Cenk Sahin

Analysis of symbol-design strategies for intrapulse radar-embedded communications

The design of communication symbols that may be embedded on an intrapulse basis into the backscatter generated by a high-power, pulsed radar is considered. This framework requires the asynchronous detection of transmitted symbols in a high-interference environment that degrades the capabilities of conventional intercept receivers. The impact of symbol design and filter structure upon the successful detection of covert symbols by the intended receiver and a hypothetical partially clairvoyant intercept receiver is examined.

A novel approach for embedding communication symbols into physical radar waveforms

Due to constantly increasing demand from commercial communications, defense applications are losing spectrum while still striving to maintain legacy capabilities, not to mention the need for enhanced performance. Consequently, ongoing research is focused on developing multi-function methods to share spectrum between radar and military communication. One approach is to incorporate information-bearing communication symbols into the emitted radar waveforms. However, varying the radar waveform during a coherent processing interval (CPI) causes range sidelobe modulation (RSM) that results in increased residual clutter in the range-Doppler response, thus leading to reduced target visibility. Here a novel approach is proposed to embed information into radar emissions while preserving constant envelope waveforms with good spectral containment. Information sequences are implemented using continuous phase modulation (CPM) and phase-attached to a polyphase-coded frequency-modulated (PCFM) radar waveform, the implementation of which is also derived from CPM. The resulting communication-embedded radar waveforms therefore maintain high power and spectral efficiency. More importantly, the adjustable parameterization of the proposed approach enables direct control of the degree of RSM by trading off bit error rate (BER) and/or data throughput.

Filter design to address range sidelobe modulation in transmit-encoded radar-embedded communications

In a companion paper a continuous phase modulation (CPM) based approach has been introduced to embed information sequences into physical radar emissions, yielding spectrally-efficient constant-envelope waveforms. In addition, the CPM-based approach enables direct control of the degree of range sidelobe modulation (RSM), which occurs due to the changing waveform structure during the coherent processing interval (CPI), by trading off bit error rate (BER) and/or data throughput. When not properly addressed, RSM translates to residual clutter in the range-Doppler response, and hence degraded target visibility. Here receive filter design to mitigate RSM is addressed. The objective for such filters is to produce pulse compression responses that are similar despite the pulse-to-pulse change in waveforms. Three different filter designs are proposed and compared by simulation, where it is found that coherence can be enhanced (and thus RSM reduced) at the expense of higher range sidelobes.

The capacity of SOQPSK-TG

In this paper we compute the capacity and the pragmatic capacity of aeronautical telemetry shaped-offset quadrature phase-shift keying (SOQPSK-TG). In the pragmatic approach, SOQPSK-TG is treated as a modulation scheme as opposed to an encoder, and no iterations are performed between the SOQPSK-TG demodulator and the outer binary decoder. We also evaluate the capacity of SOQPSK-TG constrained on a reduced complexity detection scheme based on the pulse amplitude modulation (PAM) representation of continuous phase modulation (CPM). The spectral efficiency of the PAM based SOQPSK-TG (SOQPSK-TG-PAM) is computed and shown to be superior to that of military-standard SOQPSK (SOQPSK-MIL) despite having the same detection complexity. We also show that the natural mapping of SOQPSK-TG between input bits and SOQPSK-TG waveforms maximizes the pragmatic capacity. Any other mapping such as differential encoding, reduces the pragmatic capacity. Finally, we present the performance results of a SCCC SOQPSK-TG pragmatic scheme, a LDPC SOQPSK-TG pragmatic scheme, and a SCCC scheme with SOQPSK-TG as the inner code. The LDPC scheme performs within 1.05 dB of the capacity curve for various coding rates.

On the finite blocklength performance of HARQ in modern wireless systems

Future wireless communications will face the dual challenge of supporting large traffic volume while providing reliable service for various kinds of delay-sensitive traffic. In the light of this challenge, this paper investigates the throughput performance of hybrid automatic repeat request (HARQ) systems under finite blocklength constraint. We present a framework to compute the maximum achievable rate with HARQ over the Rayleigh fading channel for a given probability of error. In the proposed framework, the operation of HARQ over the Rayleigh fading channel is modeled as a finite-state Markov chain. The state transition probabilities of the proposed Markov model are estimated from the fading characteristics of the wireless channel as well as the dispersion associated with different channel state sequence realizations. With this framework we are able to link the HARQ throughput performance to the characteristics of the underlying physical channel as well as the system design parameters such as modulation and transmit power. Furthermore, we discuss the relationship between the system throughput, and the number of HARQ rounds. The results show that the required number of HARQ rounds to take full advantage of HARQ depends on the choice of modulation, and varies as a function of the signal-to-noise ratio (SNR).

