Showing papers on "Spectral efficiency published in 2002"

Spectral efficiency in the wideband regime

Abstract: The tradeoff of spectral efficiency (b/s/Hz) versus energy-per-information bit is the key measure of channel capacity in the wideband power-limited regime. This paper finds the fundamental bandwidth-power tradeoff of a general class of channels in the wideband regime characterized by low, but nonzero, spectral efficiency and energy per bit close to the minimum value required for reliable communication. A new criterion for optimality of signaling in the wideband regime is proposed, which, in contrast to the traditional criterion, is meaningful for finite-bandwidth communication.

Multiple-access multiple-input multiple-output (MIMO) communication system

Abstract: Techniques to achieve better utilization of the available resources and robust performance for the downlink and uplink in a multiple-access MIMO system. Techniques are provided to adaptively process data prior to transmission, based on channel state information, to more closely match the data transmission to the capacity of the channel. Various receiver processing techniques are provided to process a data transmission received via multiple antennas at a receiver unit. Adaptive reuse schemes and power back-off are also provided to operate the cells in the system in a manner to further increase the spectral efficiency of the system (e.g., reduce interference, improve coverage, and attain high throughput). Techniques are provided to efficiently schedule data transmission on the downlink and uplink. The scheduling schemes may be designed to optimize transmissions (e.g., maximize throughput) for single or multiple terminals in a manner to meet various constraints and requirements.

Adaptive modulation and MIMO coding for broadband wireless data networks

Abstract: Link adaptation techniques, where the modulation, coding rate, and/or other signal transmission parameters are dynamically adapted to the changing channel conditions, have emerged as powerful tools for increasing the data rate and spectral efficiency of wireless data-centric networks. While there has been significant progress on understanding the theoretical aspects of time adaptation in LA protocols, new challenges surface when dynamic transmission techniques are employed in broadband wireless networks with multiple signaling dimensions. Those additional dimensions are mainly frequency, especially in multicarrier systems, and space in multiple-antenna systems, particularly multiarray multiple-input multiple-output communication systems. We give an overview of the challenges and promises of link adaptation in future broadband wireless networks. We suggest guidelines to help in the design of robust, complexity/cost-effective algorithms for these future wireless networks.

Performance of multiantenna signaling techniques in the presence of polarization diversity

Abstract: Multiple-input multiple-output (MIMO) antenna systems employ spatial multiplexing to increase spectral efficiency or transmit diversity to improve link reliability. The performance of these signaling strategies is highly dependent on MIMO channel characteristics, which, in turn, depend on antenna height and spacing and richness of scattering. In practice, large antenna spacings are often required to achieve significant multiplexing or diversity gain. The use of dual-polarized antennas (polarization diversity) is a promising cost- and space-effective alternative, where two spatially separated uni-polarized antennas are replaced by a single antenna structure employing orthogonal polarizations. This paper investigates the performance of spatial multiplexing and transmit diversity (Alamouti (see IEEE J. Select. Areas Commun., vol.16, p.1451-58, Oct. 1998) scheme) in MIMO wireless systems employing dual-polarized antennas. In particular, we derive estimates for the uncoded average symbol error rate of spatial multiplexing and transmit diversity and identify channel conditions where the use of polarization diversity yields performance improvements. We show that while improvements in terms of symbol error rate of up to an order of magnitude are possible in the case of spatial multiplexing, the presence of polarization diversity generally incurs a performance loss for transmit diversity techniques. Finally, we provide simulation results to demonstrate that our estimates closely match the actual symbol error rates.

OFDM code-division multiplexing in fading channels

Abstract: In this paper, orthogonal frequency-division multiplexing-code-division multiplexing (OFDM-CDM) is presented and investigated as alternative to conventional OFDM for high rate data transmission. An additional multipath diversity gain can be obtained with OFDM-CDM by spreading each data symbol in frequency and time. The optimum reliability information for the Viterbi (1979) decoder is derived for OFDM-CDM systems, and the tradeoff between spreading and channel coding in OFDM systems is presented. By using efficient interference cancellation or joint detection techniques, it can be shown that OFDM-CDM outperforms conventional OFDM with respect to bit error rate (BER) performance and bandwidth efficiency.

