Multiple transmit and receive antennas can be used to form multiple-input multiple-output (MIMO) channels to increase the capacity by a factor of the minimum number of transmit and receive antennas. In this paper, orthogonal frequency division multiplexing (OFDM) for MIMO channels (MIMO-OFDM) is considered for wideband transmission to mitigate intersymbol interference and enhance system capacity. The MIMO-OFDM system uses two independent space-time codes for two sets of two transmit antennas. At the receiver, the independent space-time codes are decoded using prewhitening, followed by minimum-Euclidean-distance decoding based on successive interference cancellation. Computer simulation shows that for four-input and four-output systems transmitting data at 4 Mb/s over a 1.25 MHz channel, the required signal-to-noise ratios (SNRs) for 10% and 1% word error rates (WER) are 10.5 dB and 13.8 dB, respectively, when each codeword contains 500 information bits and the channel's Doppler frequency is 40 Hz (corresponding normalized frequency: 0.9%). Increasing the number of the receive antennas improves the system performance. When the number or receive antennas is increased from four to eight, the required SNRs for 10% and 1% WER are reduced to 4 dB and 6 dB, respectively. Therefore, MIMO-OFDM is a promising technique for highly spectrally efficient wideband transmission.

We evaluate the performance of a previously proposed dynamic frequency hopping (DFH) when applied to cellular systems with a limited total bandwidth. We also illustrate a practical implementation for DFH deployment using network-assisted resource allocation (NARA). The performance evaluation is accomplished by system-level simulations of a system with 12 carriers and 1/1 frequency reuse, based on the EDGE-Compact specification. Voice-only circuit-switched operation is assumed. The fading channel, multicell interference, voice activity, and antenna sectorization are modeled. We present the performance of dynamic frequency hopping compared to random frequency hopping and fixed channel assignment by showing the distributions of the word error rates. The sensitivity to occupancy, Rayleigh fading assumptions, number of carriers, voice activity, and measurement errors are studied. We also compare the uplink and downlink performance. The results indicate that DFH can significantly improve the performance compared to random frequency hopping. For example, at a 2 % frame error rate with 90% coverage, the capacity improvement of DFH is almost 100% when compared with fixed channel assignment, and about 50% when compared to random frequency hopping. The amount of improvement for the uplink direction is smaller than the improvement for the downlink direction, especially for higher occupancies.

In this paper, orthogonal frequency division multiplexing (OFDM) for MIMO channels (MIMO-OFDM) is considered for wideband transmission to mitigate intersymbol interference and enhance system capacity. The MIMO-OFDM system uses two independent space-time codes for two sets of two transmit antennas. At the receiver, the independent space-time codes are decoded using pre-whitening followed by maximum-likelihood (ML) decoding or ML decoding based on successive interference cancellation. Computer simulation shows that for 4-input and 4-output systems transmitting data at 4 Mbits/sec over a 1.25 MHz channel, the required SNRs for a 10% and 1% WER are 10.5 dB and 13.8 dB, respectively, when each codeword contains 500 information bits and the channel's Doppler frequency is 40 Hz.

We investigate the performance of voice and best-effort data traffic over Enhanced General Packet Radio Services (EGPRS) using tight reuse configurations (e.g. 1/3 or 1/1), with the aim of identifying suitable bearer designs and optimal system configurations for these services. We find that voice and data have somewhat conflicting preferences: voice capacity is optimized using random frequency hopping and quality-driven power control: data throughput is optimized using no frequency hopping and no power control. These somewhat contradictory requirements impose a challenge in the system design. We investigate two simple traffic integration strategies: (1) segregating the resource for the two services and using the optimal configuration in each segment; and (2) integrating the traffic by allowing voice and data to share the same resource, and use a compromised configuration for the entire system. We find that if the majority of traffic is voice, traffic integration has an advantage since data can use any residual voice capacity. On the other hand, if the majority of traffic is data, segregation provides higher spectrum efficiency. Furthermore, the integrated traffic option becomes increasingly sub-optimal with increasing data traffic load. We conclude that the system design should depend upon the expected mix of traffic (including traffic types in addition to voice and best-effort data) and the desired engineering complexity, and that further work is needed to understand the possible design options. One possible design option is to soft segregate the resource, e.g., to allow data to use the voice segment on an overflow basis. Similar strategies may be good avenues for future work.

