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We have investigated transmitting wireless code division multiple-access (CDMA) signals from a remote antenna to a centralized base station over an analog optical link. We present specifications for transmission of standard (IS.95) CDMA signals and demonstrate that unisolated uncooled lasers can meet these requirements. Tests were conducted using both a 1.3-/spl mu/m Fabry-Perot laser and a 1.55-/spl mu/m distributed-feedback laser (DFB).

Transmitter diversity is an effective technique to improve wireless communication performance. In this paper, we investigate transmitter diversity using space-time coding for orthogonal frequency division multiplexing (OFDM) systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and we derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as high as 1.475 bits/s/Hz, the required signal-to-noise ratio is only about 7 dB for a 1% bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40-Hz Doppler frequency. In summary, with the proposed channel estimator, combining OPDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels.

This paper introduces a new approach for joint convolutional coding and decision feedback equalization (DPE). To minimize error propagation, the DFE uses a combination of soft decisions and delayed tentative decisions to cancel intersymbol interference (ISI). Soft decisions are obtained by passing the DFE output through a (soft) nonlinear device. This simple method is shown to perform almost as well as an optimum soft feedback approach on wireless channels with diversity. Tentative decisions from the Viterbi decoder are used to cancel ISI due to multipath with large delays, thus remedying the increasing effect of error propagation in channels with large delay spreads. We consider the use of this soft/delayed feedback DFE (S/D-DFE) technique in broadband wireless channels (with delay spreads up to several tens of the symbol period) typical in high-bitrate mobile data applications. Simulation results indicate that the proposed joint coding and S/D-DFE technique performs to within 1-2 dB [in required signal-to-noise ratio (SNR)] of an ideal coded DFE without error propagation. When combined with antenna diversity and a reduced-complexity DFE concept with adaptive feedforward tap assignment, it provides high packet throughput against Rayleigh fading, severe delay spreads, and high Doppler rates.

This paper presents an adaptive tap assignment technique for improving the performance of a previously reported reduced-complexity decision-feedback equalizer (DFE) for broadband band wireless systems. The spacings of individual feedforward taps of the DFE are made selectable so that, when the channel consists of a sparsely distributed multipath with a large delay spread (e.g. "hilly terrain" (HT) delay profiles), the equalizer span can be increased without increasing the total number of taps. We propose simple tap selection algorithms and show that they provide: (1) performance gains over a contiguous-tap approach in various outdoor delay profiles and (2) improved robustness against fast fading.

This paper analyzes a radio access system that has multiple antennas spatially distributed throughout each cell, instead of a single antenna at each cell. The same signal is simulcast by each antenna in the cell. The signals at a given cell could be multiplexed and modulated for radio transmission at a single node, then transmitted on a coaxial cable distribution system to and from the distributed antennas in a cell. Frequency translation and radio frequency (RF) amplification may need to be performed at each antenna, except for indoor applications with short coaxial cables. This paper presents calculations of signal-to-interference ratios (SIR) and handoff performance for cells with distributed antennas. It is shown that the use of distributed antennas can substantially increase the SIR above a system with a single antenna per cell, and this increase is quantified. It is also shown that increasing the number of antennas results in better overall handoff performance, with a lower number of handoffs and higher received signal power. Minimizing the number of handoffs will reduce the impact of wireless services on network elements.

For pt.I see ibid., vol 41, p.1626-34 (1993). Power control is essential in the use of direct-sequence code division multiple-access (CDMA) techniques. Early system-level performance analyses of a CDMA approach to wireless mobile and personal communications have assumed the ability of power control to equalize the absolute signal powers of CDMA users received at each base station. The present paper studies a more practical, although analytically more complicated, uplink power control technique that uses measurements of the received signal-to-interference ratio (SIR) instead. A combination of discrete-event link simulation and analysis of the obtained SIR statistics is used to explore the previously little-known behavior of a CDMA system using SIR-based power control and to obtain performance estimates for such a system under various operating assumptions. The overall results indicate that power control based on SIR has the potential for somewhat higher system performance than power control based on absolute signal strength assumed in the early analyses. >

A combined burst synchronization and error detection scheme is studied for a time-division multiple access (TDMA) portable radio system. Analysis, computer simulation, and experimental results of the false acceptance rate of the system using the combined scheme are presented. It is found that for a (161,147) code used in this study, which is capable of correcting +or-6-b sync slippage, the error detection capability of the code is degraded by 3 b on a slow Rayleigh fading channel using coherent QPSK demodulation with differential decoding. However, lower implementation complexity and higher efficiency are obtained than with separate burst synchronization and error detection processes. >

