Showing papers on "Spectral efficiency published in 1996"

Variable-rate variable-power MQAM for fading channels

Abstract: We propose a variable-rate and variable-power MQAM modulation scheme for high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 5-10-dB power gain relative to variable-power fixed-rate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.

CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers

Abstract: This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E/sub b//N/sub 0/ performance similar to coherent demodulation (e.g., E/sub b//N/sub 0/=4.3 dB at bit-error rate (BER)=2/spl middot/10/sup -4/ on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers.

Adaptive digital predistorter for power amplifiers with real time modeling of memoryless complex gains

Abstract: When the signals resulting from linear modulation methods, like M-ary QAM, are passed through a nonlinear power amplifier, their fluctuating envelope causes distortion and spectral spreading. In order to avoid these effects, while maintaining both power and spectral efficiency, the use of linearization techniques is necessary. This paper presents a digital predistorter with real time modeling of AM-AM and AM-PM characteristics of a power amplifier (PA). The input and output lowpass equivalent complex envelopes of the amplifier are sampled, scaled and updated into a lookup table to provide the predistorted signal. An improvement of 45 dB of out-of-band power is obtained when simulating with Signal Processing WorkSystem (SPW). The proposed technique is robust and efficient since no iterative procedure is needed, hence the convergence time is eliminated.

Variable length burst transmission over the physical layer of a multilayer transmission format

Abstract: Apparatus is provided for communicating data packets in variable length bursts over a physical layer in a multilayer data communication scheme. Each burst contains information data (40, 50, 60, 64, 74, 78, 82) and overhead (30, 32, 34, 36, 38, 42, 44, 62, 66, 76, 80, 84). The overhead includes forward error control (FEC) data (42, 62, 66, 76, 80, 84). Different burst modes (Figs. 2, 3, 4, 5) are provided to enable a trade-off to be made between bandwidth efficiency and data transmission robustness. The burst modes provide different combinations of modulation (such as QPSK and 16-QAM), symbol rates, FEC coding levels and frame and preamble structure. The apparatus is particularly suitable for use in upstream communications over hybrid fiber coax cable television plants.

A reconfigurable modem for increased network capacity and video, voice, and data transmission over GSM PCS

Abstract: A reconfigurable global mobile standard (GSM) compatible radio modem interface which doubles the number of simultaneous video and voice transmissions per channel is presented for personal communication systems (PCSs). The result is that an average video frame rate of up 14 frames per second (fps) can be supported within a single TDMA time slot, doubling the number of conventional GMSK multimedia transmissions per GSM channel. The design employs an in-circuit reconfigurable (ICR) cross-correlated Feher's (see Englewood Cliffs, NJ: Prentice-Hall, 1995). quadrature phase shift keyed (FQPSK) signal processing technique to support a data rate of 357.5 kb/s in a 200 kHz bandlimited GSM channel. This innovative new network modem is reconfigured in-circuit for high bit rate data transmission or voice operation that is compatible with existing GSM equipment. Spectrum efficiency /spl eta//sub f/ (b/s/Hz) is investigated in a nonlinear amplified (NLA) environment-providing a 6 to 9 dB advantage in power efficiency for increased battery life of hand-held terminals. Results show that RF power efficient nonlinear amplified spectrum efficiency is increased from 1.35 b/s/Hz to 1.85 b/s/Hz. Bit error rate (BER) performance is evaluated in a Gaussian and Rayleigh fading channel, and the merits of coherent demodulation in microcellular PCS are examined. Our results show that the GSM compatible configuration of a specific cross correlated FQPSK-KF implementation offers up to 2 dB improvement in E/sub b//N/sub 0/ over conventional GMSK BT/sub b/=0.3. The PCS network capacity /spl eta//sub T/ (Erlangs/Hz m/sup 2/) may be increased 37% over GMSK BT/sub b/=0.3.