It is shown that the capacity of a single, isolated, multiple transmit and receive antenna array link, with given interference, is maximized by transmitting an independent data stream from each antenna for a quasistatic and flat Rayleigh fading channel with independent fading coefficients for each path. However, if such links mutually interfere, in some cases the overall system capacity can be increased by transmitting fewer streams.

In this paper we present results from the first field test to characterize the mobile multiple-input multiple-output (MIMO) radio channel. We measured the capacity, normalized to a single antenna system for different antenna configurations using 4 transmit and 4 receive antennas at both the base station and terminal in a mobile environment. We compare results for a base station rooftop antenna array consisting of dual-polarized spatially separated antennas, a vertically-polarized multibeam antenna array, and a dual-polarized multibeam antenna array. The field test results show that close to the theoretical 4 times the capacity of a single antenna system can be supported in a 30 kHz channel with dual-polarized, spatially-separated base station and terminal antennas.

We propose a framework for network-assisted radio resource management in wireless data networks. This type of radio resource management techniques offer implementation and capacity benefits compared to conventional, interference-measurement based, dynamic channel assignment (DCA) algorithms. The basic idea is to use interbase signaling to shift most of the burden of the resource allocation from the air interface to the backbone infrastructure. By exchanging channel assignment as well as other relevant information in real time through the backbone network, each base can calculate the impact of a resource assignment on the system. As a result, rapid interference measurements, which are typically needed to implement DCA schemes, are replaced by a limited amount of path loss measurements and the computation of interference conditions by the base stations. This significantly reduces the measurement and over-the-air signaling requirements, and can also provide an opportunity for a better optimization of the system performance. We focus on two specific algorithms: network-assisted least-interference-based dynamic packet assignment (NA-LI-DPA) and network-assisted dynamic packet assignment with throughput optimization (NA-DPA). NA-LI-DPA closely resembles a least-interference-based dynamic channel assignment algorithm, and NA-DPA attempts to further improve the overall system throughput. The algorithms, as defined, are appropriate for a best-effort data service, where the primary goal is to provide a higher throughput. However, it will be clear from the discussion that it is also feasible to alter the algorithms to optimize performance metrics other than throughput, e.g., to ensure a certain quality of service. We show through simulation that, for a system like enhanced general packet radio service (EGPRS) system, NA-DPA can provide a throughput that is 50% higher than random packet assignment, and 25% higher than that obtained by conventional DCA algorithms.

Clustered OFDM can provide in-band diversity gain for wideband wireless communication systems. It is a promising technique for high rate packet data access over wideband mobile wireless channels. Due to the smaller size of each cluster for clustered OFDM than for classical (nonclustered) OFDM, edge effects can be very large. In this letter, we present new transforms for channel estimators in clustered OFDM systems. The new transforms are independent of the channel delay profiles and can effectively mitigate the edge effects. It is shown by computer simulation that the performance of clustered OFDM with the estimator using the new transforms is very close to the performance with the optimum estimator that is determined by the individual channel delay profile. Furthermore, clustered OFDM with the new estimator is almost as good as classical OFDM with transmit diversity.

OFDM is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAP) of a multicarrier signal has limited its application. Previously, we described a technique, based on partial transmit sequences, that provides improved PAP statistics with little loss in efficiency, and which is more easily implemented than the standard partial transmit sequence approach. One of the remaining challenges is how to transmit knowledge about the combining sequence to the receiver in a reliable fashion. Here, we describe a technique, using no overhead, for embedding this information within the transmitted data and reliably recovering it at the receiver.

We develop an analytical framework to quantify the effects of the spreading bandwidth on spread spectrum systems operating in multipath environments with arbitrary power delay profile. The focus of the paper is to characterize the symbol error probability (SEP) performance of a RAKE receiver tracking the L strongest multipath components in frequency-selective Rayleigh fading. By transforming the physical RAKE receiver with correlated ordered paths into the domain of a "virtual RAKE" receiver with conditionally independent virtual paths, analytical expressions for the SEP are derived in terms of the spreading bandwidth, multipath spread of the channel and the number of combined paths.

Clustered OFDM can provide in-band diversity gain for wideband wireless channels. It is a promising technique for high-rate wireless packet data access, such as mobile multimedia communications. Due to smaller size of each cluster for clustered OFDM than for classical(non-clustered) OFDM, edge effects can be very large. We present new transforms for channel estimators in clustered OFDM systems. The new transforms are independent of the channel delay profiles and can effectively mitigate the edge effects. Computer simulation shows that the performance of clustered OFDM with the estimator using the new transforms is very close to the performance with the optimum estimator that depends on the channel delay profile. For the typical-urban or hilly-terrain delay profiles, clustered OFDM using the new transform based estimator is almost as good as classical OFDM with transmitter diversity.