On coding over finite “packets” in wireless communication systems

Future wireless communications will face the dual challenge of supporting large traffic volume while providing reliable service for various kinds of delay-sensitive traffic. In light of this challenge, this paper investigates the throughput performance of a wireless communication system under channel coding over finite “packets.” When we apply coding over multiple “packets,” the underlying wireless channel can be effectively modeled as a finite-state Markov process. By linking the characteristics of the underlying physical channel to the parameters of the Markov process, we are able to derive the channel dispersion of the corresponding system. The channel dispersion is then used to assess the coding performance of various communication strategies. It is interesting to find that when there is a constraint on the total blocklength, coding over large “packets” will give better performance than that of coding over small “packets”.

Wideband microstrip RFID tag: Theory and design

In this paper, we present a theory for modeling and synthesizing a wideband passive UHF RFID microstrip antenna capable of providing a nearly 10 dB return loss between 860 and 960 MHz. From the data, we can conclude that the circuit models and various transforms presented here give good insight and are capable of being used to rapidly synthesize critical antenna parameters.

Channel coding over finite transport blocks in modern wireless systems

In modern wireless systems such as 3GPP LTE/LTE-Advanced, packets are partitioned into multiple transport blocks where each transport block is a group of resource elements with a common modulation and coding scheme. Accordingly, a transport block is the data unit in the physical layer of modern wireless systems. In this paper, we investigate the throughput performance of modern wireless systems under channel coding over finite transport blocks. When we apply coding over multiple transport blocks, the underlying wireless channel can be effectively modeled as a finite-state discrete-time Markov chain. We link the characteristics of the underlying physical channel to the parameters of the Markov chain, and derive the corresponding “channel dispersion.” The “channel dispersion” is then used to assess the throughput performance of various communication strategies. The results show that for a fixed packet size the system throughput increases with transport block size.

On the symmetric information rate of CPM in the finite blocklength regime

Continuous phase modulation (CPM) is a family of bandwidth-efficient signaling schemes with memory. In this paper we introduce a simulation-based method to compute a lower bound, namely the dependence testing (DT) bound, on the maximum achievable rate of general CPM schemes under finite blocklength, probability of error, and equiprobable input distribution constraints. The proposed method utilizes a posteriori probabilities (APPs) of the state transitions on the trellis that models the CPM modulator. The bound on the maximum achievable rate is then used to lower bound the spectral efficiency of CPM schemes under the finite blocklength constraint. For the numerical results, we focus on minimum-shift keying (MSK) modulation, and compute the DT bound for MSK as a function of the signal-to-noise ratio (SNR). We simulate a serially concatenated convolutional code (SCCC) using MSK as the inner code, which performs within 1.1 dB of the DT bound theoretical curve for various coding rates.

On the Queueing Performance of HARQ Systems with Coding over Finite Transport Blocks

With the growing popularity of mobile applications such as voice over IP (VoIP) and real-time conversational video, and due to the scarcity of the available radio spectrum, increasing the spectral efficiency of wireless systems under quality of service (QoS) constraints becomes increasingly important. Accordingly, in this paper we investigate the maximum achievable rate of incremental redundancy type hybrid automatic repeat request (IR-HARQ) over the Rayleigh fading channel under probability of error, and queueing delay violation probability constraints, in the finite code blocklength regime. We model the Rayleigh fading channel by a finite-state Markov channel (FSMC) where the channel in each state is modeled as additive white Gaussian noise (AWGN). We derive the dispersion of parallel AWGN channels with finite input alphabets (e.g. pulse amplitude modulation (PAM)), which is used to track the operation of IR-HARQ over the wireless channel for a given modulation scheme, and coding rate. We then use a two-dimensional Markov process tracking the time evolution of a queueing system, where data packets arriving at the queue are transmitted over the wireless channel with IR-HARQ. This Markov process is used to derive an upper bound on the queueing delay violation probability and to jointly approximate the average rate and the average probability of error of HARQ as a function of the modulation scheme and the coding rate. Finally, we present numerical results.