, P. Siohan. On the Duality of Multi-Carrier Spread-Spectrum and Single-Carrier Transmission K. Brueninghaus, H. Rohling. Mismatched Multi-Carrier Complementary Spread-Spectrum Radar and Sonar Systems I.S. Simic, et al. V. MC-SS for the Uplink of a Cellular System. An OFDM/SFH-CDMA Transmission Scheme for the Uplink L. Tomba, W. Krzymien. Uplink Spectral Efficiency of Multi-Carrier Joint Detection Code-Division Multiple-Access Based Cellular Radio Systems F. Berens, et al. Simulation of a DSSS/MCM System in a Doppler Spread Channel T.B. Welch, R.E. Ziemer. On the Performance of Asynchronous Multi-Carrier CDMA H. Ochiai, H. Imai. Performance Comparison B.

A flexible spread-spectrum multi-carrier multiple-access system for multi-media applications

Abstract: This paper presents a novel multiple access scheme designed for mobile multi-media applications. The scheme is referred to as spread-spectrum multi-carrier multiple-access (SS-MC-MA) and exploits the advantages given by the combination of the spread-spectrum technique with multi-carrier modulation, known from multi-carrier code division multiple access (MC-CDMA) schemes. In contrast to MC-CDMA, the SS-MC-MA separates the users' signals according to an FDMA scheme on the sub-carrier level. Thus, the novel scheme avoids multiple access interference (MAI) within a cell. SS-MC-MA can use the same radio interface for the up- and the down-link. Its bandwidth efficiency is comparable to the high bandwidth efficiency achievable with MC-CDMA in the down-link. By providing a high degree of flexibility, this system can be a good candidate for the future Universal Mobile Telecommunications Systems (UMTS).

A narrowband approach to efficient PCS spectrum sharing through decentralized DCA access policies

Abstract: This article addresses spectrum sharing and open access for personal communications services (PCS). Traditional regulation has allocated electromagnetic spectrum through fragmentation into mutually exclusive frequency blocks. Block allocation schemes produce trunking inefficiencies in the use of multiple narrowband RF channels. Broadband allocation schemes such as code division multiple access (CDMA) can accommodate multiple users on a single RF channel. Due to the near-far problem, the only way competing CDMA operators could share common spectrum is through collocation of cell sites, which hinders market mechanisms. An alternative approach is narrowband spectrum sharing through a decentralized dynamic channel assignment (DCA) approach, such as autonomous reuse partitioning (ARP). Multiple-operator DCA allocation schemes for low-tier PCS systems have been proposed in some countries. Under such an approach, competing operators use a common air interface to share the available spectrum. Open access can improve opportunities for competition in the provision of PCS. Other things being equal, a multi-operator DCA system outperforms that achievable by multiple carriers under a traditional fragmented spectrum, even for overlapping networks and unequal traffic loads. In this article we use a discrete event simulation to explore the spectrum efficiency implications of adopting a narrowband decentralized DCA/ARP spectrum-sharing policy. We explore as well regulatory measures so operators will deploy infrastructure instead of appropriating channels from competitors.

Method for improving co-channel interference in a cellular system

Abstract: A fractional loading scheme is used to improve the spectral efficiency of a cellular system, and therefore increase the number of users that the system can support. The fractional loading scheme allows only a fraction of the total number of available communication channels within each cell to be used simultaneously. Thus, each cell is deliberately underloaded to operate at less than its full capacity. The underloading of the individual cells reduces the spectral efficiency within each cell. However, the underloading of each cell means that there will be fewer interfering users at any given time so that the co-channel interference is reduced. This reduction in co-channel interference allows the reuse distance between co-channel cells to be reduced thereby increasing the reuse of frequencies throughout the system resulting in an increase in spectral efficiency in the system as a whole.

Performance of multi-carrier CDMA systems with channel estimation in two dimensions

Abstract: Mobile radio systems with high power and bandwidth efficiency can be designed by the combination of code division multiple access (CDMA) and multi-carrier modulation. The problem of channel estimation in the downlink of a multi-carrier (MC)-CDMA system is investigated. We study pilot-symbol-aided channel estimation in two dimensions using two cascaded one-dimensional FIR filters, dimensioned as Wiener filters for the worst case channel conditions. The performance with real channel estimation is demonstrated for various mobile radio environments and velocities. Furthermore, we examine the accuracy of a simple approximation to evaluate the system performance with real channel estimation, where the analysis of the channel estimation is separated from the evaluation of the remaining mobile radio system.

