Intensity Modulation Direction Detection Systems Using a Mach-

Although the optical up-conversion system for 60-GHz RoF can provide bet-ter performance and longer fiber transmission distances, most of them are very com-plex and impractical for real applications. Intensity Modulation Direction Detection (IMDD) RoF systems have the advantage that they are extremely simple and can of-fer seamless multi-standard system operation. In this section, a wideband OFDM-modulated IMDD RoF system using a high speed single-electrode MZM is proposed.

A simple DSB IMDD RoF system to directly transport a 7 GHz-wide OFDM sig-nal at 60 GHz is experimentally demonstrated. Ultra-high data-rate of 21 Gbps with transmission over standard single-mode optical fiber and 10m wireless distance can be achieved. The impact of chromatic dispersion is also investigated. 500m SSMF transmission which is sufficient for most in-building applications can be achieved in this 60-GHz RoF architecture.

5.4.1 Experimental Setup

The schematic of the experimental set-up is shown in FIG. 5.15. A one-step electrical up-converter was used to generate a 7-GHz-wide OFDM signal centered at 60.5 GHz, by retaining both OFDM signal sidebands following up-conversion, as shown. The OFDM signal at baseband was 3.5 GHz wide and it was generated by an AWG. The resolution of the DAC and the sampling rate of the AWG were set to 8 bits and 24 GSa/s, respectively. The IFFT length was 256, resulting in a subcarrier symbol rate of 93.75 MSym/s. The 3.5-GHz-wide OFDM signal consisted of 37 sub-carriers, which were modulated with the 8QAM data format. Therefore, the 7 GHz-wide OFDM signal at the output of the electrical up-converter consisted of a total of 74 sub-carriers with a combined data-rate of 20.8125 Gbps.

LD MZM OBPF

FIG. 5.15. Experimental setup of the simple IMDD 60-GHz RoF system using a MZM.

The high-speed MZM used to modulate the optical signal was specified for 40 Gbps but had reasonable performance around 60 GHz. The modulator was biased around the quadrature point resulting in a DSB optical signal (with carrier) as shown in insert (i) of FIG. 5.15. The optical signal source was a CW DFB laser emitting +10.5 dBm of optical power around 1550 nm.

After amplification using an EDFA and filtering for ASE noise, the intensity-modulated optical signal was transmitted to the Remote Antenna Unit (RAU) via stan-dard single-mode fibers of various lengths. At the RAU, the transmitted 60 GHz signal was generated by direct detection in the 67 GHz photodiode, amplified by a LNA with 38-dB gain, and transmitted wirelessly over a distance of 10m. In order to realize 10m wireless transmission distance a Gaussian Optics antenna (37 dBi) was used as the transmit antenna and a standard horn antenna (23 dBi) was used at the wireless terminal. No power amplifier was used at the RAU.

The 60-GHz signal received after 10-m wireless transmission was amplified by another LNA with 22-dB gain, and filtered by a band pass filter before it was

down-converted to an intermediate frequency (IF) centered at 4.5 GHz. This IF frequency was chosen so as to maintain the spectrum of the down-converted OFDM signal at 7 GHz. The down-converted signal waveforms were captured by a real-time digital oscilloscope with 50-GSa/s sampling rate for offline processing and analysis. Before capturing the results, the whole RoF system was optimized stage by stage to reduce the impact of the high PAPR of OFDM signals for the highest received SNR.

5.4.2 Results and Discussion

Figure 5.16 shows the spectrum of the 21-Gbps OFDM signal down-converted from the 60-GHz signal after optical fiber and 10m wireless transmission. The de-tected optical power was -8 dBm. It can be seen from the spectrum that the signal bandwidth is 7-GHz-wide and it is centered at 4.5 GHz. A strong peak at 4.5 GHz is observed. This is the residual LO signal from the one-step electrical up-conversion back at the Head-End Unit (HEU). The spectrum also shows that the RoF link had an amplitude flatness of about 10 dB over the entire 60 GHz band (57 GHz to 64 GHz).