Data throughputs using multiple-input multiple-output (MIMO) techniques in a noise-limited cellular environment

Abstract: We present a general framework to quantify the data throughput capabilities of a wireless communication system when it combines: (1) multiple transmit signals; (2) adaptive modulation for each signal; and (3) adaptive array processing at the receiver. We assume a noise-limited environment, corresponding to either an isolated cell or a multicell system whose out-of-cell interference is small compared with the thermal noise. We focus on the user data throughput, in bits per second/Hertz (bps/Hz), and its average over multipath fading, which we call the user spectral efficiency. First, an analysis method is developed to find the probability distribution and mean value of the spectral efficiency over the user positions and shadow fadings, both as a function of user distance from its serving base station and averaged over the cell coverage area. We assume fading conditions and receiver processing that lend themselves to closed-form analysis. The resulting formulas are simple and straightforward to compute, and they provide a number of valuable insights. Next, we run Monte Carlo simulations, both to confirm the analysis and to treat cases less amenable to simple analysis. A key contribution of this paper is a simple formula for the mean spectral efficiency in terms of the propagation exponent, mean signal-to-noise ratio at the cell boundary, number of antennas, and type of coding. Under typical propagation conditions, the mean spectral efficiency using three transmit and three receive antennas ranges from 19.2 bps/Hz (uncoded) to 26.8 bps/Hz (ideally coded), highlighting the potential benefits of multiple transmissions combined with adaptive techniques. This is much higher than the spectral efficiencies for a link using a single transmitter and a threefold receive diversity under the same conditions, where the range is from 8.77 bps/Hz to 11.4 bps/Hz. Moreover, the latter results are not nearly as practical to achieve, as they can for large signal constellations that would be highly vulnerable to impairments.

Environmental issues for MIMO capacity

Abstract: Wireless communication using multiple-input multiple-output (MIMO) systems enables increased spectral efficiency for a given total transmit power. Increased capacity is achieved by introducing additional spatial channels that are exploited using space-time coding. In this paper, the environmental factors that affect MIMO capacity are surveyed. These factors include channel complexity, external interference, and channel estimation error. The maximum spectral efficiency of MIMO systems in which both transmitter and receiver know the channel (using channel estimate feedback) is compared with MIMO systems in which only the receiver knows the channel. Channel complexity is studied using both simple stochastic physical scattering and asymptotic large random matrix models. Both uncooperative (worst-case) and cooperative (amenable to multiuser detection) interference are considered. An analysis for capacity loss associated with channel estimation error at the transmitter is introduced.

Multiple antennas in cellular CDMA systems: transmission, detection, and spectral efficiency

Abstract: Providing wireless high-speed packet data services for Web browsing and streaming multimedia applications will be a key feature in future code-division multiple-access (CDMA) systems. We study down-link CDMA schemes for providing such services using multiple antennas at the transmitter and receiver. We propose a generalization of the point-to-point narrowband Bell Labs layered space-time (BLAST) system to a wideband multiple access system which simultaneously supports multiple users through code spreading. We discuss transmission options for achieving transmit diversity and spatial separation and introduce a generalization of the vertical BLAST detector for CDMA signals. Using link level simulations, we determine the bit-error rates versus signal-to-interference ratio of the various transmitter options. We then describe a novel technique for determining the system spectral efficiency (measured in bits per second per Hertz per cell sector) by incorporating the link level results with system level outage simulations. Using four antennas at the transmitter and eight antennas at each receiver, the system can support multiple receivers at 16 times the voice rate, resulting in a system spectral efficiency an order magnitude higher than a conventional single-antenna voice system.

