Evolution of Access network

Access network means the network connects each user to the central office of service provider. Most broad access still relies on copper, like the digital subscriber line (DSL) or coaxial cable. But these technologies are limited by noise, the transmission distance becomes shorter when increasing the transmission distance. The trend of network service is triple play, the combination of Internet, TV and telephone services distributed over one network. It makes the demand of transmission bandwidth increase a lot. Today, the access networks which rely on copper are not able to maintain the demand of bandwidth. Fiber is a better solution that it can provide much higher bandwidth than copper. Optical access network such as fiber to the Home (FTTH), fiber to the building (FTTB) or fiber to the curb (FTTC) brings the fiber to the user’s home, building or the service node near the user [1],[2]. Figure. 1 shows the architecture of a passive optical

OLT

Splitter

Centrol office

ONU ONU ONU

ONU

ONU

Figure. 1 Passive optical network architecture

network. It uses a splitter to connect OLT with several ONUs. And ONU provides the interface between network and users.

Besides the demand of high bandwidth, another trend in the current access networks is mobility [3]. Wireless access network can provide ultimate service for users with Internet access anywhere and anytime. One of the most popular technologies of wireless service is WiMAX which can provide high bandwidth and mobility. Cost is an important concern to build a network system, so it needs to minimize the equipment cost and maximize the bandwidth. Cost is the reason that the passive optical network (PON) dominates in the access network market. Next let us see some high speed PON architecture.

1.1.1 TDM PON

Time division multiplexing passive optical network (TDM-PON) has been developed for a long time such as Ethernet-PON (EPON) and Gigabit-PON (GPON) [4],[5]. The TDM-PON is a one point to multi point architecture. There is an optical line terminal (OLT) at the central office (CO) to communicate with many remote optical network units (ONU) and there is a splitter to connect OLT with different ONUs.

It uses single wavelength for downstream. The downstream data broadcasts to all ONUs (Figure. 2), and each ONU identifies its own part by the address information located in the header, the preamble of Ethernet packet. For the upstream (Figure. 3), it uses another wavelength, and each remote ONU uses the same wavelength so that this ONUs must use dedicated time slot to transmit data.

OLT

Figure. 2 Downstream concept of TDM-PON

OLT

Figure. 3 Upstream concept of TDM-PON

Dynamic bandwidth allocation (DBA) is one of the advantages of TDM system. For upstream, OLT needs to control the transmission time of each ONU in order to avoid data collision, we can use the control information to allocate bandwidth dynamically. For instance, if all users need to transmit data, OLT can control each ONU transmit data tack turns. Also, if users do not transmit data at some time, OLT can allocate these vacant time slot to other ONU so that the bandwidth could be used efficiently.

There are three problems in TDM-PON. Firstly, since the TDM-PON system uses the optical splitter to connect OLT with ONU, the optical power loss of both direction signals

increase as the number of ONUs increases, in other words, the optical power of the transmitter limits the number of ONUs. Secondly, although dynamic bandwidth allocation is one of the advantages, it complicates the system. While the system needs to support more and more users, precise timing control becomes more difficult and the algorithm becomes more complex. Thirdly, it is difficult to achieve higher data rate of TDM-PON, because the data rate is limited by the speed of component.

1.1.2 WDM PON

Wavelength division multiplexing passive optical network (WDM-PON) is a point-to-point transmission technology [6]. WDM-PON uses multiple wavelengths in a single fiber to increase capacity instead of direct data rate increasing. Because WDM-PON uses multiple wavelengths, it can promote capacity of transmission easily. For example, if one wavelength has 2.5Gbps data rate, then we use four wavelengths simultaneously so that the fiber would have 10Gbps data rate.

Some researchers consider WDM-PON a potential technology for NG-PON.

The concept of WDM-PON is shown in Figure. 4 used passive optical wavelength router such as arrayed waveguide grating (AWG) to multiplex different wavelength and de-multiplex each wavelength. Each ONU uses its own wavelength, so it needs many OLT&ONU pairs and each pair has a dedicate wavelength. In this condition, the structure supports 32 users, AWG has an optical loss about 5dB which is less than a 1x32 optical power splitter.

TX:λ1

Figure. 4 Concept of WDM-PON

The critical problem of WDM-PON is the cost. The system needs a lot of different light sources. WDP-PON supports 32-ONU (Figure. 4), that is, it needs 32 different light sources for upstream as well as another 32 different light source for downstream. Using so many light sources is expensive. What’s more, each ONU needs its own bandpass filter to filter out its own signal. The structure of the system is very cost-wasting.

1.1.3 OFDM PON

Orthogonal frequency division multiplexing passive optical network (OFDM-PON) transmit data uses OFDM signal. OFDM signal uses several frequencies to increase transmission capacity instead of increasing data rate of single carrier signal. I will talk about details of OFDM signal in next chapter. Traditionally, if we want to create several different frequencies, we need several local oscillators. Adding too many oscillators would make the system complex. And OFDM system uses fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) to simplify

the structure. Thanks to the mature Very-large-scale integration (VLSI) technology, FFT and IFFT can implement with single integrated circuit (IC).

DSP Demodulation

Figure. 5 Transmission concept of OFDM system

The transmission concept of OFDM system is shown in Figure. 5. At the transmitter, it maps serial data into parallel subcarriers, and then executes IFFT to get serial digital data. After DSP modulation, it uses a digital-to-analog converter to convert the digital signal to analog signal. At the receiver, it uses an analog-to-digital converter to convert analog signal to digital signal, then executes synchronization and FFT. After FFT, it equalizes data in frequency domain.

The OFDMA-PON has a lot of advantages such as simple equalizer, which can support high order QAM modulation for OFDM signal is strong against channel response. In addition, the orthogonal characteristic of OFDM signal can make transmission more efficient, because we can adjust the modulation type of each sub-channel according to channel response. For example, if sub-channel is located at the good channel response part, we can use high order QAM modulation format (ex: 128QAM, 64QAM) [7]. On the other hand, if sub-channel is located at the bad channel response part, we can use lower order modulation format (ex: QPSK, BPSK). So the

transmission would not be limited by the channel response. I will talk about more details of OFDMA-PON in next chapter.