Fast call setup in a CDMA dispatch system

Abstract: A quality dispatch service on a CDMA based wireless system To achieve spectral efficiency and fast access for follow-up calls, in response to a request by a talkgroup member to have a group call established, the non-requesting (listening-only) members are allowed to establish a low-rate (non-voice) signaling link in the reverse direction (inbound) for the limited purpose of (i) providing forward power control information to the fixed infrastructure; (ii) requesting soft hand-off when required; and (iii) maintaining the correct reverse link power control to allow for fast channel access for a follow-on call In accordance with the illustrative embodiment, the establishment of multiple inbound signaling links in a CDMA dispatch call occurs as a background process while beginning the voice communications immediately

Establishment of multiple low-rate inbound signaling links in CDMA dispatch system

Abstract: A quality dispatch service on a CDMA based wireless system. To achieve spectral efficiency and fast access for follow-up calls, in response to a request by a talkgroup member to have a group call established, the non-requesting (listening-only) members are allowed to establish a low-rate (non-voice) signaling link in the reverse direction (inbound) for the limited purpose of (i) providing forward power control information to the fixed infrastructure; (ii) requesting soft hand-off when required; and (iii) maintaining the correct reverse link power control to allow for fast channel access for a follow-on call. In accordance with the illustrative embodiment, the establishment of low-rate inbound signaling links occurs prior to beginning the voice communications. Because a low-rate signaling link, as contemplated by the present invention, is a non-voice link which signals at a predetermined rate substantially less than that of a full-rate traffic link, proportionately less average power is required for dispatch communications than would otherwise be the case if full-rate traffic links were provided instead to non-talking talkgroup members. The present solution results in less interference to the CDMA system and thus substantially less impact on overall system capacity.

Code-phase-shift keying: a power and bandwidth efficient spread spectrum signaling technique for wireless local area network applications

Abstract: Code-phase-shift keying (CPSK) is a novel direct-sequence spread-spectrum (DS-SS) signaling system employing M different code phase shifts of a single pseudonoise (PN) code sequence for M-ary signaling. CPSK offers increasing thermal noise immunity as M increases, and totally mitigates the effect of carrier frequency tone interference. It maintains good performance in a Rician fading channel, and a RAKE receiver could be used to improve the performance in a Rayleigh fading channel. The improved power and bandwidth efficiency of CPSK makes it suitable for wireless local area network applications.

A spread-spectrum multi-carrier multiple-access system for mobile communications

Abstract: A flexible multiple access scheme based on a novel combination of the spread-spectrum technique and orthogonal multi-carrier modulation is presented in this paper. The scheme is called spread-spectrum multi-carrier multiple access (SS-MC-MA). Its concept is similar to the concept of multicarrier (MC)-CDMA. However, its basic difference is, that with SS-MC-MA the code division is used for simultaneous transmission of the data of a single user on the same sub-carriers, where with MC-CDMA, it is used for the simultaneous transmission of the data of different users on the same sub-carriers. The proposed scheme is investigated with channel coding and real channel estimation. SS-MC-MA achieves a high bandwidth efficiency for the up-link and for the down-link.

Mechanism and method for multiplexing voice and data over a signal carrier with high bandwidth efficiency

Abstract: A micro-controller having multiple operating modes, including an idle mode, an analog voice mode, a digital data mode, and a simultaneous voice and data (SVD) mode, is provided to a DCE designed to support multi-modal voice and/or data calls over a single analog-loop telephone line. The micro-controller further contains control logic for establishing multiple logical connections and voice as well as data transmission protocols over these logical connections with another DCE, when switching from the analog voice mode to the SVD mode, and for multiplexing voice and data transmissions over these logical connections. The control logic transmits voice over a logical voice connection in nominally fixed intervals. A non-voice transmission, i.e. data or information to be exchanged, can be suspended in favor of transmitting a voice transmission. During the suspension, a special character (e.g., escape) is transmitted in conjunction with a length preceding the voice transmission. After the voice transmission is complete, the data transmission is continued. By suspending (rather than ending and then restarting) the non-voice transmission, a large amount of "framing bits" are effectively eliminated and the bandwidth efficiency of the multiplexed transmission is increased. Effectively, voice information can be transmitted embedded within a data (e.g., non real time) transmission.