Cross layer techniques for adaptive video streaming over wireless networks

Abstract: Wireless multimedia delivery is becoming increasingly more important in today's networks. Unlike wired packet switched networks that suffer from congestion related loss and delay, the wireless networks have to deal with a time varying, error prone, physical channel that in many instances is also severely bandwidth constrained. As such, the solutions needed for wireless video streaming applications are fundamentally different from wired streaming. In this paper, we propose a set of end to end application layer techniques for adaptive video streaming over wireless networks. The adaptation is done both with respect to channel and data. Our approach combines the flexibility and programmability of application layer adaptations, with low delay and bandwidth efficiency of link layer techniques. Socket level simulations are presented to verify the effectiveness of our approach.

Optical orthogonal frequency division multiplexing using frequency/time domain filtering for high spectral efficiency up to 1 bit/s/Hz

Abstract: Summary form only given. We have proposed a novel optical orthogonal frequency division multiplexing technique that can overcome the spectral efficiency limitation of the conventional WDM system. This scheme permits substantial overlapping of the spectrum and can achieve the spectral efficiency up to 1 bit/s/Hz in principle. For demultiplexing, we used a newly developed optical discrete Fourier transformer (DFT) instead of electrical digital processing, which is impossible to apply in the optical frequency range. The optical DFT was realized by using a set of delay lines, a phase shifter and a coupler in the frequency domain and bit synchronization and an optical gate in the time domain. In experimental demonstration of this scheme, error-free operation was obtained with a 0.8 bit/s/Hz of spectral efficiency.

Alternative modulation formats in N/spl times/40 Gb/s WDM standard fiber RZ-transmission systems

Abstract: A comparison of carrier-suppressed return-to-zero (CSRZ) and single sideband return-to-zero (SSB-RZ) formats is made in an attempt to find the optimum modulation format for high bit rate optical transmission systems Our results show that CSRZ is superior to return-to-zero (RZ) and SSB-RZ with respect to signal degradation due to Kerr nonlinearities and chromatic dispersion in wavelength division multiplexing (WDM) as well as in single-channel 40-Gb/s systems over standard single-mode fibers (SSMF) It is shown that CSRZ enables a maximum spectral efficiency of approximately 07 (b/s)/Hz in a N/spl times/40 Gb/s WDM system with equally polarized channels Furthermore, the CSRZ format in N/spl times/40 Gb/s WDM systems shows no further signal degradation compared to single-channel transmission

Carrier interferometry networks

Abstract: Applications of CI processing to ad-hoc and peer-to-peer networking significantly improve throughput, network capacity, range, power efficiency, and spectral efficiency. CI-based subscriber units perform network-control functions to optimize network performance relative to channel conditions, network loads, and subscriber services. CI codes are used to identify and address network transmissions. Channel characteristics of communication links are employed to encode, address, and authenticate network transmissions. CI transceivers used as relays and routers employ unique characteristics of transmission paths to code and decode network transmissions. A central processor is adapted to perform array processing with signals received from, and transmitted by, a plurality of subscriber units in a wireless network.

The coding-spreading tradeoff in CDMA systems

Abstract: General definitions of spreading and coding are given based on the notion of Shannon bandwidth introduced by Massey (1994), with the goal of distinguishing these operations for signaling with bandwidth redundancy. These definitions are shown to lead to a separation result: every bandwidth redundancy scheme can be expressed as a concatenation of coding followed by spreading. The coding-spreading tradeoff problem is then studied for a code division multiple access (CDMA) system in which the receiver processes the received signal by using a user-separating front-end, which feeds into autonomous single-user decoders. Under the single-user decoding setting, it is established that the linear minimum mean square error (LMMSE) front-end multiuser detector is optimum among all front-ends that are constrained to use only spreading information. Also, conditions are given for the single-user decoders to ignore spreading information without losing optimality. An example illustrating the coding-spreading tradeoff optimization for a direct sequence CDMA system with random spreading is given. Single-cell and multicell scenarios are considered in the optimization, and a comparison is made of the spectral efficiencies that can be achieved with the conventional matched filter and LMMSE front-ends.

