Interference alignment (IA) is a promising technique to boost the Multiple-input Multiple-output (MIMO) network throughput. Concurrent data transmission links between multiple pairs of transmitters (TX) and receivers (RX) can be established in the same spectrum via cooperation. To make the channel state information (CSI) available on the transmitter side, channel reciprocity in time division duplex (TDD) networks is assumed by a large number of distributed IA algorithms. The physical channel is reciprocal, but nevertheless the channel considered by IA is not, because it also involves the radio frequency (RF) front ends of the transceiver, which are different circuits for transmit and receive modes individually. The difference between the two circuits yields mismatch between the forward and backward channels. Given the mutual coupling effect of the antenna array, the channel's non-symmetry is further complicated. In this paper, we provide the modeling of the non-reciprocity for the TDD MIMO channels and propose a corresponding calibration method to minimize the mismatch. Experimental validation has been conducted with classic distributed IA algorithms. The results highlight the indispensability of the reciprocity calibration for approaching the network capacity gain promised by IA.

We consider Interference Alignment (IA) under a Gauss-Markov time varying channel and give an exact expression for the Complementary Cumulative Distribution Function (CCDF) of the Signal to Interference and Noise Ratio (SINR) of a link at time t in terms of the long term path gains. We also give a simpler approximate expression for the CCDF of SINR in terms of the total interference power. Using these, we derive an expression for the expected rate at time t and the average system throughput for a given choice of data phase duration. We show how these expressions allow us to choose the data phase duration that maximizes the sum throughput of the network.

We present a low-complexity link adaptation algorithm for maximizing the energy efficiency (EE) of MIMO-OFDM systems. We prove that the EE is a single-peaked quasi-concave function of the transmit power and develop an iterative algorithm to find near-optimal transmit power. We further prune the search space by analyzing the singular values of the channel. The resulting link adaptation protocol achieves orders of magnitude reduction in complexity compared with the brute force search method without noticeably sacrificing the performance.

In a mobile ad-hoc network (MANET), concurrent transmission enabled via interference alignment or transmit and receive beamforming between$K$transmitters and$K$receivers can result in a significant increase in the network’s sum throughput. However, to achieve this high sum-throughput each of the$K$receivers must: 1) determine the presence of its desired transmitter; 2) correct the frequency and timing offset of the desired transmitter; 3) estimate the channel from the desired transmitter; and 4) estimate either the channel from each of the$K-1$interfering transmitters or the covariance matrix of the interferences. In this paper, we address the challenges associated with enabling concurrent links within a MANET. By exploiting the unique properties of Zadoff–Chu (ZC) sequences, we propose a practical mechanism for all receivers to achieve the required synchronization and channel estimations concurrently. We also propose a constant false alarm rate detection algorithm to detect the presence of the ZC sequences in a high-interference environment in the absence of frequency and timing lock. Simulation results verified the great performance of our approach in sequence detection, the carrier offset, timing offset, channel estimate and covariance estimates, in both frequency flat channels and frequency selective channels.

We propose an approach where each user independently seeks to minimize the amount of time that they occupy the channel. Essentially, we seek to minimize the number of transmitted symbols required to communicate a packet assuming variable-length coding with feedback. Users send a pilot sequence to estimate the channel quality and decide whether to proceed with a transmission or wait for the next opportunity. Thus a user may choose to leave the channel even though it has already gained access, in order to increase the network throughput and also save its own energy resources. This paper optimizes the number of pilots and the channel identification threshold to minimize the total number of transmitted symbols (including pilots) required to communicate the packet. We prove a sufficient condition for the optimal pilot length and the channel identification threshold. This optimal parameter pair is solved numerically and the reduction in channel occupancy is shown for various channel settings.

The analysis of a mixer-first receiver using conversion matrices is presented. Conversion matrices provide a systematic approach to analyze linear periodically time-varying (LPTV) circuits. Using conversion matrices of LPTV components in a frequency domain equivalent circuit allows analysis similar to a linear time invariant (LTI) circuit. For example, Ohm's law, Kirchhoff's voltage and current laws, impedance combination rules, etc., can all be used in such equivalent circuits. On applying this method to a mixer-first receiver, a common LPTV circuit, results already established in prior art are reproduced accurately. Further, effects of a few important non-idealities, such as clock overlaps, imperfect clock edges and parasitic components that were not considered previously, are also calculated and verified.

Cyclic redundancy check (CRC) codes check if a codeword is correctly received. This paper presents an algorithm to design CRC codes that are optimized for the code-specific error behavior of a specified feedforward convolutional code. The algorithm utilizes two distinct approaches to computing undetected error probability of a CRC code used with a specific convolutional code. The first approach enumerates the error patterns of the convolutional code and tests if each of them is detectable. The second approach reduces complexity significantly by exploiting the equivalence of the undetected error probability to the frame error rate of an equivalent catastrophic convolutional code. The error events of the equivalent convolutional code are exactly the undetectable errors for the original concatenation of CRC and convolutional codes. This simplifies the computation because error patterns do not need to be individually checked for detectability. As an example, we optimize CRC codes for a commonly used 64-state convolutional code for information length k=1024, demonstrating significant reduction in undetected error probability compared to the existing CRC codes with the same degrees. For a fixed target undetected error probability, the optimized CRC codes typically require 2 fewer bits.

This paper presents an energy efficient programmable hardware accelerator that targets multiple-input-multiple-output (MIMO) decoding tasks of orthogonal frequency-division multiplexing (OFDM) systems. The work is motivated by the adoption of MIMO and OFDM by almost all existing and emerging high-speed wireless data communication systems. The accelerator was fabricated in 65-nm CMOS technology and occupies a core area of 2.48 mm2. It delivers full programmability across different wireless standards (i.e., WiFi, 3G-long term evolution, and WiMax) as well as different MIMO decoding algorithms (i.e., minimum mean square error, singular value decomposition, and maximum likelihood) with extreme energy efficiency. The energy efficiency of our MIMO accelerator chip was compared against dedicated application specific integrated circuits for 4 × 4 QR decomposition, 4 × 4 singular value decomposition, and 2 × 2 minimum mean square error decoding. Despite the programmable nature of our design, it delivered energy efficiencies that were 18% to 28% better than the dedicated solutions reported in the literature. This paper presents the VLSI implementation of the architecture discussed in [14]-[16]. It discusses the implementation decisions and tradeoffs used to ensure minimum overall energy consumption of the resulting accelerator chip without sacrificing programmability. Given its programmability and extreme energy efficiency, the accelerator is an ideal solution for today's smart phones that implement multiple MIMO-OFDM waveforms on the same platform.