Wireless intelligent ATM network and protocol design for future personal communication systems

Abstract: 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.

Coded OFDM for broadband indoor wireless systems

Abstract: In this paper we investigate the use of OFDM for broadband indoor wireless links as a means of dealing with the multipath propagation problem. We consider three solutions capable of simplifying the system. First, we consider differential encoding, so that phase estimation is not required. The second solution consists of the use of special coding schemes that are capable of spreading the information bits among the subcarriers, so that the BER performance is not deteriorated by badly attenuated subcarriers. Two coding schemes are compared, a Walsh coding scheme and 4-state trellis-coded modulation. As a third solution, we consider a Nyquist sampling rate OFDM. We show that the Nyquist sampling rate OFDM signal, which is supposed to be dependent on a perfect receiver timing, can be made resistant to a receiver timing mismatch by using the coding schemes and by decreasing the rolloff factor of the Nyquist pulse, with extra benefits of greater bandwidth efficiency and lower added noise.

Fqpsk: a bandwidth and rf power efficient technology for telemetry applications

Abstract: A simple, low cost radio frequency (RF) power and spectrally efficient integrated transceiver/modem architecture employing Feher’s patented Quadrature Phase Shift Keying (FQPSK) is described. The FQPSK signals presented in this paper are obtained by using additional post low-pass filters in the FQPSK architecture. This implementation significantly improves the spectral efficiency of the worldwide commercially standardized Gaussian Minimum Shift Keying (GMSK) systems. The Bit Error Rate (BER) performance of FQPSK in additive white Gaussian noise (AWGN) channel has been investigated by means of computer simulation and hardware prototype measurements. The results of the hardware and software simulations are compared to GMSK and QPSK/OQPSK performance. These results show that the filtered FQPSK modulated signal passing through a non-linear amplifier (NLA) can achieve a spectral efficiency improvement of about 60% over NLA filtered OQPSK and an integrated spectral efficiency improvement of 50% over GMSK and a better BER performance. In particular, 100 kb/s to 34 Mb/s hardware experimental results over 2.4 GHz NLA (saturated) 1 Watt system confirmed that FQPSK hardware systems attain a BER=f(Eb/N0) performance within 1 dB to 2 dB of predicted theoretical results.

Wavelets for waveform coding of digital symbols

Abstract: A wavelet-based coder-decoder (codec) structure is defined for baseband waveform coding. Numerical results for bandwidth efficiency are given, and a comparison between several different wavelets is presented. Moreover, it is shown that wavelets obey the Nyquist pulse shaping condition and provide a unified framework for analog pulse shaping concepts of communications.

Optimum sectorization for CDMA 1900 base stations

Abstract: Since minimizing the co-channel interference in the conventional cellular technologies (AMPS, TDMA, and GSM) leads to better spectral efficiency, these technologies require a systematic frequency planning scheme. Generally, cells are grouped in a cluster of 7 (N=7). Each cluster reuses the same frequency to maximize bandwidth efficiency. To further minimize the co-channel interference from adjacent clusters, sector antennas are used. A cell is typically sectorized into three sectors (K=3) to achieve rapid frequency reuse. The deployment practices include using the N=7, K=3 or using the N=4, K=6 frequency reuse scheme. Because of the flexibility of the CDMA technology, restrictions of using the conventional clusterization and/or sectorization schemes may not be warranted. In fact, to maximize the utilization of existing resources such as cell-site equipment, structures, real-estate, backhauls, etc., employing a non-standard sectorization scheme will yield a more optimum configuration and a more cost-effective solution. We address such non-standard sectorizations.