Equalization methods in OFDM and FMT systems for broadband wireless communications

Abstract: Multicarrier systems are adopted in several standards for their ability to achieve optimal performance in very dispersive channels. In particular, orthogonal-frequency division multiplexing (OFDM) and filtered multitone (FMT) systems are two examples where the modulation filter has an ideal rectangular amplitude characteristic in time and frequency domains, respectively. In this letter, we propose new equalization schemes for FMT and compare their performances with OFDM. In general, FMT has a greater spectral efficiency than OFDM, due to the absence of the cyclic prefix and a reduced number of virtual carriers. However, it exhibits a higher distortion per subchannel, due to the imperfect equalization of the transmit filters. As a performance comparison, we considered both the achievable bit rate (ABR) and the bit error rate (BER) in a multipath Rayleigh fading channel. We note that while ABR gives a theoretical bound on the system bit rate, assuming the knowledge of the channel at the transmit side, the BER refers to an uncoiled system with a fixed modulation. Although FMT requires a fixed structure with a higher computational complexity than OFDM, it turns out that FMT, even with the simplest one tap per subchannel adaptive equalizer, yields a better performance than OFDM, both in terms of ABR and BER. Hence, FMT can be a valid alternative to OFDM for broadband wireless applications, also.

Outdoor BLAST measurement system at 2.44 GHz: calibration and initial results

Abstract: There are ever increasing demands for additional capacity in wireless communications to handle voice, data, and wideband Internet applications. These demands are constrained by the bandwidth that was allocated to wireless communications. The spectral efficiencies in present day wireless systems hover around 1 bit/s/Hz. Bell-labs Layered Space-Time (BLAST) is a communication technique for achieving very high spectral efficiencies in highly scattering environments using multiple transmit and receive antennas. A measurement campaign was undertaken to assess the BLAST gains in spectral efficiency in the suburban outdoor environment for stationary subscribers. The measurements employed directive antennas to better control interference from adjacent cells. The measurements were performed over a narrow band at 2.44 GHz with five transmitting and seven receiving antennas, respectively. Extensive calibration methods, assisted by simulations, were developed to assure accurate results for the BLAST capacities of the measured remote subscriber sites. Initial results indicate that BLAST capacities of C/sub B//spl ges/38 bits/s/Hz at 20% of the measured locations and C/sub B//spl ges/24 bits/s/Hz at 50% of these locations are feasible, for reasonable link parameters and negligible interference.

Method and system for increased bandwidth efficiency in multiple input - multiple output channels

Abstract: In one disclosed embodiment, an input bit stream (110) is supplied to a trellis code block (102) . For example, the trellis code block can perform convolutional coding using a rate 6/7 code. The output of the trellis code block is then modulated using, for example, trellis coded quadrature amplitude modulation (104) with 128 signal points or modulation symbols. The sequence of modulation symbols thus generated can be diversity encoded. The diversity encoding can be either a space time encoding, for example, or a space frequency encoding. The sequence of modulation symbols, or the sequence of diversity encoded modulation symbols, is fed to two or more orthogonal Walsh covers (110, 112, 114, 116) for example, replicas of the modulation symbol sequences can be provided to increase diversity, or multiplexing the modulation symbol sequences can be used to increase data transmission rate or 'throughput'. The outputs of the Walsh covers are fed as separate inputs into a communication channel.

Increasing spectral efficiency by data multiplexing using antenna arrays

Abstract: A novel data multiplexing scheme is presented. This technique uses an antenna array with K transmit antennas. In. total K information bits are multiplexed in an orthogonal fashion. The special property of the multiplexing technique is that, for each symbol duration, only one out of K antennas is transmitting using BPSK modulation. Furthermore, the symbol duration is equivalent to the bit-duration due to the parallelism introduced by the K antennas. The receiver is able to detect the transmitting antenna and by using this information the demultiplexing is carried out. It is demonstrated that the proposed method achieves about the same spectral efficiency as 8PSK, but with a relaxed bit-energy-to-noise ratio of about 2.5 dB. Furthermore, the linear channel capacity gain (error-free transmitted bits) over the single antenna BPSK transmission scheme is found to be in the range of 2.5-3.