In this paper, we present a practical link adaptation algorithm for MIMO-OFDM systems. The proposed solution is based on sounding the channel periodically and choosing the optimal mode that will maximize the throughput or energy efficiency (EE) while satisfying the application's quality of service (QoS) requirements. In addition, we address practical problems such as the computational complexity of the algorithm and determination of sounding period by novel closed form solutions and iterative algorithms. Realistic link level simulations confirm that the proposed solution achieves significant improvement over existing link adaptation algorithms when the aim is to maximize the throughput, and provides orders of magnitude gain in energy efficiency compared to poorly chosen fixed modes when used for energy efficiency maximization purposes. The resulting algorithms are implemented on a real time MIMO-OFDM testbed, and we report on the experimental results.

We present a low-complexity link adaptation protocol for maximizing energy efficiency (EE) of MIMO-OFDM systems. We prove that the energy efficiency is a single-peaked quasiconcave function of transmit power and develop an iterative algorithm to find near-optimal transmit power. We further prune the search space by analyzing the singular values of the channel. The resulting link adaptation protocol achieves orders of magnitude reduction in complexity compared to the brute force search method without noticeably sacrificing the performance.

A new, hybrid RF/digital phase noise cancellation technique is presented. The proposed phase noise cancellation technique estimates the phase noise using an RF delay-line-discriminator (DLD), digitizes the DLD output, performs proper digital signal processing (DSP) and then compensates for the phase noise at the digital baseband. This new technique is applicable to both transmitter and receiver and is independent of the modulation and waveform. As part of the DSP, an adaptive linear prediction filter (ALPF) is applied to compensate the analog delay associated with the DLD, low-pass filters, and data conversion. Such delay compensation enhances the phase noise cancellation bandwidth. 30 dB of phase noise cancellation is achieved in simulation using realistic phase noise and component models.

We present a practical link adaptation algorithm for MIMO-OFDM systems. The proposed solution is based on sounding the channel periodically and choosing the optimal mode that will maximize the throughput or energy efficiency while satisfying the application's quality of service (QoS) requirements. In addition, we address practical problems such as computational complexity of the algorithm and determination of sounding period, by novel closed form formulations and iterative algorithms.

We formulate an adaptive power allocation problem where we minimize the energy-per-goodbit (EPG) of a system employing a multiple-input multiple-output maximum ratio combining (MIMO-MRC) scheme. Closed-form results are obtained for the optimum transmit power and minimum EPG as a function of the number of antennas employed and the quality of the channel. To assess the performance of the solution in a statistically varying channel, we obtain the cumulative distribution function (CDF) of the minimum EPG in closed form. We explore the energy-efficiency trade-off between enhanced diversity and the increased circuit power consumption of multiple antennas. In particular, we use the EPG CDF in a numerical example to find the most energy-efficient number of antennas for a given probability of outage. We consider both Rayleigh and Rician MIMO fading channels.

Reliable spectrum sensing is the key requirement for secondary users to opportunistically utilize the spectrum in the absence of primary users. To increase the secondary user's throughput and efficiency, it has to be able to detect the needed white band out of all possible bands in the shortest possible time. To speed up the spectrum sensing process and to minimize the power consumption overhead, detectors have to sweep a wide band in a parallel fashion. In this paper, we analyze the effect of the I/Q mismatch on the performance of the wide-band energy detector as the simplest detection method. We demonstrate that the false alarm probability can go beyond 0.5 for 2° phase mismatch or 0.1dB gain mismatch between I and Q paths when there is 30dB near-far effect between the mirror primary users. Afterward, we suggest an adaptive threshold method that improves the performance knowing the I/Q imbalance parameters a priori.

We propose an approach for minimizing the total power consumed by a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing communication system. The system under consideration supports a wide range of communication scenarios over a flat fading channel. A communication scenario is defined by throughput requirement, transmission distance, and maximum packet error rate (PER) constraints. Power consumption minimization is achieved by the joint configuration of hardware and physical layer system parameters according to the communication scenario. The configurable parameters are the modulation and coding scheme, antenna configuration, operating bandwidth, power amplifier back-off, analog-to-digital converter resolution, phase-locked loop phase noise power, transmission power, and transmit duty cycle. The optimal parameter configuration is found through a brute-force search over a simulation-based database linking performance with power consumption. Results show that power consumption can be reduced by a factor of 24.6 for a single-input single-output system and 3.79 for a MIMO system at a PER of 10%.

Secondary wide-band users have to guarantee they will not disturb the in-band narrow-band primary licensed users. There are techniques proposed to limit the interference by secondary wide-band users in the specified in-band narrow bands. Active Interference Cancelation (AIC) can flexibly solve this problem in the digital domain and hence remove the need for extra expensive analog hardware. The main challenge is the degradation in the final depth of a nulled band by radio impairments such as PA non-linearity, phase noise, and I/Q imbalance. In this paper, we analyze the effect of phase noise on the performance of active interference cancelation techniques and then propose a mitigation technique. Our analysis shows that phase noise can degrade the nulling depth performance by 11.4-19.6dB at the edge of nulled band. We propose an adaptive pre-filtering technique to improve the nulling performance by 3-11dB depending on system configuration. The proposed scheme utilizes the existing analog-to-digital converter in the device's receiver path for providing feedback when in transmit mode. This can leads to higher power consumption in transmit mode due to this additional analog-to-digital conversion in the feedback path. To address this limitation, we propose an adaptive tracking filter with low duty-cycle on/off periods. We show that sacrificing 0.5dB in the null-depth can reduce the ADC power consumption by 95%.

A practical protocol jointly considering PHY and MAC for MIMO based concurrent transmissions in wireless ad hoc networks, called Contrabass, is presented. Concurrent transmissions refer to simultaneous transmissions by multiple nodes over the same carrier frequency within the same interference range. Contrabass is the first-to-date open-loop based concurrent transmission protocol which implements simultaneous channel training for concurrently transmitting links without any control message exchange. Its MAC protocol is designed for each active transmitter to independently decide to transmit with near optimal transmission probability. Contrabass maximizes the number of successful concurrent transmissions, thus achieving very high aggregate throughput, low delays and scalability even under dynamic environments. The design choices of Contrabass are deliberately made to enable practical implementation which is demonstrated through GNURadio implementation and experimentation.

We propose a novel technique for the controlled suppression of aliasing interferers during analog-to-digital conversion. The technique exploits the aliasing inherent in the sampling operation to perform interference cancellation. This is achieved by optimally spreading and weighting the aliasing bands in the frequency domain prior to the analog-to-digital converter (ADC). The resulting filtering response scales automatically with the ADC bandwidth and can be digitally reconfigured to target specific interference profiles to maximize the signal-to-interference-and-noise ratio (SINR). The proposed technique eliminates the need for complex reconfigurable filters in a varying spectral content scenario while maintaining minimal ADC bandwidth and resolution requirements.