In this paper we evaluate the performance of dynamic frequency hopping (DFH) applied to cellular systems with a limited total bandwidth. The evaluation is done by system-level simulations of a system with 12 carriers and 1/1 frequency reuse, based on the EDGE-Compact system (EDGE-enhanced data rates in existing digital cellular systems). Voice-only circuit-switched operation is assumed. Fading channel, multi-cell interference, voice activity and antenna sectorization are modeled. We present the performance of dynamic frequency hopping compared to random frequency hopping and fixed channel assignment by showing distributions of word error rates. Sensitivity to occupancy, Rayleigh fading assumptions, number of carriers, voice activity and measurement errors are studied. The results indicate that DFH can significantly improve performance compared to random frequency hopping. For example, at a 2% frame error rate with 90% coverage, the capacity improvement of DFH is almost 100% when compared with fixed channel assignment, and about 50% when compared to random frequency hopping.

We present physical and MAC layer techniques for high-speed wireless access to the Internet based on OFDM. Using dynamic packet assignment (DPA) for interference avoidance based on two examples of resource definition with different time and frequency units, we demonstrate that OFDM has significant flexibility to provide high transmission rates on demand with different available bandwidths. Innovations in radio link design, such as joint channel estimation and maximum likelihood decoding as well as interference suppression can be readily applied to coded OFDM systems. Significant performance gain in packet data throughput is realized in a system with aggressive frequency reuse.

We investigate the possibility of supporting voice over EGPRS as a potential evolutionary path for 2/sup nd/ generation wireless TDMA systems. We discuss different voice bearer designs for EGPRS radio access networks. We demonstrate how the operating mode can be chosen at each layer in the protocol stack to form appropriate voice bearers, and quantify the performance of these bearers. Different system control options are examined, including frequency reuse factor, MAC layer operating mode, frequency hopping, power control, and dynamic channel assignment. We find that a baseline EGPRS system using random channel assignment and no power control can support up to 27 Erlang/Site/MHz, which is significantly higher than the capacity of current 2/sup nd/ generation TDMA systems. In addition, the packet-switched nature of EGPRS makes it a very flexible system. We conclude that EGPRS offers an excellent evolutionary path for 2/sup nd/ generation TDMA systems.

We propose a network-assisted radio resource allocation algorithm for wireless data networks with synchronized base stations, which support variable rate transmission. The algorithm, Soft-Bar, may be used in both mobile and fixed wireless systems to assign the channel, transmission mode and transmit power for each user, and may be implemented in a distributed manner. The basic idea of the algorithm is to use measurements at the base and the terminal, and limited, real-time inter-base signaling, to control the interference generated by the few dominant interferers for each user. We show that in an EDGE-like system using 1/3 reuse, compared to conventional random packet assignment, Soft-Bar can (1) increase the system throughput by more than 50% for a given delay constraint; or (2) increase the number of users by more than 30% for a given user outage requirement, while at the same time improving the user experience. In addition, it can facilitate QoS provisioning in a straightforward manner.

We examine the feasibility of supporting integrated services, such as packet voice, Web browsing, and best-effort data, using Enhanced General Packet Radio Services (EGPRS). Among many factors that need to be considered, we focus on enhancing the radio link control/medium access control (RLC/MAC) layer design to support such services. We first describe the relevant capabilities offered by the current standard, and then outline additional capabilities that are needed at RLC/MAC layer to support real-time services. In summary, these needed capabilities are: (1) fast uplink access during an ongoing session; (2) fast resource assignment for both uplink and downlink; and (3) the ability to differentiate services at the base station sub-system. To realize these capabilities, we propose enhancements to the RLC/MAC layer, including: (1) a new set of packet control channels; (2) uplink access protocols that utilize these control channels; and (3) a new access request identifier. These changes would enable EGPRS to offer a wide range of services through a single packet based network.