This flatness would have an adverse impact on system performance if single carrier modulation was used. However, this uneven response can be compensated for due to the multi-carrier characteristic of the OFDM signals. FIG. 5.17 (a) shows the clean constellation diagram of the received 8QAM OFDM signal for the detected optical power of -8 dBm with only 60-m fiber transmission. The SNR and Error Vector Mag-nitude (EVM) corresponding to this constellation diagram were 16 dB and 12.6 %, respectively.

Since both sidebands of the optical signal were transmitted, fiber chromatic dispersion-induced fading of the generated 60-GHz carrier after fiber transmission was expected. In order to investigate the nature of the fading and its impact on the performance of the OFDM IMDD RoF system, the intensity-modulated optical sig-nal was transmitted over various fiber lengths and the average SNR of the received

1 2 3 4 5 6 7 8 -70

-60 -50 -40 -30

-20 0.06 km

0.16 km 0.26 km 0.36 km 0.46 km 0.56 km 0.66 km 0.76 km 0.86 km

Level (dBm)

Frequency (GHz)

FIG. 5.16. Electrical spectrum of the down-converted OFDM signal.

signal analyzed. 10-m wireless transmission distance was included in all cases. FIG.

5.18 shows the impact of chromatic dispersion on the sensitivity of the IMDD RoF system. No penalty was observed for transmission over 160 m standard single-mode fiber. A small 1-dB optical power penalty measured at the SNR equal to 14 dB was observed after transmission over 360-m fiber. The dispersion penalty grew to 3.5 dB after 560-m fiber transmission. The constellation diagram corresponding to 560-m fiber transmission with -8 dBm detected optical power is shown in FIG. 5.17 (b). In this case, the measured SNR and EVM were 11.1 dB and 22.6 %, respectively. The progressive deterioration of the system performance at longer fiber spans was caused by progressive increase in the signal fading suffered by sub-carriers inside the 7-GHz-wide OFDM signal as shown in FIG. 5.16. The signal after 860-m fiber transmission could not be recovered.

A detailed study of the impact of chromatic dispersion on the wide-band OFDM signal was conducted by transmitting the RoF signal over fiber spans ranging from 60 m to 3.56 km, with a length resolution of 100m. FIG. 5.19 shows the measured

FIG. 5.17. Constellation diagrams of the recovered 21-Gbps 8QAM OFDM signals with (a) 60 m and (b) 560 m fiber transmission distances.

system performance in terms of the average SNR. An apparent “fading cycle” of 2 km was observed. The “fading cycle” of the OFDM signal was similar to that of an un-modulated CW signal at 60.5 GHz. However, as shown in FIG. 5.19, the OFDM signal faded earlier and recovered later (>200m) than the CW signal due to the former’s wide spectrum of 7-GHz. The detail of the chromatic dispersion induced fading of the DSB signal will be discussed in Appendix B.

5.4.3 Summary

A simple DSB IMDD RoF system was experimentally demonstrated to directly transport a high data-rate (21 Gbps) OFDM signal at 60 GHz over 500 m of standard single-mode fiber and 10 m wireless distance. Experimental results confirm that using OFDM modulation format rather than single-carrier modulation formats significantly reduces the impact of serious uneven frequency response found in ultra-wideband IMDD RoF systems. Considering the average system SNR, a “fading cycle” of 2km was observed, which was similar to the fading cycle of a um-modulated CW signal at

-16 -14 -12 -10 -8 -6 -4 4

6 8 10 12 14 16 18 20

60 m 160M 360M 560M

SNR (dB)

Receiver Power (dBm) 4 dB

FIG. 5.18. SNR vs. Received Optical Power with different fiber distances and 10-m wireless transmission distance.

60.5 GHz. The dispersion-induced fading issue can be avoided by using SSB modu-lation schemes with more complexity or dispersion compensation techniques, but the 500m fiber transmission distance achieved without any dispersion compensation is sufficient for most in-building applications.

5.5 Intensity Modulation Direction Detection System Using an Electro-absorption