We present a novel multiple-input multiple-output (MIMO) decoder accelerator and its associated integrated design environment. The accelerator architecture allows tradeoffs in decoding algorithm, antenna configuration, modulation scheme, and bandwidth at run-time via user programming. The accelerator delivers an improvement over a general purpose digital signal processor (DSP) reaching three orders of magnitude for matrix processing and linear MIMO decoding. The hardware architecture is user-configurable through ten independently set parameters. The parameterization allows independent control over the size and structure of the processing core as well as the structure, size, and access scheme of data memory. We provide a custom high level script and a scalable machine level instruction set and compiler. The elements of hardware configuration and programmability are combined in a user-friendly design flow that takes the MIMO decoder designer from simulation to hardware with dedicated-hardware-like performance in no time.

For a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system that is being pushed into power amplifier (PA) saturation, this letter investigates power-optimized PA clipping. Our goal is to identify the optimum clipping level for a MIMO-OFDM system that delivers the desired bit error rate (BER) with minimum power consumption in the PA. We present a complete theoretical framework resulting in an analytical expression for the BER of a MIMO-OFDM system subject to PA clipping. PA power saving is addressed by the total degradation metric, which shows that as much as 6dB power reduction can be achieved by proper choice of the clipping level.

In this paper we formulate a power loading problem for the spatial subchannels (parallel channels) of a single-carrier MIMO-SVD system. The power loading solution is designed to minimize the energy-per-goodbit (EPG) of the MIMO-SVD system. The optimum power loading obtained by solving a nonlinear fractional program, has a closed-form expression and is found by applying a water-filling procedure. It is observed that the optimum total transmit power (water level) depends not only on the MIMO channel realization, but also on the ratio of circuit power cost (which depends on the number of antennas) to transmit power cost (which depends on path loss and other factors). We study the statistical performance (using simulation) of the solution in Rayleigh and Rician flat-fading channels. Using outage EPG as a measure of performance, we determine the MIMO configuration (from a set of allowed configurations) that yields the minimum outage EPG. It is observed that the average number of spatial subchannels utilized (which indicates preference for diversity or multiplexing) depends on the ratio of circuit power cost to transmit power cost and fading type (Rayleigh or Rician). For both cases, the results show that both multiplexing and diversity obtained by MIMO systems are critical for energy efficiency.

Two fundamental problems related to multi-carrier modulation are considered in this paper. The first is finding the best modulation schemes in all subcarriers to minimize the total energy consumption required to send a given number of bits between two wireless nodes. The second is finding the best modulation schemes to maximize the number of bits transmitted between two nodes under total energy constraint. Discrete bit loading algorithms have been used to solve both problems over frequency selective channels. None of these algorithms takes the circuit energy into consideration. In this paper, we introduce new discrete bit loading algorithms to solve both problems with total energy defined as the sum of transmission energy and circuit energy. Compared to the conventional discrete bit loading algorithms, the new algorithms can achieve savings in the order of 9 dB in the energy minimization problem. 180% increase in the number of transmitted bits can also be achieved with the same total energy when compared to the existing algorithms.

the results of a field trials campaign in a park, a wide street, a narrow street and office environments are presented in this paper. The trials are carried out using a real time 4x4 MIMO-OFDM testbed working in the 2.4 GHz ISM band and a 20 MHz bandwidth. The testbed supports both spatial multiplexing and transmit and receive diversity. We compare the system performance using different number of spatial streams in different environments. We also determine the maximum number of spatial streams that can be supported in the indoor and outdoor environments respectively. We found that the maximum achievable throughput for the system in the indoor environment is almost double the maximum achievable throughput in the outdoor environment at the same receive power. This is mainly due to the richness of scattering elements in an indoor setting. Our outdoor trials showed that supporting more than two spatial streams will not introduce any throughput gain.

In this paper, we develop an energy-efficient power allocation algorithm for the parallel channels of an OFDM system. This algorithm provides the optimum solution to a nonlinear fractional program involving an objective function called energy-per-goodbti (EPG). The EPG objective function models the impact of both transmit power and constant circuit power consumption. The energy minimization problem formulation is quite general and subsumes both maximize rate (MR) and maximize margin (MM) problems as specific cases. As a result, the energy efficiency viewpoint provides a convenient and unified perspective of the various water-filling solutions. Using a numerical example, we show that the energy-efficient solution is quite different from the MM or MR solutions and can provide several dBs of performance improvement. We also study the impact of rounding to discrete constellation sizes.

Reliable spectrum sensing is one of the main challenges in the development of cognitive radios. To achieve the high probability of detection at very low SNRs, some feature detection schemes have been proposed in the literature based on the cyclo-stationarity theorem. One limitation of such methods is their high sensitivity to RF and analog impairments. This paper focuses on analysis and compensation of sampling clock frequency offset (SCFO) between the DAC (at transmitter side) and the ADC (at detector side). We show that for an SCFO of 20 ppm and for a pilot-aided cyclo-stationary detector, the performance degradation in detection thresholds reach 7 dB and 12 dB for 50 ms and 100 ms sensing periods, respectively. We then present a method to compensate for such significant degradations in the presence of SCFO. Our analysis and simulations show that using this technique, we can achieve a performance within 1.5 dB of the no-offset ideal scenario.

Reliable spectrum sensing is the key requirement for secondary users to utilize the spectrum in the absence of primary users. Given the widespread deployment of OFDM modulation in recent and future multimedia broadcasting and wireless broad-band services, reliable spectrum sensing of OFDM waveforms is of great interest for future cognitive radio systems. This paper presents a new detection method for OFDM waveforms that exploits the available embedded pilot tones without any simplifying assumption or constraints on the pilots. Analytical performance characterization is presented and supported by simulation results for both AWGN and fading channels. This study shows that the new method can improve the performance by 5dB in comparison with other feature detectors.

This paper presents a low power VLSI implementation of a novel Multiple-Input Multiple-Output (MIMO) decoder which combines Fixed Complexity Sphere Decoder (FSD) algorithm, real-valued lattice formulation and Pair-wise sorted QR decomposition (P-SQRD) searching approach to simultaneously improve the throughput, bit error rate (BER) and complexity. Two-stage approximate sorting scheme with minimum data swapping is adopted to realize a power efficient architecture. This ASIC is implemented in IBM 90 nm 8 metal layer standard CMOS technology with core area of 1.3 mm2. This design supports 4×4 antenna array with flexible modulations from BPSK to 16-QAM. At 0.8V core power supply, the estimated peak data rate exceeds 1.44Gbps. The estimated energy efficiency is 15.4 pJ/bit which is 50% better than the other state of the art SDs.

Reliable spectrum sensing is the key requirement for secondary users to utilize the spectrum in the absence of primary users. Given the widespread deployment of OFDM modulation in recent and future multimedia broadcasting and wireless broadband services, reliable spectrum sensing of OFDM waveforms is of great interest in cognitive radios. Although feature detection methods outperform energy detection ones, however they are generally more sensitive to RF impairments. In this paper, we have investigated the effects of a non-ideal receiver RF chain on the performance of the Pilot-Aided Cyclo-Stationary Detection (PACSD). Our study demonstrates how the performance of PACSD degrades due phase noise and carrier frequency offset while it is more tolerant to I/Q imbalances.

