Sandeep Chennakeshu

Decision feedback equalization for digital cellular radio

The authors study an adaptive decision feedback equalizer (DFE) for application in the USA digital cellular radio telephone system. A synchronous DFE and a fractionally spaced DFE are adaptive and use a fast recursive least squares algorithm to track rapid channel variations. Simulation results indicating the performance sensitivity to time delay spread, Doppler shift, and timing jitter are presented. A DFE using a complex fast-Kalman adaptation algorithm is presented, and its bit error rate performance evaluated. The fast Kalman equalizer is found to possess good tracking ability and can track channel variations at vehicle speeds of 50 mph (80 km/h). Sensitivity to sample timing jitter can be reduced by using a DFE with fractionally spaced feedforward taps.<<ETX>>

Differential detection of pi /4-shifted-DQPSK for digital cellular radio

The detection of pi /4-shifted-DQPSK modulation using a tangent-type differential detector with an integrated symbol timing and carrier frequency offset correction algorithm is discussed. pi /4-shifted-DQPSK modulation has been proposed for use in a high-capacity, TDMA-based digital cellular system being developed in the US; differential detection could potentially allow the production of low-complexity mobile units. Results obtained using the proposed IS-54 TDMA frame structure for base to mobile transmissions are presented. Theoretical and simulation bit-error-rate (BER) results are presented for static and Rayleigh fading channels. BER results are provided as a function of E/sub b//N/sub 0 /and C/I, where the interferer is a second pi /4-shifted-DQPSK signal. Additional results are provided which show the BER sensitivity to Doppler frequency shifts, time delay spread, and carrier frequency offsets. >

Capacity analysis of a mixed mode slow frequency hopped cellular system

The capacity of a mixed mode slow frequency hopped time division multiple access (TDMA) based cellular system via a theoretical development of outage probability is analyzed. Expressions for outage probability are provided for mobile to base transmission considering voice activity, power control, and antenna diversity. The outage probability expressions are derived as functions of mobile locations. These expressions are used to evaluate system capacity (users/megahertz/cell) based on simulating mobile locations for a system with 37 hexagonal cells.

Spectral shaping using coded modulation for mobile radio

Spectral efficiency for land mobile radio (LMR) channels is primarily limited by adjacent channel interference (ACI). Conventional digital modulation schemes do not achieve current specifications for ACI at high data rates, in the absence of powerful interference mitigation schemes. A novel trellis coded modulation scheme (TCM) scheme was presented by S. Chennakeshu et. al., (see Proc. Fifth Tirennia International Workshop, Tirennia, Elsevier, p.329-40, 1991) that has the potential to provide high data rates and at the same time, mitigate adjacent channel interference. The main idea was that the TCM scheme must be optimized to provide the desired spectral shaping. The authors explore this idea further and show how the various components of the system can be optimized to provide the desired gains in spectral shaping.<<ETX>>

An adaptive lattice decision feedback equalizer for digital cellular radio

A study of an adaptive decision feedback equalizer (DFE) based on a lattice structure, for application in the US digital cellular radio telephone system, is conducted. The scheme uses a least-squares algorithm that is capable of tracking rapid channel variations. The equalizer exploits the order-recursive nature of the lattice structure to adaptively vary the number of taps, thereby achieving a relatively uniform bit error rate (BER) performance over a wide range of delay spreads. Simulation results illustrate the equalizers sensitivity to delay spread variation, Doppler shift, and sample timing jitter. Comparisons are made with the performance of a fractionally spaced DFE that uses a complex fast Kalman algorithm. Issues of complexity and finite precision implementation are addressed.<<ETX>>

Performance of coded slow-frequency-hopped TDMA cellular systems

The outage probability as a function of the number of users/MHz/cell is adopted as the performance measure for evaluating the capacity of a time division multiple access (TDMA) based slow frequency hopped cellular system. A Gilbert-Elliott model to arrive at an estimate of system capacity is used. The Gilbert-Elliott model and expressions for codeword outage probability are derived. Results are provided for mobile to base transmission considering voice activity, power control, antenna selection diversity, and channel coding. The analysis presented is general, but results are restricted to a shortened Reed-Solomon code.

Performance of differentially detected pi /4-shifted DQPSK with diversity

The application of postdetection selection combining diversity in land mobile radio systems is addressed. Selection of an antenna-receiver pair is based on metrics that are functions of the signal-to-impairment power ratio at the outputs of differential detectors. That is, postdetection selection metrics are used. Because these schemes operate on the detector output, they can be used with hard-limited IF signals. The schemes are evaluated using the North American digital cellular channel structure and modulation. Simulation results are reported which show performance improvements in the presence of cochannel interference, multipath, and Doppler.<<ETX>>

Performance of a spectrally efficient modem for land-mobile radio

The spectral efficiency in land mobile radio (LMR) systems is limited by stringent adjacent-channel interference protection ratio (ACIPR) specifications of at least 60 dB. High spectral efficiency can be achieved using digital modulation techniques. Linear modulation schemes exhibit high spectral efficiencies but require linear power amplifiers. Power amplifiers typically used in LMR are nonlinear which degrade the ACIPR for conventional linear modulation schemes. The paper presents simulated performance results of a trellis-coded linear modulation scheme that is designed to be less sensitive to the nonlinearity exhibited by the power amplifier. Simulation results indicate that an ACIPR of 60 dB can be achieved. An E/sub b//N/sub 0/ of 14 dB is required to achieve a bit error rate of 3% with 3 users per 25 kHz channel, with an average data rate of 9.6 kbits/second/user.<<ETX>>

Improving ACIPR for land mobile radio systems using coded modulation

The spectral efficiency for land mobile radio (LMR) systems is primarily limited by adjacent channel interference (ACI). Conventional digital modulation schemes do not meet current ACI protection specifications, unless augmented by powerful interference mitigation techniques. High spectral efficiency and ACI protection can be achieved by using trellis codes that are specifically designed to maximize the adjacent channel interference protection ratio (ACIPR). A spectral optimization is done in conjunction with appropriate transmit and receive filters and constraints on peak-to-average transmit power ratio and minimum distance of the code. The theory of spectral shaping to maximize ACIPR is discussed, and examples are presented to illustrate the potential of the approach for LMR systems.<<ETX>>