Adaptive digital access protocol: a MAC protocol for multiservice broadband access networks

Abstract: The authors describe a protocol that can adapt to the changing demands of a mix of synchronous transfer mode (STM) and asynchronous transfer mode (ATM) applications and efficiently allocate bandwidth to a variety of bursty traffic sources. In the case of a hybrid fiber-coaxial (HFC) network, the protocol resides in customer premises equipment (CPE) and a common head-end/central-office (HE/CO) controller. A medium-access control (MAC) processor provides for dividing the time domain for a given digital bitstream into successive frames, each with multiple STM and ATM time slots. Within the STM region of a frame, variable-length time slots are allocated to calls (e.g., telephony, video telephony) requiring different amounts of bandwidth. In the upstream channels, a contention access signaling time slot is also provided in the STM region for call control and setup requests. Within the ATM region, fixed-length time slots accommodate one individual ATM cell. These ATM time slots may be reserved for a user for either the duration of a call or a burst of successive ATM cells, or shared via a contention process. At least one contention time slot is available for signaling messages related to ATM call control and setup requests. The MAC-layer protocol, its relation to circuit- and ATM-amenable applications, and its performance with respect to throughput, latency, and bandwidth efficiency for several service scenarios are examined.

Spectral efficiency and optimal base placement for indoor wireless networks

Abstract: In this paper, we address the problem of optimizing the spectral efficiency of cellular indoor wireless networks by adjusting the location and power of the base-stations. Focusing on the downlink, we derive general network access criteria for mobiles on the indoor floor for systems that employ omnidirectional antennas and adaptive antennas arrays at the base-stations, in order to show and explain the advantages of the use of spatial diversity. Multiple access capability measures that depend only on energy are defined for both schemes. They are then used as the cost function for the solution to the optimal base-station placement problem, for a single-frequency system. Both continuous and combinatorial approaches have been applied to the solution of the optimization problem, and near-optimal solutions have been obtained. We show that the use of adaptive arrays yields greater capacity when increased cell-area overlap is allowed. The optimization methods, channel prediction methods, and a graphic user interface are parts of an integrated software environment that we developed in support of our investigation and which is described.

A flexible modem structure for increased network capacity and multimedia transmission in GSM PCS

Abstract: A reconfigurable Global Mobile Standard (GSM) compatible radio modem interface which doubles the number of simultaneous video and voice transmissions per channel is presented for personal communication systems (PCS). The result is that an average video frame rate of up 14 fps can be supported within a single TDMA time slot, doubling the number of conventional GMSK multimedia transmissions per GSM channel. The design employs an in-circuit reconfigurable cross-correlated FQPSK signal processing technique. This innovative network modem is reconfigured in-circuit for high bit rate data transmission or GSM compatible operation. Spectrum efficiency /spl nu//sub f/ (b/s/Hz) is investigated in a nonlinear amplified (NLA) environment-providing a 6 to 9 dB advantage in power efficiency. Our results show that the GSM compatible configuration of a specific cross correlated FQPSK-KF implementation offers up to 5.1 dB improvement in E/sub b//N/sub o/, power efficiency, and network capacity /spl nu//sub T/ (erlangs/Hz m/sup 2/) may be increased 76% over GMSK BT/sub b/=0.3.

Dynamic reservation multiple access (DRMA): a new multiple access scheme for personal communication systems (PCS)

Abstract: To improve the spectrum efficiency of integrated voice and data services in Personal Communication System (PCS), several reservation-type multiple access schemes, such as Packet Reservation Multiple Access (PRMA), Dynamic Time Division Multiple Access (D-TDMA), Resource Auction Multiple Access (RAMA), etc., have been proposed. PRMA uses the data packet itself to make a channel reservation, and is inefficient in that each unsuccessful reservation wastes one slot. However, it does not have a fixed reservation overhead and offers shorter access delay. On the other hand, fixed reservation overhead is unavoidable in both RAMA and D-TDMA. Compared to D-TDMA and PRMA, RAMA is superior in the sense that its slot assignment is independent of the traffic load. But its implementation is difficult. With these observations, a new reservation protocol, called Dynamic Reservation Multiple Access (DRMA), is proposed in this paper. With this new protocol, the success probability of channel access is greatly improved at the expense of slightly increased system complexity. It solves the problem of inefficiency in PRMA, but without introducing the fixed reservation overhead as in D-TDMA and RAMA. In addition, it is more suited to the dynamic behavior of the integrated traffic because there is no fixed boundary between voice and data slots (which is mandatory in D-TDMA and RAMA). Our numerical results indicate that its performance is superior to the existing reservation protocols, especially in the integrated traffic scenario. Moreover, the soft capacity feature is exhibited when the traffic load increases.