In this paper, we present a multi-input-multi-output (MIMO) decoder accelerator architecture that offers versatility and reprogrammability while maintaining a very high performance-cost metric. The accelerator is meant to address the MIMO decoding bottlenecks associated with the convergence of multiple high-speed wireless standards onto a single device. It is scalable in the number of antennas, bandwidth, modulation format, and most importantly, present and emerging decoder algorithms. It features a Harvard-like architecture with complex vector operands and a deeply pipelined fixed-point complex arithmetic processing unit. When implemented on a Xilinx Virtex-4 LX200FF1513 field-programmable gate array (FPGA), the design occupied 43% of overall FPGA resources. The accelerator shows an advantage of up to three orders of magnitude (1000 times) in power-delay product for typical MIMO decoding operations relative to a general purpose DSP. When compared to dedicated application-specific IC (ASIC) implementations of mmse MIMO decoders, the accelerator showed a degradation of 340%-17%, depending on the actual ASIC being considered. In order to optimize the design for both speed and area, specific challenges had to be overcome. These include: definition of the processing units and their interconnection; proper dynamic scaling of the signal; and memory partitioning and parallelism.

We present a link adaptation strategy for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based wireless communications. Our objective is to choose the optimal mode that will maximize energy efficiency or data throughput subject to a given quality of service (QoS) constraint. We formulate the link adaptation problem as a convex optimization problem and expand the set of parameters under the control of the link adaptation protocol to include: number of spatial streams, number of transmit/receive antennas, use of spatial multiplexing or space time block coding (STBC), constellation size, bandwidth, transmit power and choice of maximum likelihood (ML) or zero-forcing (ZF) for MIMO decoding. Additionally, we increase the fidelity of the energy consumption modeling relative to the prior art. The resulting solution allows us to easily and quickly search the space of possible system parameters to deliver on the QoS with minimal energy consumption. Moreover, it provides us insight into where crossovers occur in the choice of the radio parameters. Application of the results to a generic MIMO-OFDM radio shows that the proposed strategy can provide an order of magnitude improvement in energy efficiency or data throughput relative to a static strategy.

This work presents a theoretical framework for determining the optimal clipping level for a multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) system. We start by modeling of the clipping noise, and propose a maximum likelihood (ML) receiver for the resulting signal. The bit error rate (BER) for this ML receiver is then derived for MIMO spatial multiplexing, space time coding, and cyclic delay diversity schemes. A search through the tradeoff space between the BER performance penalty and power amplifier (PA) power consumption advantage reveals optimum clipping levels of the system. Using an IEEE 802.11n like system, we show that the optimum clipping as predicted by this approach can improve PA power efficiency as much as 70%.

We introduce an importance sampling (IS) method that successfully simulates the performance of Low density Parity Check (LDPC) Codes in an AWGN channel at very low bit error rates (BERs). By effectively finding and biasing bit node combinations that are the dominant sources of error events, called trapping sets, the developed technique provokes more frequent decoder failures. Consequently, fewer simulation runs and higher simulation gains are achieved.

The detection of MIMO signals presents a formidable computation challenge due to the inherent complexity incurred by matrix-type wireless fading channels. To reduce the hardware cost, linear MIMO detection algorithms can be used instead of non-linear space-time processing. Conventionally, adaptive linear algorithms such as RLS and LMS are treated separately in implementation from matrix inversion based algorithms such as ZF and MMSE. In this paper, we identify the similarities between the QR-decomposition based forms of RLS and MMSE MIMO detection and present a unified systolic array implementation that can accommodate both. In the meanwhile, some subtle but important differences also exist. As a result, the unified array must have a size of Nrx Nr(assuming Ntles Nr) and both array inputs and array operations need to be controlled accordingly. As special cases of RLS and MMSE, LMS and ZF can also be realized in the same array, making the array flexible enough for all linear MIMO detection algorithms.

In this paper, the problem of choosing constellation size and transmit power that minimizes the energy-per-goodbit in a frequency-flat, long-term static fading channel is considered. A goodbit is a bit that is received without error. A low-complexity, near-optimal algorithm to find constellation size and transmit power is presented. Although perfect channel state information (CSI) at the transmitter is assumed, fading can cause situations where the system cannot meet user constraints on acceptable performance. The implications of this type of system outage and the consequent trade-off between energy-efficiency and range are discussed. Assuming that the fading process is ergodic, the average energy-efficient spectral efficiency can be computed for various fading distributions of interest. As an application of the techniques developed, we evaluate, by simulation, the average minimum energy-per-goodbit for a Rayleigh fading channel and note that over a short range of distances (1 m to 10 m), it can cost at most 1 dB more than for the corresponding non-fading AWGN channel. We also show that the restriction of using only a square-QAM constellation does not incur a large penalty in cost.

In this paper, an energy minimization problem for a quadrature amplitude modulation (QAM) system with automatic repeat request (ARQ) is considered. We formulate an optimization problem with constellation size and transmit power as the variables. We give an efficient and accurate geometric programming (GP) approximation to the problem and illustrate the solution using a numerical example. We learn that the transmit power or equivalently bit error rate (BER) should be carefully chosen for minimal energy. Our analysis provides guidelines for setting the BER in an energy optimal way for various packet lengths.

The 11 papers in this special issue focus on adaptive, spectrum agile and cognitive wireless networks. Some of the topics covered include: fundamental performance aspects of spectrum sharing; spectrum sensing capabilities in cognitive radios; detection and estimation approaches to spectrum sharing; MAC design in opportunistic spectrum access; and competitive spectrum sharing and resource allocation using concepts from game theory and pricing.

This paper presents an architecture that utilizes spectrum sensing with a Weaver architecture receiver to ease the requirements on the RF front-end components. With the ability to sense the environment, large interferers that overwhelm small desired signals can be avoided. The learning and adapting capabilities are enabled by means of a flexible receiver architecture employing variable local oscillators (LOs) at RF and intermediate frequency (IF). Avoiding large interferers can reduce image rejection ratio (IRR) requirements for a given performance and results in a greater tolerance to the Q of filters and receiver mismatches. The design approach is applied to an IEEE 802.11a receiver and the results show that for a given performance level, the proposed design requires an IRR that is 40 dB less than that required for conventional Weaver receivers

Theoretical capacity calculations and corresponding simulations show significant capacity/throughput gains from MIMO systems. Whether these gains are achievable in a real system, deployed in a practical environment, depends on a variety of factors, such as the choice of the communication algorithms, analog impairments and the "quality" of the wireless channel to sustain MEMO communications. In this paper, a 5.25 GHz broadband MIMO-OFDM testbed is described along with field measurements conducted with it. The MIMO-OFDM communication algorithms and also the impact of analog impairments on the performance of the system are described. Detailed system calibration results are described which serve as a baseline for results of field measurements. The results of wireless measurements are compared with the theoretical capacity, computed with the channel estimates obtained during the demodulation process. The average achievable capacity in the indoor wireless environment is shown to be 9.97 bps/Hz (bits per sec per Hz) while the capacity loss due to analog impairments and the choice of algorithms is about 2.33 bps/Hz. Also, field measurements conducted with the system in various environments are presented comparing the average throughput/capacity achieved in each of these environments.