Coding, Modulation and Multiuser Decoding for DS-CDMA Systems

Abstract: The general trend both in wireless and fixed network communication systems is flexibility in services, data rates, desired delay and performance. Future systems should not be designed for specific services but be more general and allow for a multitude of services, both now existing and those not yet invented. Considered in this thesis is the design of a flexible direct-sequence code-division (DS-CDMA) system, supporting many services, and using advanced receiver structures and coding techniques. The first part focus on modulation methods to achieve multiple data rates in a DS-CDMA system. Considered are the multi-processing-gain, the multi-modulation, and the multicode schemes. It is shown that the multicode and multi-processing-gain schemes have about the same performance, while the multi-modulation scheme performs significantly worse for high-rate users. Disadvantages are: for the multi-processing-gain the low suppression of external interference for the high-rate users; and for the multicode scheme the high variations in the envelope for high-rate users which results in decreased spectral efficiency, increased bit error rate, and less power efficient amplifiers. The envelope variations can, however, be significantly reduced by the introduction of a high-rate non-linear block code --- a precoder. To further increase the the number of possible source data rates, a rate matching scheme using a family of rate-compatible convolutional codes is presented. The codes are variable rate codes all decodeable by a single decoder, and almost as good as the best known fixed rate codes. Other possible applications for the codes are unequal error protection and hybrid ARQ. Multiuser detection is a vital part in a DS-CDMA system to reduce the multiple-access interference and the near-far effect. Extensive simulations, however, show that errors in the estimates of channel parameters such as propagation delays and carrier phases drastically reduce the near-far resistance. Also treated are joint receiver designs. The derived receivers are jointly optimized source, channel, and multiuser decoders, where it is assumed that the source encoders are vector quantizers. Performance evaluations show significant performance gains with respect to separately optimized receivers, using the optimum multiuser detector in tandem with the optimum channel and source decoders. Furthermore the presented decoders are soft-in, soft-out decoders that directly map the matched filter outputs onto the source space. The mapping is minimum mean-squared error (MMSE) optimum, thus not containing any decision devices or other information destroying functions.

Extensions of turbo trellis coded modulation to high bandwidth efficiencies

Abstract: We extend turbo trellis coded modulation (TTCM) with punctured component codes to allow higher bandwidth efficiency. Two techniques, that can also be combined, were investigated that achieve this: the inclusion of uncoded bits in TTCM in conjunction with high order modulation (e.g. 64-QAM), and the replacement of two dimensional component codes (e.g. Ungerboeck codes) with multi-dimensional codes. In the first approach, the inter-set distance of the partitioning tree can be used to fix the number of coded and uncoded bits, which will strongly determine the complexity of the decoder. Simulation results are presented for 8-PSK with 2.5 bits per symbol and 64-QAM with 5 bits per symbol; they indicate that the scheme can achieve good bit error rates close to Shannon's limit.

Capacity increase in cellular PCS by smart antennas

Abstract: This paper considers the increase in spectral efficiency of a TDMA PCS system employing smart antennas at the base station. Two different strategies are analyzed: spatial filtering for interference reduction (SFIR) and space division multiple access (SDMA). Our results indicate that a cluster size of N/sub cl/=1 is possible by utilizing smart antennas. The use of SDMA increases the spectral efficiency over SFIR by 80%. Additionally, SDMA adds flexibility to the whole network.

Multilevel coding for peak power limited complex Gaussian channels

Abstract: For transmission over the peak power limited complex Gaussian channel with various rates, the semi-continuous transmitter probability density function (pdf) maximizing mutual information is approximated by discrete equiprobable signal points. Since there is no need for shaping, in most cases only one bit of redundancy per two dimensions is sufficient to come very close to capacity. These results are in contrast to the average power limited channel requiring two bits of redundancy for coding and shaping. Combinations of properly designed multilevel coding (MLC) schemes with these discrete signal sets show a trade-off between bandwidth efficiency and peak power similar to the case of average power limitation.

Performance analysis of a wideband CDMA system for FPLMTS

Abstract: In this paper, the physical layer of a 3.6864 Mcps wideband CDMA system which has been proposed by ETRI for FPLMTS (future public land mobile telecommunication systems) is described. It is designed to be adequate upto a 5 MHz bandwidth in order to make it easy that frequencies are allocated to carriers by multiples of 5 MHz bands and make the specification of pulse shaping filter more loose. 8 kbps CS-ACELP (conjugate structure algebraic CELP) is adopted as a main vocoder algorithm and 32 kbps ADPCM can be used. In the reverse link, the continuous pilot scheme is introduced to cope with discontinuous data transmission and to have a symmetrical H/W component to the forward link. In order to maintain the service quality with heavy signaling data, signaling activity with a dedicated signaling channel is introduced for spectrum efficiency. The user information data of upto 128 kbps can be transmitted by using QPSK data/QPSK spreading, variable spreading factor, and code pair assignment. Based on the 3.6864 Mcps system, the multiband 0.9216/3.6864/14.7456 Mcps system for multilayered cell environments is under consideration for FPLMTS.