A multiplexing scheme for H.323 voice-over-IP applications

Abstract: Voice communications such as telephony are delay sensitive. Existing voice-over-IP (VoIP) applications transmit voice data in packets of very small size to minimize packetization delay, causing very inefficient use of network bandwidth. This paper proposes a multiplexing scheme for improving the bandwidth efficiency of existing VoIP applications. By installing a multiplexer in an H.323 proxy, voice packets from multiple sources are combined into one IP packet for transmission. A demultiplexer at the receiver-end proxy restores the original voice packets before delivering them to the end-user applications. Results show that the multiplexing scheme can increase bandwidth efficiency by as much as 300%. The multiplexing scheme is fully compatible with existing H.323-compliant VoIP applications and can be readily deployed.

Adaptive loading for OFDM/SDMA-based wireless networks

Abstract: The two major obstacles toward high-capacity indoor wireless networks are distortion due to the indoor channel and the limited availability of bandwidth which necessitates a high spectral efficiency. A combined orthogonal-frequency division multiplexing/spatial-diversity multiple access (OFDM/SDMA) approach can effectively tackle both obstacles. The channel distortion due to multipath propagation is easily mitigated with OFDM while the bandwidth efficiency can be increased with the use of SDMA. In order to keep the network's cost acceptable, simplified SDMA processors are preferred over the exponentially complex optimal maximum-likelihood processors. However, these simplified processors perform significantly worse than the optimal ones in terms of average bit error rate (BER). In this paper, we show that by adapting the constellation sizes applied on the individual subcarriers to the channel conditions, the performance of OFDM/SDMA processors can be significantly enhanced. In the uplink, we derive simple closed-form equations for the optimal constellation sizes for both a simple linear minimum mean-square error (MMSE) detector and for a nonlinear MMSE decision feedback equalizer (DFE) detector. Furthermore, for each detector, we introduce a simplified loading algorithm which lowers the computational and signaling complexity substantially at a small performance penalty. In the downlink, we study the dual precoders of the uplink detectors, respectively, the linear MMSE precoder and the nonlinear Tomlinson-Harashima (TH) MMSE-based precoder. For both precoders, we derive expressions for the optimal and the simplified constellation sizes. Additionally, we show that in time-division duplexing systems, the constellation distribution of a set of dual detectors/precoders is identical for up- and downlink, which effectively halves the computational complexity of adaptive loading. In the fully loaded uplink, the proposed adaptive loading algorithm results in a gain of 9 dB for a BER=10/sup -3/ for the linear MMSE detector and a gain of 4.5 dB for the nonlinear MMSE-DFE detector. In the fully loaded downlink, a gain of 6.3 dB is achieved for the MMSE precoder and 5.5 dB for the TH-MMSE precoder.

Method and system for improved spectral efficiency of far field telemetry in a medical device

Abstract: Radio frequency bandwidth from a far field transmitter of an implantable medical device is adjusted by controlling the data rate, the output power level, the transmitter supply voltage or combinations thereof. Adjustments are made on the basis of a sensed temperature, absence or presence of a lead or by way of wave shaping. A Surface Acoustic Wave (SAW) resonator based transmitter exhibits stable bandwidth in a temperature controlled operating environment. For temperatures outside of the range of human body temperature, the output of the SAW-based transmitter is adjusted to provide reduced bandwidth.

Outage probability and spectrum efficiency of cellular mobile radio systems with smart antennas

Abstract: Relying on the distribution of noncentral multivariate F variates, we investigate the outage probability and spectrum efficiency performance of cellular systems with smart antennas. We consider interference-limited systems in which the number of interferers exceeds or is equal to the number of antenna elements, and we present closed-form expressions when the desired signal is subject to Rician-type fading and interfering signals exhibit Rayleigh-, or, more general Nakagami-type fading. When applicable, these new expressions are compared to those previously reported in the literature dealing with the performance of cellular systems without smart antenna capabilities and the performance of cellular systems with optimum combining when both the desired and interfering signals are subject to Rayleigh-type fading. Corresponding numerical results and plots are also provided and discussed.