We introduce an Importance Sampling (IS) method that successfully simulates the performance of Low density Parity Check (LDPC) Codes in an AWGN channel at very low bit error rates (BERs). By effectively finding and biasing bit node combinations that are the dominant sources of error events, called trapping sets, the developed technique provokes more frequent decoder failures. Consequently, fewer simulation runs and higher simulation gains are achieved. Regardless of the block size of an LDPC code, only a few dominant trapping set classes cause decoder failures at low BER regions. Therefore, the proposed technique allows the performance evaluation for any size LDPC code at very low BER regions with remarkable simulation gains. For BERs of 10-20, we observed simulation gains on the order of 1014.

This paper presents an architecture that utilizes spectrum sensing with a Weaver architecture receiver to ease the requirements on the RF front-end components. With the ability to sense the environment, large interferers that overwhelm small desired signals can be avoided. The learning and adapting capabilities are enabled by means of a flexible receiver architecture with variable local oscillators (LOs) and tunable bandpass filters. Avoiding large interferers can reduce image rejection (IR) requirements for a given performance level and results in greater tolerance for low-Q filters and receiver mismatches. The design approach is applied to an IEEE 802.11a receiver and has shown significant performance improvements compared to conventional approaches despite an 18 dB lower image rejection.

MIMO-OFDM is being considered for communication systems where high throughput and spectral efficiency are important factors. Analog impairments like I/Q mismatch and phase noise significantly degrade and limit the performance of communication systems. In this paper, we analyze the impact of I/Q mismatch and phase noise in MIMO-OFDM systems as a function of the number of antennas. We show the improvement in performance that is possible when these impairments are cancelled. We also discuss the impact of correlated and uncorrelated phase noise in MIMO-OFDM systems. Finally, we show the results of I/Q mismatch and phase noise cancellation in wireless measurements performed using a 2times2 MIMO-OFDM testbed

Cooperative relays have recently been proposed and studied for mobile ad hoc networks. It has been shown that under perfect symbol synchronization, parallel relays with space-time modulation can yield more than 10 dB power savings over conventional serial relays in a highly mobile environment. We analyze the effect of synchronization errors on the performance of parallel relays, and also study several effective methods to reduce the negative impact of synch errors. Our study shows that when the synch errors are much smaller than a symbol interval, the performance of parallel relays deteriorates gracefully. We have also found that well designed space-time coding techniques, such as TR-STC and ST-OFDM, can be highly effective in combating large synch errors with only marginal reduction of data rate.

In this work, we introduce a unique technique that improves the performance of the BP decoding in waterfall and error-floor regions by reversing the decoder failures. Based on the short cycles existing in the bipartite graph, an importance sampling simulation technique is used to identify the bit and check node combinations that are the dominant sources of error events, called trapping sets. Then, the identified trapping sets are used in the decoding process to avoid the pre-known failures and to converge to the transmitted codeword. With a minimal additional decoding complexity, the proposed technique is able to provide performance improvements for short-length LDPC codes and push or avoid error-floor behaviors of longer codes

A MIMO OFDM receiver with interference cancellation requires a channel estimation and matrix inversion on a sub-carrier basis. This is computationally intensive and can prevent a practical implementation. In this paper, we propose new interpolation algorithms for this type of receiver. To eliminate the need for a matrix inversion for each OFDM sub-carrier, the weight matrix is interpolated. Algorithms to determine the interference cancellation detection order are also introduced. The computation requirement of this receiver is greatly reduced while the performance degradation is acceptable. For example, for a normalized product of the rms delay spread by the interpolation factor below 6.25%, the degradation compared to the optimal solution is 2dB. The results also demonstrate that the impact of the sub-optimal interference cancellation detection order is non-significant compared to the coefficient matrix interpolation error and that using interference cancellation in a MIMO OFDM system with interpolation significantly improves the performance.

This paper describes the development and implementation of a 5.25 GHz radio transceiver for a MIMO testbed. The transceiver is built with off the shelf components on several printed circuit boards. A modular concept enables expansion of the system from a simple 1/spl times/1 SISO to a complex 8/spl times/8 MIMO setup. Calibration curves and MIMO wireless measurements of the fully functional testbed are presented.

The implementation of wideband MIMO systems poses a major challenge to hardware designers due to the huge processing power required for MIMO detection. To achieve this goal with a complete VLSI solution, channel coding and MIMO detection are preferably separated so that each of them can be fitted into a single chip. In this paper, a comparative study is presented regarding various uncoded adaptive and non-adaptive MIMO detection algorithms. Intended to serve as a reference for system designers, this comparison is performed from several different perspectives including theoretical formulation, simulated BER/PER performance, and hardware complexity. All the simulations are conducted within MIMO-OFDM framework and with a packet structure similar to that of the IEEE 802.11a/g standard. As the comparison results show, the RLS algorithm appears to be an affordable solution for a wideband MIMO system targeted at gigabit wireless transmission. As a direct result of this work, an ASIC for a 25 MHz wideband 8 /spl times/ 8 MIMO-OFDM system using RLS has been designed and fabricated.

This paper presents a complete study and characterization of a real-time frequency-hopped, frequency shift-keyed testbed capable of transmitting data at 160 kb/s, with hopping rates of up to 80 Khops/s operating in the 900 MHz band. The system provides the highest hopping rate reported to date and sets a new trend for FHSS communications with superior low probability of interception/detection and anti-jamming (LPI/LPD/AJ) capabilities. The architecture features a direct digital frequency synthesizer to enable high-rate hopping, and a frequency correlator-based demodulator, plus all digital timing and frequency recovery algorithms to minimize complexity. Furthermore, single sideband modulation was used to achieve spectral efficiency. The testbed is software configured and provides the user with full control over the diversity combining techniques, symbol interleaving, packet structure, and acquisition protocols. A total of 5850 independent experiments were carried out under various receiver configurations and wireless environments. The results underscore the dramatic potential for a system that optimally combines high-rate hopping, interleaving, and equal gain combining to combat severe propagation conditions, including multipath fading and intentional jamming.

