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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.

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.