Multiuser subcarrier and bit allocation along with adaptive cell selection for OFDM transmission

Abstract: A dynamic multiuser subcarrier-and-bit allocation algorithm with low computational complexity for wideband OFDM downlink transmission is proposed to exploit the multiuser diversity in this paper. Our objective is to maximize the overall spectral efficiency in terms of the number of bits per subcarrier while simultaneously satisfying the requirements on each user's data rate, bit error rate (BER), and the total transmit power. Our results show that the proposed dynamic subcarrier-and-bit allocation algorithm offers about 6 dB performance gain over OFDM using adaptive modulation with fixed subcarrier allocation. Since outages may occur when users happen to be located near the boundary of the cell, a novel adaptive cell selection scheme is proposed to further reduce the probability of outage. Our results show that the probability of outage can be reduced by one order of magnitude.

Spectral efficiency of FDMA/TDMA wireless systems with transmit and receive antenna arrays

Abstract: The ever growing need for higher capacity in wireless systems has fueled the interest in exploiting the spatial dimension-through the use of antennas arrays-to improve the utilization of the available radio spectrum. As a result, a large number of space-time techniques have been proposed wherein arrays are used to mitigate interference and enhance signal levels. More recently, information theory has shown that, with spatial data multiplexing, very large spectral efficiencies can be attained in multipath channels using transmit and receive antenna arrays. In this paper, the system benefit of using transmit and receive arrays in multicell scenarios is evaluated as a function of both the propagation environment and the number of antennas. Our results confirm the potential for very large system spectral efficiencies associated with the use of transmit and receive arrays, in particular in interference-limited rich-multipath conditions wherein the ability to perform interference mitigation-leading to tighter spectral reuse-and spatial data multiplexing grows with the number of antennas. In environments free of multipath, the potential is smaller but still very significant, associated with interference mitigation and signal enhancement only, since spatial data multiplexing is no longer possible.

Apparatus and associated method for communicating multimedia information upon communication link

Abstract: Apparatus, and associated method, converts real-time multimedia information generated pursuant to an RTP protocol into a form amenable for transmission upon a radio channel, such as a radio channel defined in a cellular communication system When converted, the informational content of the multimedia information is transmitted in a manner that achieves spectral efficiency and low, constant delay Once transmitted over the radio channel, the multimedia information is converted back into form corresponding to the RTP protocol before being sent to the receiving station

Channel capacity of WDM systems using constant-intensity modulation formats

Abstract: In conclusion, constant-intensity modulation eliminates the effects of both self phase modulation (SPM) and cross phase modulation (XPM), potentially increasing the limits to spectral efficiency in WDM systems. In the linear regime at high SNR, the spectral efficiency of constant-intensity modulation is 1.1 bit/s/Hz larger than half the Shannon limit. In practice, the improvement yielded by constant-intensity over unconstrained modulation formats depends on the relative strengths of XPM and FWM. A representative 10 span WDM system shows an improvement from 2.3 to 2.8 bit/s/Hz.

LDPC Coded OFDM with Alamouti/SVD Diversity Technique

Abstract: Two transmit two receive space-time processing with LDPC coding is evaluated for OFDM transmission. The two methods for space-time processing are Alamouti's combining and the SVD technique. The channel estimates are calculated and provided to the diversity combiner, the SVD filters and LDPC decoder. Noise variance estimates are provided to the LDPC decoder. Using the proposed scheme we can obtain a BER of 10−5 at an SNR of 2.6 dB with spectral efficiency of 0.4 bits/sec/Hz and 14.5 dB with a spectral efficiency of 4.2 bits/sec/Hz.