Enhanced Data Rates for Global Evolution (EDGE) is currently being standardized as an evolution of GSM in Europe and of IS-136 in the United States as an air interface for high speed data services for third generation mobile systems. In this paper, we study space-time processing for EDGE to provide interference suppression. We consider the use of two receive antennas and propose a joint equalization and diversity receiver. This receiver uses feedforward filters on each diversity branch to perform minimum mean-square error cochannel interference suppression, while leaving the intersymbol interference to be mitigated by the subsequent equalizer. The equalizer is a delayed decision feedback sequence estimator, consisting of a reduced-state Viterbi processor and a feedback filter. The equalizer provides soft output to the channel decoder after deinterleaving. We describe a novel weight generation algorithm and present simulation results on the link performance of EDGE with interference suppression. These results show a significant improvement in the signal-to-interference ratio (SIR) performance due to both diversity (against fading) and interference suppression. At a 10% block error rate, the proposed receiver provides a 20 dB improvement in SIR for both the typical urban and hilly terrain profiles.

We develop an analytical framework to quantify the effects of the spreading bandwidth (BW) on spread spectrum systems operating in dense multipath environments in terms of the receiver performance, the receiver complexity, and the multipath channel parameters. The focus of the paper is to characterize the symbol error probability (SEP) performance of a RAKE receiver tracking the L strongest multipath components in wide-sense stationary uncorrelated scattering (WSSUS) Gaussian channels with frequency-selective fading. Analytical SEP expressions of the RAKE receiver are derived in terms of the number of combined paths, the spreading BW and the multipath spread of the channel. The proposed problem is made analytically tractable by transforming the physical RAKE paths, which are correlated and ordered, into the domain of a "virtual RAKE" receiver with independent virtual paths. This results in a simple derivation of the SEP for a given spreading BW and an arbitrary number of combined paths.

Orthogonal frequency division multiplexing (OFDM) is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAP) of a multicarrier signal has limited its application. Two promising techniques for improving the statistics of the PAP of an OFDM signal have previously been proposed: the selective mapping and partial transmit sequence approaches. Here, we summarize these techniques and present suboptimal strategies for combining partial transmit sequences that achieve similar performance but with reduced complexity.

Transmitter diversity and down-link beamforming can be used in high-rate wireless networks with OFDM for capacity improvement. In this paper, we compare the performance of delay, permutation and space-time coding transmitter diversity for OFDM systems, and its impact on high-rate packet data wireless networks, where relatively high block error rates, such as, 10%, on the wireless links are acceptable using ARQ. As an alternative, we also consider using the same number of transmitter antennas for down-link beamforming in the direction of the desired mobiles. Our investigation indicates that the delay transmitter diversity with QPSK modulation and adaptive antenna arrays can provide good quality of service (QoS) with a low retransmission probability while space-time coding transmitter diversity can provide high peak-rate. However, down-link beamforming together with adaptive antenna arrays, if it is applicable, appears to be a better alternative to transmitter diversity.

The rapid growth of wireless voice subscribers; the growth of the Internet; and the increasing use of portable computing devices suggest that wireless Internet access will rise rapidly over the next few years. Transmission rates are currently only about 10 kb/s for large cell systems. Third generation wireless access and the evolution of second-generation systems will provide bit rates of 50 to 384 kb/s in macrocellular systems. This paper discusses packet data transmission rates of 2 to 5 Mb/s in macrocellular environments and up to 10 Mb/s in microcellular and indoor environments as a complementary service to evolving second and third generation wireless systems. Dynamic packet assignment for high efficiency resource management and packet admission; OFDM at the physical layer with interference suppression, space-time coding and frequency diversity; as well as smart antennas to obtain good power and spectral efficiency are discussed.

We develop an analytical framework to quantify the effects of the spreading bandwidth on spread spectrum systems operating in dense multipath environments. The focus of the paper is to characterize the symbol error probability (SEP) performance of a RAKE receiver tracking the L strongest multipath components in frequency-selective fading. By transforming the physical RAKE receiver with correlated ordered paths into the domain of a "virtual RAKE" receiver with independent virtual paths, analytical expressions for the SEP are derived in terms of the spreading bandwidth, multipath spread of the channel and the number of combined paths.

Transmitter diversity and down-link beamforming can be used in high-rate data wireless networks with orthogonal frequency division multiplexing (OFDM) for capacity improvement. We compare the performance of delay, permutation and space-time coding transmitter diversity for high-rate packet data wireless networks using OFDM modulation. For these systems, relatively high block error rates, such as 10%, are acceptable assuming the use of effective automatic retransmission request (ARQ). As an alternative, we also consider using the same number of transmitter antennas for down-link beamforming as we consider for transmitter diversity. The investigation indicates that delay transmitter diversity with quaternary phase-shift keying (QPSK) modulation and adaptive antenna arrays provides a good quality of service (QoS) with low retransmission probability, while space-time coding transmitter diversity provides high peak data rates. Down-link beamforming together with adaptive antenna arrays, however, provides a higher capacity than transmitter diversity for typical mobile environments.