Multi-user direction of arrival and channel estimation for time-slotted CDMA with joint detection

Abstract: In cellular mobile radio systems, the directional inhomogeneity of the radio channel can be exploited to increase the spectral efficiency via the implementation of smart antennas. In this paper, we investigate the optimum and a simplified smart antenna concept for a time-slotted CDMA mobile radio system termed joint detection CDMA (JD-CDMA). These new combined direction of arrival and channel estimation algorithms and the conventional detection scheme are evaluated via Monte Carlo simulations. They reveal that significant performance improvements can be achieved by exploiting relationships between the estimated directions of arrival and the channel impulse response vectors. Compared to the optimum solution, the new suboptimum approach offers a considerable reduction of the signal processing complexity without severe degradations of the system's performance.

Cellular evolution into wideband services

Abstract: An evolution of cellular is presented-an evolutionary time-division multiple access scheme, that combines GSM compatibility with high performance and wideband service capabilities. Several evolutionary steps are considered, including linear, higher level modulation. It enables higher bit rate services to be mixed with low bit rates in a combined scheme. The performance of the evolutionary scheme in terms of spectral efficiency is improved through the new modulation and diversity techniques. A high flexibility is achieved in the implementation and deployment of the scheme, since services ranging from low to high bit rates can co-exist on the same carrier. This evolutionary path allows for successive introduction of new services and new features into an existing GSM system.

A CDMA data overlay of the GSM network

Abstract: The feasibility of providing a CDMA overlay of the GSM network is considered. The services to be provided by the proposed network are complimentary to those offered on the current GSM system. It is shown that without some form of mutual interference suppression the overlay is not viable, since neither systems has adequate carrier to interference ratio (CIR). However, relatively simple notch filters in the CDMA transmitter and receiver can increase the overall spectral efficiency by a factor of 3.32 for a 1 dB deterioration in baseline GSM CIR.

Performance analysis of link adaptation in wireless personal communication systems

Abstract: We analyze the link adaptation (LA) performance of a wireless personal communication system supporting two types of voice connections, full-rate and half-rate. Full-rate connections are used when the channel conditions are poor. When the channel conditions are good a full-rate connection is switched to a half-rate for improving spectral efficiency. For analysis simplicity, we consider flat uniform propagation conditions where, for a given propagation exponent, the received powers are determined by distances. Using this propagation model, users in the outer region of the cell will use full-rate channels while users in the proximity of the base station will use half-rate channels. The numerical results are extracted by forming a multi-dimensional Markov chain model of the proposed scheme and are compared with those obtained through a comprehensive computer simulation for call blocking probability, hand-off failure probability and carried traffic. A 42% increase in capacity is achieved when 75% of the users can use halfrate channels.

Multi-code CDMA with variable sequence-sets

Abstract: To increase the spectrum efficiency in mobile communications, multi-code code-division multiple-access (MC-CDMA) systems have been proposed. An adaptive MC-CDMA scheme is discussed which allows one to support variable source data-rates by variation of the set of code-sequences assigned to each user. Especially in multimedia scenarios with different data-rates and different requirements on the transmission performance, this technique seems to be superior to known techniques even when fading channels are considered.

A CDMA based bidirectional communication system for hybrid fiber-coax CATV networks

Abstract: Hybrid fiber-coax CATV networks are considered a promising infrastructure for the implementation of future interactive services. Efficient utilization of the available CATV spectrum requires a communication system which is dedicated to the specific properties of these networks. This paper presents the physical layer of a bidirectional communication system for HFC CATV networks which is based on CDMA, a technique that has a certain robustness to the ingress noise found in the CATV bandwidth. Because the application of asynchronous CDMA results in a poor spectral efficiency, a transmission scheme based on synchronous CDMA is adopted. Using synchronous CDMA and QPSK modulation, a capacity of 64 channels of 64 kbit/s each can be achieved in a bandwidth of 6 MHz. It is found that small synchronization errors can be tolerated without significant performance degradation. Next, a description is given of the cable modems responsible for the communication between subscribers and a CATV headend. The design exploits the typical CATV network configuration to enable a cost-effective hardware realization. Sensitivity of system performance to linear distortion is investigated by computer simulation. Especially linearly varying group delay and uneven amplitude response are shown to be potential causes of performance degradation.