This paper reports the development of a configurable, non-real-time, 2/spl times/2 multi-antenna testbed operating at 5.25 GHz with a bandwidth of 25 MHz.. MIMO-OFDM modulation and demodulation are performed offline. The unit is fully calibrated and was used in extensive indoor wireless measurements at UCLA. The paper reports measurements for SISO, SIMO 1/spl times/2 and MIMO 2/spl times/2 setups in line of sight and nonline of sight channels. These measurements show that LOS channels are not necessarily detrimental for MIMO performance. Further experiments show that with -3 dBm of transmit power uncoded data rates of 140 Mbit/s are achievable.

Tracking algorithms for carrier and sampling clock frequency offset are proposed for pilot assisted MIMO-OFDM in frequency selective channels. The methods are based on maximum likelihood estimation theory applied to observations of received pilot subcarriers at the output of the FFTs of the receiver. Simulation results show that larger MIMO configurations benefit from lower estimator variances, providing increased synchronization accuracy at low SNRs, or allowing for a reduction of the number of pilot subcarriers.

Filter banks are efficient and essential signal processing blocks for design and implementation of multi-rate multi-band communications and signaling. In this paper we analytically study and derive the optimum choice of design parameters and filter bank structure to minimize power consumption and implementation cost for a programmable multi-rate transmit filter bank for OFDM. The optimization is performed on two fronts. We first perform system-level power and complexity analysis to define the optimum choice of filter parameters. Then through a hardware-level optimization, an efficient filter bank structure is introduced that results in at least a factor of 4 power reduction and also a complexity reduction from 6.95GOPS to 1.73GOPS for the multi-rate filter bank over the baseline design.

We propose a new technique for initial channel estimation in packet-mode MIMO-OFDM. A maximum likelihood estimator uses observations of the channel spectra obtained by means of a carefully designed multi-transmitter channel probing sequence. The sequence length is controlled by two competing parameters, which allow for an explicit and fine trade-off between the channel estimation overhead and the quality of the resulting estimates. Compared to the training sequence used in the IEEE 802.11a standard, our technique requires 6 to 8 times less training symbols in order to attain channel estimates of equal accuracy.

MIMO-OFDM systems are gaining prominence in high data rate applications due to their increased spectral efficiency, in MIMO-OFDM systems, I/Q mismatch causes interference between the frequency mirror subcarriers. This can significantly degrade and limit the performance of the system. We present a mathematical analysis of the effects of I/Q gain, delay and phase mismatches in MIMO-OFDM systems. We show the impact of I/Q mismatch when more antennas are added. We also derive a linear optimal solution to cancel all three I/Q mismatches and an RLS based adaptive filter for the same. We show simulation results and results on an experimental testbed.

Detection of MIMO signals presents a formidable computation challenge due to the inherent complexity incurred by matrix-type wireless fading channels. When combined with OFDM, the MIMO detection over frequency-selective fading channels is divided into that over each flat-fading subchannel. Since this is usually done on per subchannel basis, it implies significant hardware cost. This work proposes the use of linear interpolation to exploit the correlation among adjacent subchannels. Since the weight matrices on non-pilot subchannels are linearly interpolated from those of pilot subchannels, the computation overhead is greatly reduced. This is particularly effective when the subchannel bandwidth is much smaller than the coherence bandwidth of the channel. Simulation using MMSE MIMO detection shows that, by keeping the product of interpolation factor and normalized RMS delay spread within a certain limit, linear interpolation can result in minimum degradation of system performance up to a practical SNR level of 20 dB to 25 dB.

An OFDM receiver ASIC specifically targeting mobile applications has been designed and fabricated. The receiver ASIC incorporates the digital front-end receiver, pre/post-FFT processing units, a 1024-point complex (I)FFT processor, channel estimator and corrector, all digital synchronization loops, and control and configuration interface. In this paper we describe the VLSI implementation and optimization for the ASIC that entails a compact design with low power dissipation. Finite-precision effects are analyzed to minimize the computation requirements for major signal processing blocks. Performance and implementation cost tradeoff on time/frequency synchronization and channel estimation are also analyzed. The final receiver ASIC dissipates only 32 mW while providing a date-rate of 8 Mb/s (uncoded) in highspeed mobile environments, making the chip suitable for packet-based mobile applications.

The first step in the design and implementation of a MIMO-OFDM system is to analyze the characteristics of the baseband modulated signal. This is crucial in order to minimize the complexity of the digital signal processing. This paper demonstrates the generalized derivation of the minimum word-length requirements for the main data-path in OFDM systems based on the analytical evaluation of the normalized power distribution, clipping noise effect, and essential dynamic range. We have formulated the minimum word-length requirements for the main data path in both transmitter and the receiver. The theoretical analyses are also supported by system level simulations.

In this paper, a novel architecture for a fine timing-tracking loop is presented. The loop is a second order feedback loop utilizing an interpolating filter to relax the requirements on the analog to digital section of the receiver. A complete analysis of the interpolating filter, loop filter along with a system simulation based on direct sequence spread spectrum are presented.

A new receiver structure able to deliver high data rates in a multiple-input multiple-output (MIMO) frequency selective wireless environment is proposed and investigated. The optimal solution for the finite length MIMO decision feedback equalizer (DFE) with cancellation is derived and used to illustrate the potential of this architecture for space-time communications. LMS and RLS adaptive algorithms are also presented for the MIMO architecture. The convergence and performance of these algorithms is confirmed through simulation results. The proposed adaptive solutions do not require channel identification, are less computationally intensive than the optimal solution and allow the proposed MIMO receiver to adapt to channel changes.

In this paper, a compact architecture for direct digital frequency synthesis (DDFS) is presented. It uses a smaller lookup table for sine and cosine functions compared to existing architectures, with minimal hardware overhead. The computation of the sinusoidal values is performed by a parabolic interpolation structure, thus only interpolation coefficients need to be stored in the read-only memory (ROM). A DDFS with 64 dBc SFDR, 10-bit output resolution and 32 bit phase accumulator requires only 104 bits of ROM storage. The ROM size is consistently less than 1 Kbits for SFDR up to 85 dBc.

A forward error correction (FEC) strategy and a medium access control (MAC) protocol that are thoroughly tailored to complement and support a high-speed asymmetric physical layer design based on equalization and precoding is presented and fully discussed. Both proposals exhibit a high degree of adaptability and flexibility, which allows for increased data throughput while providing a wide range of quality-of-service requirements. Fast link layer adaptation is made possible through the joint design of link and physical layers. The adaptive FEC algorithm is based on the use of variable-rate trellis coded modulation with fast channel estimation, while the MAC protocol employs a centralized, dynamic slot allocation technique. The overall system design is shown to achieve high spectral efficiency, while minimizing energy consumption at the portable unit.