Orthogonal frequency-division multiplexing (OFDM) is one of the promising techniques for future mobile wireless data systems. For OFDM systems with cochannel interference, adaptive antenna arrays can be used for interference suppression. This paper focuses on a key issue for adaptive antenna arrays, that is, parameter estimation for the minimum mean square error (MMSE) diversity combiner (DC). Using the instantaneous correlation estimation approach developed in the paper, an original parameter estimator for the MMSE-DC is derived. Based on the original estimator, we propose an enhanced parameter estimator. Extensive computer simulation demonstrates that the MMSE-DC using the proposed parameter estimators can effectively suppress both synchronous and asynchronous interference in OFDM systems for packet and continuous data transmission.

Orthogonal frequency division multiplexing (OFDM) modulation is a promising technique for achieving the high-bit-rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3 dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator to channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

We investigate optimum spatial-temporal equalization for diversity receiving systems with co-channel interference. We first present the structure of the optimum spatial-temporal decision-feedback equalizer and linear equalizer, and derive closed-form expressions for the equalizer parameters and mean-square-error (MSE) for one-antenna systems. Then, we generalize the results to multiple-antenna systems through a single channel equivalent model. Finally, we obtain the general configuration of the minimum MSE spatial temporal equalizer for bandlimited systems, and show its application by a simulation example.

The advanced cellular Internet service (ACIS) is targeted for applications such as Web browsing with a peak downlink transmission rate on the order of 1-2 Mbits/s using a wide-area cellular infrastructure. In order to provide bandwidth on demand using scarce radio spectrum, the medium-access control (MAC) protocol must: 1) handle dynamic and diverse traffic with high throughput, and 2) efficiently reuse limited spectrum with high peak rates and good quality. Most of the existing approaches do not sufficiently address the second aspect. This paper proposes a dynamic packet assignment (DPA) scheme which, without coordinating base stations, allocates spectrum on demand with no collisions and low interference to provide high downlink throughput. Interference sensing and priority ordering are employed to reduce interference probability. A staggered frame assignment schedule is also proposed to prevent adjacent base stations from allocating the same channel to multiple mobiles at the same time. Simulation results based on a packet data traffic model derived from wide-area network traffic statistics, which exhibit a "self-similar" property when aggregating multiple sources, confirm that this method is able to reuse spectrum efficiently in a large cellular system having many users with short active periods. Distributed iterative power control further enhances spectrum efficiency such that the same channel can be simultaneously reused in every base station.

Orthogonal frequency-division multiplexing (OFDM) modulation is a promising technique for achieving the high bit rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3-dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MMSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator-to-channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

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.

Advanced cellular Internet services (ACIS) are targeted for applications such as web browsing with a peak downlink rate on the order of 1-2 Mb/s using the wide-area cellular infrastructure. PCS or new spectrum allocations by the FCC could be used for ACIS. In order to provide bandwidth on demand using scarce spectrum, the medium access control (MAC) protocol must: (1) handle dynamic and diverse traffic with high throughput and (2) efficiently reuse limited spectrum with high peak rates and good quality. Most of the existing approaches do not address the second aspect. In this paper, we first outline the problems for media access for ACIS and review some existing approaches, followed by a discussion of the proposed methodology. The remaining part focuses on a channel assignment method which combines the concepts of interference sensing and priority ordering to dynamically reuse spectrum with low interference probability. Preliminary simulation results based on a packetized voice model indicate that this method is able to reuse spectrum efficiently in a large cellular system having many users with short active periods.

We investigate parameter tracking of spatial-temporal equalization for IS-136 TDMA cellular/PCS systems. We present three parameter tracking algorithms: the diagonal loading minimum mean-square-error algorithm, which uses the diagonal loading to improve tracking ability, the 2-stage tracking algorithm, which uses diagonal loading in combination with a reduced complexity architecture, and the simplified 2-stage tracking algorithm, which further reduces the complexity to one M/spl times/M and one 3/spl times/3 matrix inversion for weight calculation with M antennas.

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.