This work describes the use of a receiver with a time-reversal structure for low-complexity decision feedback equalization of slowly fading dispersive indoor radio channels. Time-reversal is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the equalizer is a time-reverse of the actual channel impulse response. Selective time-reversal operation, therefore, allows a decision feedback equalizer (DFE) with a small number of forward filter taps to perform equally well for both minimum-phase and maximum-phase channel characteristics. The author evaluates the theoretical performance bounds for such a receiver and quantifies the possible performance improvement for discrete multipath channels with Rayleigh fading statistics. Two extreme cases of DFE examples are considered: an infinite-length DFE; and a DFE with a single forward filter tap. Optimum burst and symbol timing recovery is addressed and several practical schemes are suggested. Simulation results are presented. The combined use of equalization and diversity reception is considered. >

Describes a low-complexity equalization technique for improving the reliability of portable radio links in the presence of multipath time delay spread. A technique that operates on hard-limited received signals, with only the phase information available, is presented. Suboptimum receivers based on a maximum likelihood estimation criterion are discussed, including a sequence estimator and a decision feedback phase detector. A low-complexity adaptive phase equalizer structure using decision-directed phase tracking is proposed. The equalizer does not require the multiplication operations required in most conventional equalization algorithms. The author also proposes a receiver that includes a time-reversal structure and a joint estimator for optimum timing recovery and equalizer training. The time-reversal structure plays a crucial role in maximizing the compensation capability of the phase equalizer. The combined use of phase equalization and diversity reception is also considered. >

The architecture and implementation of a time-division multiple-access digital portable communication system using short bursts are considered. An efficient and robust TDMA frame structure that requires very low overhead for burst demodulation and synchronization while being robust against fading and carrier frequency offset is described. Based on this frame structure, an experimental radio link was implemented by using a hardware architecture that is suitable for digital signal processing and very large-scale integration implementation. Implementation of demodulation, synchronization, error detection, and fading recovery using combined software and off-the-shelf hardware is described, followed by a summary of the performance results. >

Power control in the use of direct-sequence code-division multiple-access (CDMA) techniques is addressed. Previous performance analyses of CDMA wireless mobile and personal communications have assumed the ability of power control to equalize the absolute signal powers of users received at each base station. A more practical, although analytically more complicated, uplink power controls technique that uses measurements of the received signal-to-interference ratio. (SIR) instead is studied. A combination of discrete-event link simulation and analysis of the obtained SIR statistics is used to explore the behavior of a CDMA system using SIR-based power control and to obtain performance estimates for such a system under various operating assumptions. The overall results indicate that power control based on SIR has the potential for somewhat higher system performance than power control based on absolute signal strength. >

This paper discusses the performance of a slow-frequency-hopped time-division multiple-access (SFH-TDMA) technique, which has been proposed as a "high-tier" extension of a low-complexity TDMA architecture optimized for low-power pedestrian applications. The SFH-TDMA technique considered uses QPSK modulation and rate-1/2 convolutional coding. Numerical results for a wide range of fading rates are obtained through analytical calculation of the effective signal-to-noise ratio combined with a simulation approach which incorporates measured multipath channels and actual frequency correlation among contiguous hopping channels. The results indicate that the SFH-TDMA technique can tolerate root-mean-square (RMS) delay spread up to several bit periods without a need for adaptive equalization, but also point to the need for fast power control when the fading is slow and the RMS delay spread is much smaller than the bit period. This work is targeted toward understanding the implications to local exchange networks of wireless technology alternatives that could provide access to those networks. >