Bandwidth renegotiation for VBR video over ATM networks

Abstract: A scheme for delivery or variable bit-rate (VBR) video over asynchronous transfer mode (ATM) networks where bandwidth can be renegotiated during the duration of a call between the video source and the network is considered. Renegotiation can be initiated by either the video source or the network. The video bandwidth requirement is characterized by a usage parameter control (UPC) consisting, in general, of peak rate, burst length, and sustained rate. A baseline design is outlined where rate-control adjusts the source's rate while a new UPC is requested from the network. When granted, the new UPC allows the source to maintain its target quantization and delay requirements. Rate control epochs may be extended when the network blocks UPC requests or sets a lower UPC value to temporally deal with congestion. Simulation results are presented for VBR MPEG video. The results show that with a moderate renegotiation rate the scheme tracks the bandwidth requirements of the source. As a result, the video quality and bandwidth efficiency can be maintained.

Advances in satellite CDMA transmission for mobile and personal communications

Abstract: An ubiquitous network for multimedia personal communications (Personal Communications Network (PCN)) with small, individual low-cost terminals is one of the most ambitious worldwide projects for the 21st century that are being pursued nowadays. In the development of such PCN, Geostationary, Medium, and Low Earth Orbiting (GEO, MEO, LEO) satellite constellations will play a fundamental role to provide worldwide coverage for most services required by the end-user. The characteristics of efficiency and flexibility inherently required by that scenario suggest, amidst other possibilities, to take into special consideration a radio interface based on code division multiple access (CDMA) to ensure, in addition to the features mentioned above, a sufficient grade of power and spectral efficiency of the relevant satellite radio link. The aim of this paper is a review of the current status of those issues in the field of satellite CDMA transmission systems design that, in our opinion, appear fundamental to the successful operation of an efficient PCN. In particular, we survey the techniques for multiplexing, coding and transmission of direct-sequence spread spectrum (DS/SS) signals, and we touch upon the techniques for the minimization of the self-noise effect, and the related topics of power-control and multiuser detection. We also shortly address in this respect some technological aspects related to an efficient modem design via digital signal processing techniques. The final part of the paper deals more specifically with some typical issues of satellite transmission, namely the minimization of the detrimental effects of the nonlinear satellite transponder and of multipath propagation; the applicability of diversity reception to a multisatellite network is also addressed as possible means of performance boost.

Effect of carrier phase noise and frequency offset on the performance of multicarrier CDMA systems

Abstract: Kaiser (1995) showed that OFDM-CDMA outperforms DS-CDMA in radio fading channels in terms of spectral efficiency, under the assumption of ideal carrier recovery and chip synchronization. However, residual carrier frequency offset and phase noise degrade the system performance significantly if suitable countermeasures are not taken. In this work we consider the downlink of a cellular multicarrier CDMA system. In particular, we evaluate the impact of the carrier frequency offset and Wiener phase noise on the system performance considering typical indoor radio channels. Moreover, a frequency detector is used to reduce the impairment produced by the carrier frequency offset. Results are given in terms of BER obtained by means of simulations and analytical computations.

Iterative interference cancellation for FFH/MFSK MA systems

Abstract: Iterative interference cancellation is introduced to improve system spectral efficiency in fast frequency hopping multiple access systems. The proposed algorithm iteratively cancels interference from users whose symbols have already been estimated. Both an interference-only channel and a frequency selective fading channel are considered. Noncoherently detected M-ary frequency shift keying is employed and all users are time-aligned at the receiver. Analytical results and results obtained from computer simulations are presented. It is shown that iterative interference cancellation may either substantially reduce the bit error probability or allow for a large increase in the number of simultaneously transmitting users.

