Cost-efficient OFDM generation at 60-GHz by heterodyne technique with direct modulation and envelope detector

Abstract

A cost-effective radio over fiber system to up-convert and transmit multigigabit signals at 60 GHz is presented. A low intermediate frequency OFDM signal is used to directly modulate a laser, which is combined with an independent unmodulated laser. The generated millimeter wave frequency can be adjusted by tuning the frequency separation between the lasers. Since no external modulator is required, this technique is low-cost and it is easily integrable in a single chip. In this paper, we present numerical results showing the feasibility of generating an IEEE 802.15.3c compliant 3.5-Gbps 60-GHz OFDM. We show that received signal quality is not limited by the lasers' linewidth but by the relative intensity noise. Keywords: Radio over fiber, OFDM, heterodyne detection. INTRODUCTIONThe use of millimeter-wave (mm-wave) bands has been proposed by many authors [1-4] as a solution for the increasing wireless bandwidth demand because of two main reasons: (i) the higher atmospheric and free-space path losses allow higher frequency reuse factor and, (ii) huge portions of the spectrum are still unlicensed worldwide [1,2]. However, migration to mm-wave frequencies results in higher number of base stations (BSs) and more complex and power consuming electronics [1]. Radio over fiber (RoF) has emerged as a cost-effective alternative to communicate mm-wave signals [5]. One of the challenges of RoF systems is the generation of the optical signal that after the photodetection results in the desired mm-wave signal. On the other hand, even if multipath effect in the wireless channel is reduced at mm-wave frequencies compared to lower bands [3], it still limits the maximum channel capacity, especially in non-line of sight communication, common in indoor networks. Because of that, high-bit rate wireless personal area network (WPAN) standards, operating in the band of 60 GHz, such as ECMA387 [6] and IEEE 802.15.3c [7], have adopted orthogonal frequency division multiplexing (OFDM) in their high capacity operation mode. In addition to robustness to multipath effect by simple equalization and inclusion of cyclic prefix (CP), OFDM offers high spectral efficiency and the possibility to adapt the modulation format of each subcarrier for the channel response.In recent years, a plethora of optical mm-wave generation techniques have been proposed to generate OFDM signals [8,9], especially in the frequency band around 60 GHz. Most of the reported optical generation techniques for upconversion of OFDM signals require external modulation [10][11][12][13][14][15], either an intensity modulator or phase modulator, whose cost increases as its bandwidth does. In order to relax the bandwidth requirements of the electro-optical component, as well as the frequency of the required electronics, frequency duplication has been proposed in [10,11] where a Match-Zehnder modulator (MZM) is biased at the null point, suppressing the optical carrier. A higher frequency multiplication scheme has been reported in [14] where a low bandwidth MZM is followed by a phase ...

Knowledge-based framework for the design of millimeter-wave (60 GHz) radio over fiber land networks

“…An increasing number of techniques has been proposed in the last years to perform this task based on different approaches, such as directly modulated lasers [58,59], externally modulated lasers [45,60,61], multi wavelength sources [21,[62][63][64], and heterodyne of independent lasers [65][66][67][68].…”