This paper presents reduced-complexity equalization techniques for broadband wireless communications, both outdoors (fixed or mobile wireless asynchronous transfer mode (ATM) networks) and indoors [high-speed local-area networks (LANs)]. The two basic equalization techniques investigated are decision-feedback equalization (FE) and delayed decision-feedback sequence estimation (DDFSE). We consider the use of these techniques in highly dispersive channels, where the impulse response can last up to 100 symbol periods. The challenge is in minimizing the complexity as well as providing fast equalizer start-up for transmissions of short packets. We propose two techniques which, taken together, provide an answer to this challenge. One is an open-loop timing recovery approach (for both DFE and DDFSE) which can be executed prior to equalization; the other is a modified DFE structure for precanceling postcursors without requiring training of the feedback filter. Simulation results are presented to demonstrate the feasibility of the proposed techniques for both indoor and outdoor multipath channel models. The proposed open-loop timing recovery technique plays a crucial role in maximizing the performance of DFE and DDFSE with short feedforward spans (the feedforward section of DDFSE is a Viterbi sequence estimator). A feedforward span of only five is quite sufficient for channels with symbol rate-delay spread products approaching 100. The modified DFE structure speeds up the training process for these channels by 10-20 times, compared to the conventional structure without postcursor precancellation. The proposed techniques offer the possibility of practical equalization for broadband wireless systems.

The authors discuss and characterize factors affecting the use of error-correcting codes with bit interleaving for a channel model which includes antenna diversity in interference-limited fading radio environments. The coding gain required to compensate for the bandwidth expansion due to channel coding is investigated for binary phase-shift keying (BPSK) with coherent and with differential detection, and for orthogonal signaling with noncoherent detection. In some radio communication systems where the overall interleaving delay that can be tolerated by a user is limited, it is found that channel coding may not be practical from the viewpoint of spectrum efficiency. >

A feedback power control approach that allows power commands to be updated at a higher rate than the rate of multipath fading is investigated. The signal and interference statistics as received at the base stations after power control are obtained for a simulated direct-sequence code-division multiple-access (CDMA) system, which includes multiple base stations with diversity receivers and a large number of power-controlled users continuously moving at various speeds. It is shown that often-used analyses based on perfect average power control lead to optimistic capacity results (by 25% to 60%) because interference is underestimated by 1 to 2 dB. >

An investigation is made of the performance of a CDMA (code-division multiple access) radio system with feedback power control. Feedback power control delay, power delay profile and channel fading rate are used as parameters to characterize the channel performance. Tradeoffs between the use of single and multiple correlators per antenna for the multipath dispersive channel are shown. It is found that in some delay-spread environments, it may be better to turn off one of the correlators or lock all the correlators onto the strongest path. >

A low-complexity equalization technique for improving the reliability of portable radio links in the presence of multipath time delay spread is described. A technique that operates on hard-limited received signals, with only the phase information available, is presented. The use of passband hard-limiters in radio front end circuits reduces the complexity of portable radio receivers. An adaptive phase equalizer structure using decision-directed phase tracking which does not require the multiplication operations required in most conventional equalization algorithms is proposed. A receiver that includes a time-reversal structure and a joint estimator for optimum timing recovery and equalizer training is described. The time-reversal structure plays a crucial role in maximizing the compensation capability of the phase equalizer. The combined use of phase equalization and diversity reception is also considered. Computer simulations confirm the feasibility of the proposed receiver structure. >

The author discusses the use of a receiver with a time-reversal structure for improving the performance of a low-complexity decision-feedback equalizer (DFE) in slowly fading dispersive indoor radio environments. Time-reversal operation is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the DFE becomes a time reverse of the actual channel impulse response. The author describes useful performance bounds for such a receiver and studies important topics, including optimum symbol and burst timing recovery, and the combined use of equalization and diversity reception. Simulation results for a proposed receiver implementation that incorporates the time-reversal concept are presented. It is shown that a DFE with a very short forward filter can be effectively used to eliminate the effects of multipath time dispersion up to several symbols in duration. >

This paper describes a broadband wireless data access system using a microcellular infrastructure. This system can be envisioned as an overlay data system which shares the same network infrastructure as a microcellular voice system, such as the Personal Access Communications System (PACS). Unlike microcellular voice systems, however, the proposed system provides a high peak data rate of 2 Mbps, can operate with a group frequency reuse factor of one, only requires handoffs between the group areas of sixteen radio ports, and can start with a minimum of 1 MHz spectrum in each direction. We quantify the performance of the proposed system under different air interface assumptions, each of which has a different required complexity and achievable performance. The proposed system can provide adequate performance (i.e., 1% packet error rate) with only the use of either half rate channel coding or dynamic time slot assignment, for a 95% target coverage. Further performance improvement is possible with the use of SINR-based feedback power control.