Performance of BLAST over frequency-selective wireless communication channels

Abstract: Currently one of the most promising techniques for realizing high spectral efficiencies over wireless links is Vertical-Bell Laboratories Layered Space-Time (V-BLAST). This technique employs multi-element antenna arrays at both the transmitter and receiver to increase the spectral efficiency. In contrast to previous work, we consider the performance of V-BLAST in a frequency-selective fading channel. In particular, we investigate the effect of delay spread on V-BLAST with various QAM modulation formats and different numbers of transmit and receive antennas for two types of delay spread distributions. Comparisons with a flat-fading channel are also provided.

TDD FDD air interface

Abstract: Downlink and uplink frequencies (F1, F2) in a wireless access system (100) are time-shared by adjacent sectors, but remain dedicated to downlink or uplink transmission and may utilize FDD-only bandwidth within the MMDS spectrum. TDD wireless access equipment need only be modified by introducing a frequency change at the normal TDD guard point (422), with respective downlink or uplink periods (321/322) and (323/324) for adjacent sectors offset to form overlapping frames. Cyclo-stationary processing, block equalization, and burst timing coordination allow the boundary between downlink and uplink portions of both frames to be set dynamically, improving spectral efficiency. Fast frequency switching within an allotted physical slot (421) enables synchronization of time-sharing the dedicated frequencies (F1, F2) to be maintained among sectors and cells. Duplex spacing (313) between downlink and uplink frequencies (F1, F2) for a given sector and adjacent sectors, combined with in-depth filtering of received signals, prevents spurious out-of-band transmission signal strength from reaching an interference level.

An efficient space-frequency coded wideband OFDM system for wireless communications

Abstract: A space-frequency coded OFDM system for high-speed transmission over wireless links is proposed. The analytical expression for the error event probability of such space-frequency coded OFDM system is derived in slow, spatial and frequency selective fading channels. The design criteria of TCM codes for the proposed system are then developed and discussed. It is shown that the proposed space-frequency coded OFDM with low trellis complexity can efficiently exploit the available diversity resources of the fading channel, while in conventional space-frequency coded OFDM systems, space-time trellis codes with very high trellis complexity are required to make full use of the diversity resources. Simulation results show that at a FER of 10/sup -2/, the proposed scheme outperforms the traditional space-frequency coded OFDM by 2-4 dB at an equal trellis complexity and spectral efficiency.

Multiuser Serially Concatenated Continuous Phase Modulation

Abstract: A multiuser communication system using serially concatenated and randomly interleaved continuous phase modulation (SCCPM) over the additive white Gaussian noise (AWGN) channel is investigated. The users, which may be asynchronous, are allowed to have individual energy levels as well as carrier frequencies and phases. This model incorporates multiple-access signaling similar to direct-sequence code division multiple-access (DS-CDMA), trellis-coded multiple-access (TCMA), and frequency division multiple-access (FDMA) with arbitrary spectral overlap, as well as non-intentional co-channel or adjacent channel interference of the same signaling type. First, the system is analyzed through analytical upper bounds on the average bit error probability for a given user under maximum-likelihood (ML) detection, where the R> average is over the ensemble of systems over all sets of interleavers. It is shown that in a properly designed system, the bit error probability vanishes for infinite interleaver sizes and a sufficiently large channel signal-to-noise ratio (SNR), regardless of the signal correlation between the users. Thus, even with equal modulation, energy levels, and carrier frequencies and phases, the users can be detected adequately provided they employ random interleaving. The second part of the analysis concerns iterative decoding of multiuser SCCPM. A convergence analysis based on EXIT charts is presented, along with decoding threshold estimates. It is observed that in systems with no frequency offset, the performance of ML detection does not always carry over to iterative decoding. On the other hand, for many other systems excellent performance can be obtained both in terms of power efficiency (bit error rate as a function of SNR) and spectral efficiency (bandwidth). In particular, systems are demonstrated with performance within 1 dB of the average-power limited Shannon capacity at 1 bit/s/Hz, and within 2.3 dB at 2 bits/s/Hz.