A broadband multiple access protocol for STM, ATM, and variable length data services on hybrid fiber-coax networks

Abstract: This paper presents a broadband multiple access protocol for bidirectional hybrid fiber-coax (HFC) networks. The Adaptive Digital Access Protocol (ADAPt+) supports a full range of subscriber services via HFC networks with tree and branch topologies. The protocol efficiently accommodates different circuit- and packet-based access modes, such as synchronous transfer mode (STM), asynchronous transfer mode (ATM), and inherently variable length (VL) native data (for example, the Internet protocol [IP]). ADAPt+ allocates the available bandwidth efficiently and adapts to the changing traffic mix. In addition, the paper describes the medium access control (MAC) protocol for upstream and multiplexing/demultiplexing for downstream communication, its applicability to STM, ATM, and other native data applications, and its performance with respect to throughput, latency, and bandwidth efficiency. While discussed in the context of an HFC network, many aspects of ADAPt+ have relevance to wireless, fiber to the curb (FTTC), and fiber to the home (FTTH).

Capture division packet access: a new cellular access architecture for future PCNs

Abstract: The authors describe a new cellular access architecture, known as capture-division packet access, which is a packet-oriented architecture able to support the constant bit rate traffic and variable bandwidth on demand necessary for multimedia traffic. The approach integrates the multiple access and channel reuse issues to achieve a high degree of spectral efficiency, and presents general advantages even if used for delay-constrained circuit-oriented traffic. Unlike CDMA and TDMA, wherein the effective data rate of each connection is typically a small fraction of the total radio channel allocated for PCN, the CDPA approach allows each user to access the entire channel, if necessary, for brief periods of time (packet access). Spectrum sharing is accomplished by exploiting the different path losses suffered by the various signals as they appear at the base stations (the capture effect), with co-channel interference abated through time diversity (colliding users do not successively retry in the same time interval). Results suggest that abating co-channel interference by random retransmission may be more effective than spatial isolation at cells using the same channel, as is usual in FDMA-TDMA systems.

Known and novel diversity approaches as a powerful means to enhance the performance of cellular mobile radio systems

Abstract: The main requirements to be met by third generation mobile radio systems are high cellular spectrum efficiency and high flexibility. The authors focus on high cellular spectrum efficiency, which is difficult to achieve due to the time variance and frequency selectivity of the mobile radio channel and due to interference. It is known that the degrading effects of these adverse characteristics of the mobile radio channel and of interference can be mitigated by diversity. The way how diversity influences cellular spectrum efficiency is derived in general. As a reference point, the types of diversity used in GSM are analyzed. In GSM, the potential for diversity enhancement inherent in code-division multiple-access (CDMA) is not exploited. A joint detection code-division multiple-access (JD-CDMA) system concept aimed at third generation mobile radio systems has been proposed which introduces a CDMA feature into systems based on time-division multiple-access (TDMA) and frequency-division multiple-access (FDMA) like GSM and also advanced TDMA (ATDMA). The gains achievable by different types of diversity in GSM as well as in the JD-CDMA system concept are investigated. It is shown that considerable gains can be achieved by different types of antenna diversity and by exploiting the additional diversity potential of CDMA. Therefore, third generation standards should be flexible in order to allow the use of as many types of diversity as possible to enhance the cellular spectrum efficiency.

A critical review of CDMA

Abstract: The pros and cons of CDMA as a multiple access scheme for third generation cellular mobile radio systems are considered. The main criteria are spectral efficiency and capacity, but also flexibility and costs. In these radio systems the multiple access scheme should be chosen adaptively depending on location, momentary system load and desired services. CDMA offers the advantages of frequency diversity and interferer diversity, which enhance spectral efficiency and capacity. CDMA with single user detection is very flexible and supports variable data rates and services. However, this advantage is cut back by the increased interference. In the case of less flexible interference reducing CDMA schemes, intracell interference is eliminated which increases the spectral efficiency and capacity.

Measurement-based parameter adaptation of wideband spatial mobile radio channel models

Abstract: A prerequisite for the performance analysis and optimization of future mobile radio systems applying directional diversity or adaptive array techniques is the availability of realistic channel models which explicitly take into account the spatially inhomogeneous distribution of partial waves. Unfortunately, recent proposals for such spatial channel models based on a statistical approach have not yet been verified to be realistic. Moreover, the behaviour of said models is determined by more or less arbitrarily choosing the model parameters. Wideband measurements of channel impulse responses which include unambiguous information about the directions-of-arrival in the azimuth are used to adapt parameters of a statistical spatial channel model. Results for the spectral efficiency of the uplink in a joint detection CDMA mobile radio system using directional diversity are presented which were derived by simulations based on such a parameterized spatial channel model.