This paper reports on field measurements of point-to-point indoor high-speed (10 Mbit/s to 30 Mbit/s at 5 Mbaud) wireless communications realized using a flexible multilevel quadrature amplitude modulation (M-QAM) testbed that features real-time equalization and smart antenna-array technology. The results from an extensive set of measurements, 59262 trials in all, performed without cochannel interference under various receiver configurations and wireless environments are presented and analyzed. The results underscore the dramatic potential for a system that optimally combines equalization and a smart antenna array. For example, using only 10 mW of transmit power, the system delivered 30 Mitts at an uncoded bit error rate (BER) of 10/sup -3/ with 5% outage at a coverage radius of 20 in. For a lower data rate of 10 Mbitts, the coverage radius was increased to 32 in, the uncoded BER dropped below 10/sup -7/, and the outage improved to 1%. The field measurements indicate that a 4-tap feedforward-filter decision-feedback equalizer with eight feedback-filter taps is sufficient to mitigate the intersymbol interference for typical indoor environments. They also show a significant gain when using a smart antenna array. For example, when transmitting between rooms at a 2% outage probability, the signal-to-noise ratio (SNR) improves by 8.3 dB when using two antennas instead of one antenna. Doubling the number of antennas to four provided an additional SNR improvement of 5.2 dB. The paper also presents simulation results that confirm the performance trends observed from the field measurements.

In this paper, we present the system design of an integrated receiver for the next generation high-speed mobile data communications based on OFDM. First an overview of the OFDM system suitable for mobile communications is described. The effects of non-ideal transmission conditions of the OFDM system including channel estimation errors, symbol timing offset, carrier and sampling clock offset, phase noise and time-selective fading are analyzed. We then propose an integrated receiver in which all of these issues relevant to the mobile transmission environment are addressed. Novel techniques for symbol timing and frequency synchronization are proposed. The architecture of the receiver and resulting implementation complexity are then analyzed. The overall performance degradation of the proposed receiver is simulated to be within 2-2.5 dB of the ideal receiver in various channel conditions.

This paper presents a computationally efficient, low-power algorithm for the decoding of space-time block codes using a previously reported maximum likelihood algorithm (Tarokh et al. 1998). The new algorithm arrives at the same decision with less computation, and therefore reduces power dissipation for an actual implementation. Based on the previous algorithm, we develop a more efficient algorithm by utilizing the properties of the constellation signals and reported decision metrics. In most cases, the new algorithm results in more than 50% reduction in the computation complexity. The algorithm is illustrated for BPSK, QPSK, PSK or QAM.

We consider the performance of a turbo code with 2 transmit antennas and 2 receive antennas for flat fading channels. We consider robustness within and among families of channels with the same singular values (SVs). When the two SVs are similar, the bit error rate performance is robust in terms of proximity to the channel capacity. When the 2 SVs differ significantly, robust performance can be achieved by a time-varying linear transformation (TVLT). Iterative decoding is the same as for standard parallel concatenated convolutional codes, except that the extrinsic information about encoder outputs are also exchanged.

This paper describes the implementation of a novel direct digital frequency synthesizer (DDFS) based fast frequency-hopping frequency shift keyed (FSK) testbed capable of transmitting 160 kbps with a hopping rate up to 80 khops/s. The testbed is equipped with a variety of user-defined parameters in order to fully characterize its performance over the physical channel. Initial measurements using the testbed were carried out in different wireless environments and indicate that higher hopping rates can significantly improve radio link quality when proper diversity combining is utilized.

A frequency-shift keying (FSK) receiver has been designed for deep space applications which exhibits potential for ultra low power performance. The receiver is based on a novel, almost all-digital architecture. It supports a wide range of data rates and is very robust against large and fast frequency offsets due to Doppler. The architecture utilizes subsampling and 1-bit data processing together with a discrete Fourier transform-based detection scheme to enable power consumption dramatically lower than implementations reported in the literature. Novel and power-efficient algorithms are derived for frequency and timing tracking. Most of the power saving techniques are applicable to a variety of applications, but some are achieved by taking advantage of the asymmetric power constraints for the receiver and the transmitter as well as the absence of adjacent channel interferers. The worst-case bit-error rate (BER) performance of the receiver is just 2.5 dB below that of the optimal uncoded noncoherent FSK receiver at a BER of 10/sup -6/ and better for lower BERs.

A new flexible architecture is proposed for word-serial filtering and variable rate decimation/interpolation. The architecture is targeted for low power applications requiring medium to low data rate and is ideally suited for implementation on either an ASIC or an FPGA. It combines the small size and low power of an ASIC with the programmability and flexibility of a DSP. An efficient memory addressing scheme eliminates the need for power hungry shift registers and allows full reconfiguration via a single ROM. The decimation ratio, filter length and filter coefficients can all be changed in real-time. The architecture takes advantage of coefficient symmetries in linear phase filters and in polyphase components.

The problem of identifying the optimum adaptive channel code rate that achieves a target error performance in a wireless precoded link operating over slowly varying channels is addressed. It is found that pragmatic trellis coded modulation (PTCM) with adaptive QAM constellation is a natural fit to the problem at hand. It supports variable coding rates while providing decoder simplicity. Moreover, it enables the efficient exploitation of the unique features of a precoded system. The article also describes a simple technique for fast code rate adaptation based on channel estimation algorithms that combine the use of intra-frame and inter-frame physical layer information. As a consequence of the implementation of these adaptive techniques, a significant increase in data throughput is achieved, as verified in simulations.

An FSK receiver has been designed for deep space applications which exhibits potential for ultra low power performance. The receiver is based on a novel, almost all-digital architecture. It supports a wide range of data rates and is very robust against large and fast frequency offsets due to Doppler. The architecture utilizes subsampling and 1-bit data processing together with a DFT based detection scheme to enable power consumption dramatically lower than implementations reported in literature. Novel and power efficient algorithms are derived for frequency and timing tracking. Most of the power saving techniques are applicable to a variety of applications, but some are achieved by taking advantage of the asymmetric power constraints for the receiver and the transmitter as well as the absence of adjacent channel interferers. The worst case BER performance of the receiver is just 2.5 dB below that of the optimal uncoded non-coherent FSK receiver at a BER of 10/sup -6/ and better for lower BERs.

In this paper, the use of base station antenna arrays in wireless code division multiple access (CDMA) systems is studied. Different beamforming algorithms are simulated based on the cdma2000 uplink physical layer specifications. The performance of these algorithms is compared as a function of the number of active users under realistic conditions. The paper also presents a post-processing multi-user-beamforming (MUB) algorithm to be applied at the base station. Simulation results show an improvement in performance after applying MUB.