A broadband wireless technique based on equalized quaternary phase shift keying (QPSK) proposes to increase the data rate capabilities of current mobile radio links from tens of kbps to several Mbps. The key-enabler is a reduced complexity decision feedback equalization technique that provides a delay spread tolerance in mobile broadband channels of up to several tens of symbol periods. This paper presents an implementation study that includes a complexity estimation of this equalization technique. In doing so, we also introduce a new cyclic correlation technique that minimizes the packet overhead required for channel estimation. The decision feedback equalizer (DFE) is implemented in VHSIC hardware description language (VHDL). Design verification, which compares the performance of the implemented DFE with that obtained from computer simulation, confirms the feasibility and practicality of the proposed equalizer concept. Complexity estimates are provided for different design options. An optimum design yields a required gate count of approximately 600 K gates and a power consumption of about 0.6 W.

Transmitter diversity is an effective technique used to improve wireless communication performance. We investigate transmitter diversity using space-time coding for OFDM systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as large as 1.475 bits/sec/Hz, the required signal-to-noise ratio is only about 7 dB for a 146 bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40 Hz Doppler frequency. In summary, combining OFDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels.

This paper presents an adaptive tap assignment technique for improving the performance and robustness of a reduced-complexity decision-feedback equalizer (DFE) for broadband wireless mobile communications. The spacing of individual feedforward taps of the DFE is made variable such that, when the channel consists of sparsely-distributed multipath with a large delay spread (e.g., "hilly terrain" delay profiles), the feedforward taps can be sparsely assigned to increase the equalizer span without increasing the total number of the feedforward taps. We provide an analysis of the optimum tap assignment problem and propose a practical channel-estimation-based tap assignment algorithm which can be executed prior to equalizer training. Simulation results are obtained for various outdoor delay profiles with dispersions ranging up to several tens of symbol periods and while using only a small number of feedforward taps (e.g., 5 taps), compared to a conventional contiguous-tap DFE, the proposed tap-selectable DFE is found to give relatively stable performance (measured in terms of required SNR) over all profiles, and also to provide improved robustness to fast fading.

We present specifications for transmission of code-division-multiple-access (CDMA) signals over an optical link, and demonstrate that unisolated, uncooled Fabry-Perot lasers can meet these requirements. Since full testing using a CDMA base station is not practical, a simple test using two tones to simulate the RF channel has been developed and used.

Broadband analog transport facilities using fiber or fiber/coax cable can play a significant role in the evolution of the network infrastructure for personal communications services (PCSs). Low-power PCS systems require a dense grid of radio ports to provide connectivity to the telephone network. Analog transport has a number of important advantages over digital transmission facilities, including the flexibility to support a variety of air interface formats, shared infrastructure cost with other services such as video distribution, and centralized call processing allowing the use of low cost and simple radio ports. A simulcast technique can be used in such systems to permit low rates of handoff (no handoff within each simulcast area) and sharing of hardware resources among multiple radio ports. This paper provides a detailed model and a simulation analysis of the cochannel interference and noise performance as well as the resource sharing benefit of a simulcast PCS system. Several potential PCS air interfaces are considered, including time division multiple access (TDMA) and code division multiple access (CDMA) techniques. Our investigation shows that the impact of multiple antenna noise in a simulcast system is offset by the improved signal-to-interference (SIR) ratio brought about by distributed antennas. Even with distributed antennas, multiple antenna noise places a limit on the maximum number of radio ports that can be assigned to each simulcast group. This limit, however, is shown to have little impact on the achievable resource sharing benefit of simulcasting (i.e., grouping beyond this limit has diminishing returns). A saving of 40% to 60%, in terms of the required central hardware resources, is typical for both TDMA and CDMA systems in suburban environments.

This paper presents the multiport system concept for interconnecting the radio ports of a microcellular personal communications services (PCS) network or wireless local loop. The radio ports are low-cost since they have minimal functionality. Air interface signals are transported without demodulation from radio ports to a central hub. The scheme exploits the availability of existing fiber/coax infrastructure. A novel feature is that the radio ports are dynamically assigned to capacity-sharing groups depending on the traffic needs. Within a group, the radio ports simply simulcast the radio signals and share dedicated cable spectrum and call-processing hardware. This proposal, with its resource sharing and dynamic capacity allocation benefits, is applicable to various air interface formats. The proposal features a low start-up cost upon service introduction and low cost of service in low-traffic and low-penetration areas. It also allows graceful upgrade to the capacity allowed by small microcells without requiring any modification of the radio ports or transport infrastructure.