The PACS radio interface is the leading low-tier candidate for standardization in North America. This radio interface was originally conceived to serve pedestrian and fixed-distribution applications; there has been significant recent interest in extending this technology into high-mobility environments. In such environments, rapid channel variations significantly degrade the performance of the preselection diversity scheme proposed for use in PACS handsets. This paper quantifies this degradation and proposes a modified preselection diversity scheme using shorter diversity measurement lengths. This scheme yields good performance in both low-mobility and high-mobility environments.

For CDMA (code-division multiple-access) wireless communications systems, orthogonal signaling with noncoherent detection has been proposed when a reference phase is not available. In this paper, orthogonal signaling using Walsh signals is considered. Performance of the channel using error-correcting coding together with 1) bit-by-bit interleaving or 2) Walsh word-by-word interleaving is investigated. Simulation results show that for several types of fading and non-fading channels with practical amounts of interleaving and with practical power control, the second technique with complete soft decision decoding that has added complexity in the decoder gives significant reduction in the required signal to interference ratio (SIR) over the first technique.>

We combine autonomous algorithms for dynamic channel assignment (DCA) and power control in a TDMA/FDMA wireless system as a medium access control (MAC) protocol. The DCA algorithm determines paired radio channels that experience the least interference and are least likely to cause interference. The power control algorithm uses local estimations of signal to interference ratio (SIR) at a receiver to iteratively command power adjustment on the desired transmitter. A common control frequency, which is frame-synchronized among base stations, provides all necessary information for DCA without blind slots. Computer simulations are used to evaluate system performance. Results from computer simulations demonstrate good spectrum efficiency and robustness. Although studied under a specific set of parameters, this type of MAC protocols can be applied in different wireless communications environments.>

We combine autonomous algorithms for dynamic channel assignment (DCA) and power control in a TDMA/FDMA wireless system as a medium access control (MAC) protocol. The DCA algorithm determines paired radio channels that experience the least interference and are least likely to cause interference. The power control algorithm uses local estimations of signal to interference ratio (SIR) at a receiver to iteratively command power adjustment on the desired transmitter. A common control frequency, which is frame-synchronized among base stations, provides all necessary information for DCA without blind slots. Computer simulations are used to evaluate system performance. Results from computer simulations demonstrate good spectrum efficiency and robustness. Although studied under a specific set of parameters, this type of MAC protocol can be applied in different wireless communications environments.>

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.>

A low-overhead burst coherent demodulation method that jointly estimates symbol timing and carrier frequency offset and then performs diversity selection is studied. It coherently demodulates individual bursts of TDMA (time division multiple access) symbols by operating solely on random data within the burst without requiring training sequences. Its performance is robust against frequency offset between transmitter and receiver, thereby eliminating the need for a highly stable frequency reference. The performance of this demodulation method in a fading channel can be further improved by using a diversity selection technique based on a quality measure derived as part of the joint timing/frequency offset estimation process. Simulations and experiments have confirmed that two-branch diversity using this method can provide reliable speech communication using TDMA with a transmission rate of 450 kb/s for a portable radio channel with an RMS delay spread of 555 ns or less.>

Simulation and experimental results for a hardware implementation of a low-overhead digital coherent burst demodulator for time-division multiple-access (TDMA) radio systems using short bursts are analyzed. This implementation is based on digitizing a receiving signal after a low-frequency IF amplifier and performing all demodulation functions using digital signal processing techniques. Demodulation with very low overhead for TDMA is made possible by storing a burst in memory. A novel forward loop plus backward loop structure performs carrier recovery. Symbol-timing and carrier-frequency-offset estimations are performed by block processes, using the error signal resulting from differential demodulation. Bursts as short as 32 symbols can be demodulated without overhead. Since the demodulator is entirely digital and has the potential for low-power VLSI implementation, it is suitable for a portable radio system. Other advantages of this implementation include the availability of a channel quality measure derived from symbol timing estimation and the possibility of providing differential detection when it is desirable.>

The authors present a real-time single DSP prototype of a 16-kb/s subband speech coder, using a single digital signal processor, for application to digital portable radio communications. The system architecture, coding algorithm, firmware, and hardware of this coder are addressed. Careful attention has been given to the many special design requirements for interfacing to a time-division multiple-access radio link. The coder uses nonlinear quantization improvement in speech quality with negligible added complexity. A single-board full-duplex prototype, based on a TMS320C25, has been designed and delivers good speech quality.>