This paper reports on the development of a flexible 5-MBd equalized M-QAM testbed for high-speed wireless data communications. The unit operates in real time and was field tested in typical indoor environments. A total of 3600 independent experiments were conducted using the testbed where one of 4-, 16-, 64-QAM constellations were transmitted, and the performance as a function of adaptive equalization and antenna selection diversity was studied. The experimental results presented here help put previous simulation and analytical results into perspective and demonstrate some of the performance bounds associated with a practical implementation. The field trial results show that with only 10-mW of transmit power, reliable 10-Mbit/s data communication can take place in between rooms with a coverage radius of 17 m, and better than 15% outage at an uncoded bit error rate (BER) of 10/sup -3/. The addition of two-branch antenna selection diversity to the system would allow 10-Mbit/s transmission at better than 3% outage and 20-Mbit/s transmission at 10% outage in the same environment. Moreover, 30-Mbit/s data transmission is feasible when both the transmitter and receiver are located within the same room, albeit at higher outage levels. In general, the results demonstrate the tremendous impact that adaptive equalization can have on the achievable performance of indoor links. Average SNR was improved anywhere from 8 dB to 12 dB depending on the particular environment and data rate. The impact on the outage performance, however, was much more dramatic.

A flexible and reconfigurable signal processing ASIC architecture has been developed, simulated, and synthesized. The proposed architecture compares favorably to classical DSP and FPGA solutions. It differs from general-purpose reconfigurable computing (RC) platforms by emphasizing high-speed application-specific computations over general-purpose flexibility. The proposed architecture can he used to realize any one of several functional blocks needed for the physical layer implementation of data communication systems operating at symbol rates in excess of 125 Msymbols/s. Multiple instances of a chip based on this architecture, each operating in a different mode, can be used to realize the entire physical layer of high-speed data communication systems. The architecture features the following modes (functions): real and complex FIR/IIR filtering, least mean square (LMS)-based adaptive filtering, discrete Fourier transforms (DFT), and direct digital frequency synthesis (DDFS) at up to 125 Msamples/s. All of the modes are mapped onto a common, regular data path with minimal configuration logic and routing. Multiple chips operating in the same mode can be cascaded to allow for larger blocks.

The accuracy with which simulation techniques can be used to predict the performance of an experimental high speed M-QAM testbed in typical indoor wireless environments is studied in this paper. Ray tracing techniques were used to produce impulse responses for specific transmit and receive antenna locations where actual measurements were taken using a highly versatile equalized M-QAM testbed operating at 10 Mbps, 20 Mbps and 30 Mbps. The results obtained using a simulation of the testbed plus the ray-traced channel are compared with those obtained with experimental field trials. The paper presents the results of such comparisons for a total of 2,322 trials, where a trial is defined as a simulation run plus a field test for a given data rate and antenna location. The study shows that when quantization noise is factored into the simulation, the performance predicted using the simulation approach closely matches the experimental data.

A novel set of techniques for microscopic space diversity in high-speed wireless systems that employ decision feedback equalization (DFE) to mitigate ISI is postulated and evaluated via simulations. Adaptation on a packet basis rather than on a symbol basis is shown to be a simple and effective choice in indoor, low-mobility environments. Antenna selection based on slicer signal-to-noise ratio and largest-ray antenna combining are shown to improve the performance of a DFE-based link by margins that are similar to those obtained by conventional selection and combining techniques operating over flat fading channels. Furthermore, in the context of asymmetric systems that employ DFE on the reverse link and channel precoding on the forward link, the proposed techniques positively impact both links without adding any circuitry to the downlink receiver.

High data rates and portability are conflicting requirements in the design of reliable indoor wireless data communication systems. Asymmetric system design addresses this problem by exploiting the major differences in the availability of resources (i.e., energy, space) that exist between base stations and portable units. Such an approach seeks to concentrate most of the signal processing tasks involved in the operation of the two-way wireless link at the base station. This paper presents and discusses the implications of a set of techniques for asymmetric physical layer system design based on the use of channel precoding for forward transmission. System performance is assessed via simulations using a realistic time-varying channel model. Simplified two-way antenna diversity implemented exclusively at the base station and the key issue of automatic gain control at the receive side of the precoded link are also addressed and evaluated. Simulation results confirm that the adoption of these techniques enables reliable digital communication at a data rate of 20 Mbit/s on both links while significantly reducing the power consumption of the portable unit.

This paper provides an overview of present trends in the development of integrated circuit technology for applications in the wireless communications industry. Through advanced circuit, architectural and processing technologies, ICs have helped bring about the wireless revolution by enabling highly sophisticated, low cost and portable end user terminals. Two broad categories of circuits are highlighted. The first is RF integrated circuits and the second is digital baseband processing circuits. In both areas, the paper presents the circuit design challenges and options presented to the designer. It also highlights the manner in which these technologies have helped advance wireless communications.

An elegant means by which high-speed burst wireless transmission can be accomplished with small amounts of overhead is through a novel technique referred to as clustered-OFDM. By using OFDM modulation with a long symbol interval, this scheme overcomes the complex and costly equalization requirements associated with single carrier systems. Moreover, the need for highly linear power amplifiers typically required in OFDM systems is alleviated through the use of multiple transmit antennas combined with nonlinear coding. The clustering technique also leads to a natural implementation of transmit diversity. This paper reports on the implementation of a clustered-OFDM transmitter which was constructed using off-the-shelf components in a relatively short period of time. Despite the limitations associated with the prototyped system, it can deliver 7.5 Mbps and it is expected that this data rate could be easily tripled with existing technology. In addition to providing an introduction to clustered-OFDM, we describe the various architectural trade-offs which were made in order to reduce the hardware complexity of the boards as well as some experimental results showing the operation of the transmitter.

An all digital-QAM system is studied here for high-speed data transmission on digital subscriber loops. All elements of the system have been fully investigated and their effects on overall system performance documented. In particular, the following issues have been explicitly addressed: utility of an adaptive error-prediction (noise-prediction) filter; equalizer size and convergence; blind equalization; choice of constellation size and center frequency; combined timing/carrier recovery; and the oversampling requirements of a numerically-controlled-oscillator (NCO) for potential use in the timing and carrier recovery loop. The paper relies on simulation results to investigate and quantify the overall interactions of the various blocks. Throughout the paper emphasis has been placed on reducing the complexity of the system for implementation on an integrated circuit (IC). It is expected that the information provided here will serve as a good starting point for an IC designer to start the implementation of a transceiver ASIC

A 60-MHz 64-tap adaptive finite-impulse-response (FIR) filter chip was fabricated in 1.2- mu m CMOS. It can implement either an echo canceler or a decision feedback equalizer for 2B1Q high bit rate digital subscriber line (HDSL) transceivers. The 4.3*4.3 mm/sup 2/, 30000 transistor chip is a completely self-contained adaptive filter which incorporates the least mean square (LMS) algorithm for coefficient updating. The device can be cascaded to implement very long filter lengths, which are often required in high bit rate transceivers. At a 60-MHz clock rate, the echo canceler/decision feedback equalizer chip can accommodate symbol rates in excess of 800 kbaud.>