Time synchronisation of fixed radio ports is crucial in digital wireless personal communications. The Letter describes a centralised approach for providing each synchronisation over the wireline distribution network. Various existing digital transmission facilities are considered as candidates for the wireline transport interface. Issues related to the possible impacts of this approach on evolving distribution network architectures are discussed.

Power control is used to solve the 'near-far' problem which arises in a spread spectrum system using direct sequence code-division multiple access (CDMA). A method to estimate the signal-to-interference ratio to be used for power control is proposed and the difference in the signal-to-interference ratio between the set power control threshold and the average value actually received at the antenna over a fading multipath channel is presented.

The Letter investigates the achievable performance in a TDMA system of an autonomous power control technique which relies only on a simple fixed-step feedback adjustment algorithm. Simulation results indicate that this simple power control technique can perform nearly as well as ideal signal-to-interference ratio (SIR) balancing/call removal, yielding a possible increase in spectrum efficiency of at least three times compared to a system without power control. The work is targeted towards understanding the traffic capacity and deployment implications to local exchange networks of wireless technology that could provide access to those networks.

The Letter investigates the issue of uncorrelated multipath fading on uplink and downlink radio channels which impacts the handoff decision quality of slowly moving users in a CDMA system. Simulation results indicate a resulting capacity reduction by as much as 30% when all the users experience quasistationary, single-path Rayleigh fading. This work is targeted toward understanding the traffic capacity and deployment implications to local exchange networks of wireless technology alternatives that could provide access to those networks.

The Letter presents a summary of the results of a study on CDMA feedback power control in multipath fading radio channels. The signal and interference statistics after power control are presented for a simulated radio system which includes multiple base stations. It is shown that often used analyses based on perfect average power control lead to optimistic capacity results because interference is underestimated by 1-2 dB.

A new scheme of fractional-bit differential detection of minimum shift keying (MSK) is proposed. This technique is aimed at reducing timing fluctuation of eye patterns, which is found to be a cause of abnormally long burst errors in a frequency-selective fading channel. We have confirmed in a detailed laboratory experiment that such long-burst errors can be eliminated by employing a differential delay shorter than one-bit interval, thereby yielding remarkable improvement in the measured average bit error rates. The result, in a way, highlights the influence of eye-pattern timing fluctuation on an actual digital mobile radio system.

This paper describes a new modulation technique called DSK (double phase-shift keying) which shows an interesting antimultipath feature over mobile frequency-selective fading environments. The effect of this technique is shown for various multipath channels by detection waveform and theoretical bit error rate (BER) analyses. A result obtained from a laboratory test is also presented. In the latter part of the paper, a generalized form of DSK is described and extensive investigations are made so as to clarify the anti-multipath effect of DSK in view of the diversity concept and to further explore the possibility of a narrowband scheme. A major result shows that this technique can give a kind of diversity effect which is implicit in the arrival of signals via multiple paths with different delays.

Differential detection of PSK after an IF hard-limiter is described. This detection scheme is well known for the reason that the receiver structure is greatly simplified with saturated amplifiers and a digital-type differential detector. In addition, it can give the improved effect of reducing eye pattern timing fluctuation which is found to be a cause of excessive errors in a frequency-selective fading channel. Error rates were measured for binary and quadrature phase-shift keying. The results show great improvement over the detection scheme employing linear amplifiers and an analog differential detector.

This paper describes a new concept in modulation methods for high speed digital mobile communications. Linear modulation is introduced to allow amplitude variation in the modulated signal. This primarily brings the advantage that sharp cut-off band restriction can be performed to enhance the transmission efficiency. Also, we can adopt as a multipath strategy an amplitude-shift anti-multipath modulation technique, which is found to be more effective than its phase-shift counterpart. A linear modulation method is presented to meet the mobile radio requirement on power efficiency. We finally derive a narrow-band anti-multipath modulation scheme, with required bandwidth compatible with other narrow-band modulation techniques and with superior anti-multipath effect.

A new scheme of fractional-bit differential detection of MSK is proposed. This technique is aimed at reducing timing fluctuation of eye patterns, which is found to be a cause of abnormally long burst errors in a frequency-selective fading channel. We have confirmed in a detailed laboratory experiment that such long burst errors can be eliminated by employing differential delay shorter than one-bit interval, thereby yielding remarkable improvement in the measured average bit error rates.