QPSK/QBL-MSK waveform enhancement

Abstract: A modulator device is disclosed for a spread spectrum communication system which generates a modulated signal from incoming data. The device includes a QPSK portion for separating the incoming data into two data channels and for further spreading the two channels over a wide bandwidth according to a predetermined spreading code. A phase mapping element coupled to the QPSK portion for processing both channels in order to minimize spectral regrowth of the modulated signal by preventing the RF envelope deviation from going close to zero. Pulse shaping elements coupled to the phase mapping element for further processing each channel in order to improve the modulator spectral efficiency and minimize adjacent channel interference. A quad-mixer coupled to the pulse shaping elements for combining both channels into a recombined signal and for further modulating the recombined signal according to a predetermined carrier frequency.

Spectrum efficiency limits in mobile cellular systems

Abstract: In cellular mobile radio systems using coded modulations, the spectrum efficiency of the system is related to the cutoff rate and the cluster size. We evaluate the cutoff rate for M-ary QAM considering a coherent detection and a memoryless flat fading channel. Simulation results relate the cluster size to the signal-to-noise ratio and to the cutoff rate. We present the spectrum efficiency of the system as a function of the cutoff rate and show its limits in the classical cellular system. We apply the same procedure to examine the performance of systems using directional antennas with and without sectorization of the original cell surface. With our proposed representation, we can always determine the optimum spectrum efficiency of the modulation that maximizes the spectrum efficiencies of the different systems.

Channel symbol expansion diversity-improved coded modulation for the Rayleigh fading channel

Abstract: The uncorrelated Rayleigh channel can be regarded a worst case channel for digital radio communication. Crucial for the detection performance on this channel is the diversity order of the communication system, which shall be as large as possible. For trellis coded modulation (TCM) with symbol-interleaving the diversity order is the smallest number of distinct channel symbols along any error event. The diversity order can be further increased by bit-wise interleaving of the encoder output before mapping the bits onto channel symbols and is now the smallest number of distinct bits along any error event. The concept of bit-interleaving is generalized such that a convolutional code of lower rate (larger Hamming distance) can be used giving an even higher diversity order. The same spectral efficiency is maintained by an expansion of the channel symbol constellation. This concept is referred to as channel symbol expansion diversity (CSED). Simulation results indicate that this approach leads to coded modulation schemes with significantly better performance on the Rayleigh fading channel than the previously best known symbol- or bit-interleaved TCM-systems of comparable complexity. An interesting special case, referred to as repetition coded CSED, arises when the convolutional encoder is replaced by a repetition encoder. Theoretical analyses as well as simulation results indicate that this low-complexity system gives a large performance gain on the Rayleigh channel compared to uncoded signaling.

A multicarrier transmission scheme for wireless local communications

Abstract: This paper presents a novel multitone transmission scheme that employs a nonlinear binary code to specify multitone signal constellations. The scheme, which can be viewed as a generalization of M-ary frequency shift keying, is amenable to noncoherent demodulation, and it is therefore useful for communication over fading channels. We determine the performance of this modulation scheme when it is used in conjunction with a Reed-Solomon code for error control. Methods for determining and erasing unreliable multitone constellations are presented and their performance is evaluated when they are used in conjunction with errors-and-erasures decoding. The transmission scheme is evaluated with respect to complexity, bandwidth efficiency, and performance in the presence of Gaussian noise and frequency- and time-selective fading.

Potential impact of rate-splitting multiple access on cellular communications

Abstract: The progress in the field of multiple access techniques keeps alive the hope that multiple access systems approaching the ultimate limit in terms of bandwidth efficiency are possible. The bandwidth efficiency of a spread spectrum multiple access (SSMA) system and that of an ideal multiple access technique are compared under various scenarios pertinent to cellular communications. The scenarios are: (1) single cell and power control for both schemes; (2) single cell and no power control for the ideal system; (3) multi-cell and no power control. It is shown that in all three cases there is a considerable gap between the bandwidth efficiency of SSMA and that of an ideal system. This leads one to expect that rate-splitting multiple access (RSMA)-a new technique whose decoder is based on interference cancellation (IC)-has the potential for a significant impact even if perfect IC is not possible. It is also found that IC is very valuable even in a multicell environment, provided that there is no power control.

