Impact of partial phase decorrelation on the performance of pilot-assisted millimeter-wave RoF-OFDM systems

Abstract

It is well known that in radio over fiber (RoF) systems, the transmission performance of orthogonal frequency-division multiplexing (OFDM) is highly sensitive to phase noise. In these systems, the radio frequency (RF) signal is generated by beating a reference and a modulated signal at the base station and, therefore, the phase noise of the RF signal depends on the phase noise of both reference and modulated signals as well as on the correlation between them. In many RoF systems, the reference and modulated signals come from the same optical source and, consequently, they are affected by the same phase noise, i.e., perfect correlation. Unfortunately, chromatic dispersion of fiber progressively decorrelates the phase noise affecting both signals. This impairment is especial detrimental in RoF systems operating at millimeter waves, limiting the maximum achievable range. On the other hand, pilot-aided equalization has proven its potential to combat the impact of phase noise in OFDM signals. However, the complex interrelation between phase noise induced by partial decorrelation and pilot-aided equalization is still uncertain. In this paper, we present extensive simulation and theoretical results to assess the optical signal to noise penalty and range limitation caused by partial field decorrelation. We discovered three performance regimes in terms of the correlation degree. This finding was explained by both the profile of the power spectral density and the subcarrier phase noise. Whereas the former is a qualitative result, the latter allows to quantify the phase noise for an OFDM signal with partial decorrelation and phase noise mitigation. Our results revealed that the appearance of a third operating regime is due to pilot-assisted equalization. Finally, we found the range of RoF-OFDM systems for perfectly correlated fields at the transmitter.

Extreme Learning Machines to Combat Phase Noise in RoF-OFDM Schemes

“…Note that all stages experimenting with the RoF-OFDM signal from the central station to a certain base station were explained in our previous work [9]. The authors in [23] revealed that in terms of the time delay parameter, the heterodyning field correlates perfectly (τ = 0), decorrelates partially (0 < τ < 1/∆ν), or decorrelates completely (τ > 1/∆ν).…”

“…Depending on the subcarrier position, a symbol constellation takes the value of either the data symbol d k or the pilot symbol p k [20]. If the OFDM-based RoF system is characterized by correlated-phase fields at the central station (the direct optical detection approach), the input to the OFDM demodulator is corrupted by (i) phase noise due to laser linewidth, as well as chromatic dispersion [9] and (ii) AWGN due to thermal and shot noises [21]. After the photo detection and down-conversion processes, the RoF-OFDM signal may be written as [22]:…”

“…where φ 0 = 2π( f RF − f 0 )τ denotes the phase rotation as a consequence of the time delay parameter τ and f RF and f 0 are the radio frequency and the optical carrier, respectively. It is worth mentioning that τ models the chromatic fiber dispersion effect as long as the OFDM bandwidth is much smaller than the radio frequency [7] and can be interpreted as the phase-decorrelation degree between the mixed tones in the photo detection process [9]. ∆φ(t) = φ(t) − φ(t + τ) refers to the differential phase noise, where φ(t) is phase noise modeled from a Wiener process quantified by the laser linewidth ∆ν.…”

“…As seen, pilots are uniformly added in the OFDM frame, and by linear interpolation of these in the frequency domain, the distortion in data symbols caused by phase noise may effectively be reduced for time delays less than the OFDM symbol period [9]. In [20], the linear interpolation has shown acceptable performance together with a simple realization.…”

“…This equalizer is included in most OFDM modems with the purpose of correcting the wireless channel. The complex interrelation between the chromatic dispersion effect and the feasible pilot-based equalization has been recently studied in [9], finding three different regimes as the combined tones become phase-decorrelated. The first and the second zones correspond for to case of partially phase-correlated fields.…”