Digital coherent receivers have gained significant attention in the last decade. The reason for this is that coherent detection, along with digital signal processing (DSP) allows for substantial increase of the channel capacity by employing advanced detection techniques. In this paper, we first review coherent detection technique employed in the receiver as well as the required receiver structure. Subsequently, we describe the core part of the receiver — DSP algorithms — that are used for data processing. We cover all basic elements of a conventional coherent receiver DSP chain: deskew, orthonormaliation, chromatic dispersion compensation/nonlinear compensation, resampling and timing recovery, polarization demultiplexing and equalization, frequency and phase recovery, digital demodulation. We also describe novel subsystems of a digital coherent receiver: modulation format recognition and impairment mitigation via expectation maximization, which may gain popularity with increasing importance of autonomous networks.

Over 100 Gbit/s/ch high-speed optical transmission is required to achieve the high capacity networks that can meet future demands. The coherent receiver, which is expected to yield high frequency utilization, is a promising means of achieving such high-speed transmission. However, it requires a high-speed Analog to Digital Converter (ADC) because the received signal bandwidth would be over several tens or hundreds of GHz. To solve this problem, we propose a band-divided receiver structure for wideband optical signals. In the receiver, received wideband signals are divided into a number of narrow band signals without any guard band. We develop a band-divided receiver prototype and evaluate it in an experiment. In addition, we develop a real-time OFDM demodulator on an FPGA board that implements 1.5 GS/s ADCs. We demonstrate that the band-divided receiver prototype with its real-time OFDM demodulator and 1.5 GS/s ADC can demodulate single polarization 12 Gbit/s OFDM signals in real-time.

Continuous phase modulation (CPM) is a non-linear modulation technique whose power and bandwidth efficiency make it an attractive choice for mobile communication systems. Current research has focused on devising encoding rules for using CPM over multiple-input multiple-output (MIMO) systems in order to obtain the improved bit error rate (BER) and high data rates promised by MIMO technology. In this paper, optimal and suboptimal non-coherent receivers for a class of CPM signals called orthogonal space-time CPM (OST-CPM) are derived under a quasi-static fading channel assumption. The performance of these receivers is characterized and shown to achieve the same diversity order as that of the corresponding optimal coherent receiver.

We propose a selective detection scheme based on pulse repetition considering both BER (Bit Error Rate) performance and complexity in coherent UWB (Ultra Wide Band) systems. To take system complexity into account, the proposed scheme transmits the UWB signals by pulse repetition at the transmitter, like conventional PRC (Pulse Repetition Coding). However, to effectively improve BER performance of the system, the proposed scheme performs selective detection by estimating the SNR (Signal-to-Noise Ratio) of the received pulse-repeated signal at the UWB receiver.

In order to effectively improve the BER (Bit Error Rate) performance of noncoherent IR-UWB (Impulse Radio Ultra Wide Band) systems utilizing 2PPM (Binary Pulse Position Modulation), we propose a selective signal combining scheme which performs selective combination of received signals by estimating the SNR (Signal-to-Noise Ratio) of the energies during the pulse width interval.

During the execution of precise ranging in the time domain, the most important fact to consider is how to achieve an accurate estimate of the time corresponding to first arrival of the transmitter. However, it is difficult to extract an estimate of the time-of-arrival (TOA) through use of a simple correlator due to degradation on correlation, and in the case where the pulse repetition interval (PRI) is less than the maximum excess delay (MED). In order to enhance the correlation capability, this paper proposes a TOA estimation method that obeys a threshold predetermined in a non-coherent system using multiple-mask operation (MMO). The performance of the proposed scheme is verified by conducting simulations under two different types of channel situations. The simulation results show that the proposed scheme performs well even in a dense indoor multipath environment and with the existence of multiple simultaneously operating piconets (SOPs).

This paper proposes a new coherent CDMA channel structure for the uplink with staggered burst pilot and its detection algorithm. In the uplink, mobiles in a cell share a pilot channel by transmitting periodic bursts in nonoverlapping time slots, which enables coherent detection. We analyze the uplink capacity and derive the capacity formula. The dual mode channel estimator (DMCE) consists of pilot-based channel estimation (PBCE) and data-based channel estimation (DBCE). The proposed DMCE algorithm is very stable in the presence of Gaussian noise and Doppler shift because the pilot burst initiates the DMCE operation periodically. A negligible loss (0.068 dB) in Eb/N0 results from the introduction of the burst pilot. When compared with ideal (0 km/h) coherent detection, the required Eb/N0 in Doppler shift, corresponding to the speed of 160 km/h, is degraded less than 2.0 dB. The simulation result also shows increased channel capacity. The burst pilot can be implemented without added complexity even though some extra correlators are needed for the DMCE. This improvement is significant compared to previously published studies of coherent CDMA detectors with non-shared pilots.