A parallel combinatory OFDM system

Abstract: A parallel combinatory OFDM system (PC-OFDM) is proposed. The proposed system is able to obtain a higher bandwidth efficiency than an ordinary OFDM system. The bit error rate performance is analyzed and simulated. On a Gaussian channel and on known time invariant channels the PC-OFDM system can obtain lower bit error rates than ordinary OFDM. In some cases we can at the same time increase the bandwidth efficiency compared to an ordinary OFDM system. Further, the peak to mean power ratio of the PC-OFDM system is lower than for OFDM, which reduces the problem of designing linear power amplifiers.

Wavelet-based multi-carrier CDMA for personal communications systems

Abstract: Based on the properties of wavelet orthogonal basis, three novel wavelet-based multi-carrier CDMA (MC-CDMA) systems are proposed, and their advantages and performances are investigated by the comparison with the conventional MC-CDMA systems. For the additive white Gaussian noise (AWGN) channel, all three wavelet-based MC-CDMA systems possess the same bit error rates as the corresponding MC/BPSK-CDMA and MC/QPSK-CDMA systems, but have significant higher bandwidth efficiency than the conventional MC-CDMA systems by the suitable choice of wavelet functions. Their potential applications in the broadband wireless communications to improve bandwidth efficiency and combat the fading and narrowband interference are also discussed.

Multi-tone combinatory frequency hopping system

Abstract: A novel frequency hopping spread spectrum system is proposed in which combination tones are transmitted for each hopping step to improve spectral efficiency. Differing from conventional systems a wide variety of modulation processes are available in the system. Some of these are presented in this paper and, as an example, a result of analysis of an illustrative system is shown.

A general approach to the evaluation and characterization of packet radio networks performance

Abstract: In this work a general exact methodology for capture effect and performance evaluation of packet radio networks is presented. Both narrow-band and spread-spectrum systems are investigated and compared. Several network performance quality indexes are considered: throughput, outage probability, packet error probability, average delay time. Two particular capture effect models are investigated. It can be seen that one of the two models can be considered as a good approximation of the other if a appropriate parameter definition is made. By using this methodology various operating conditions given by fading (Rice, Rayleigh, Nakagami), shadowing, different traffic models, different modulation, and coding schemes are investigated. Comparison is performed by means of spectral efficiency parity.

The CDMA overlay concept

Abstract: We present an overview of the results which have been obtained on the concept of allowing wideband code division multiple access (CDMA) waveforms to share a common spectrum with conventional, narrowband signals. Such sharing of the frequency spectrum is referred to as a CDMA overlay, and has the potential of enhancing the overall spectral efficiency. The overlay has been demonstrated in both the PCS band and the cellular band. It is also shown that such an overlay could be successfully accomplished either by employing tunable notch filters in the transmitters and receivers of the CDMA units, or by using a multicarrier CDMA waveform.

Hybrid TDMA/CDMA multiple access protocol for multi-media communications

Abstract: A novel hybrid TDMA/CDMA multiple access protocol for multi-media communications is proposed. Its performance in terms of throughput, delay and spectrum efficiency is evaluated using computer simulation. Voice, and variable bit rate data traffic are integrated within one TDMA frame through a movable-boundary structure. Combined with DS-CDMA a new hybrid multiple access protocol is investigated that is capable of integrating different types of traffic and also gives us the capacity and flexibility of the CDMA protocol.

Usage of adaptive arrays to solve resource planning problems

Abstract: The usage of adaptive antenna arrays in cellular systems is currently being investigated by many researchers. In this paper we show by means of simulation that adaptive antenna arrays allow to reduce the reuse distance, and hence to increase spectrum efficiency. Furthermore, it is shown that, by adopting a fractional loading factor, it is possible to adopt a cluster size equal to 1, which avoids the need of frequency planning altogether, and allows a further improvement in spectrum efficiency.