wdm optical interfaces for future fiber radio systems phần 3 ppsx

Chapter 2: Literature Review

enabling the fibre feeder network to support the required large number of BSs to

service a certain geographical area.

The introduction of OSSB+C modulation as well as tandem single sideband

modulation enables increased spectral efficiency by reducing the required spectral￾band for an optical mm-wave channel, in addition to mitigating the effect of fibre

chromatic dispersion due to ODSB+C modulation format [59-61,112-122]. The

tandem single sideband modulation effectively doubles the capacity of the mm-wave

fibre-radio systems while compared to the conventional ODSB+C based systems

[121-122]. However, the use of WDM in fibre feeder networks can resolve the

challenge by enabling transport of multiple optically modulated mm-wave signals,

feeding multiple antenna BSs through one fibre [15-16, 23, 36-39] . The following

section reviews the literatures towards the implementation of WDM fibre feeder

network in mm-wave fibre-radio systems.

2.3.1 Wavelength Division Multiplexed MM-Wave Fibre-Radio

WDM is an elegant and effective way to increase the capacity of the fibre optic

feeder networks in mm-wave fibre radio systems. In the WDM incorporated feeder

networks, optical mm-wave channels, each carried by a separate wavelength, are

transmitted to/from the BSs via the CO through a single fibre that provides quantum

increase in network capacity without the need for laying new fibre [15-16, 23, 36-39,

44, 89, 92-93, 123-129]. It also simplifies the network upgrades and the deployment

of additional BSs, while support multiple interactive services for future broadband

wireless access communications [15, 36-37, 125-126].

Fig. 2.12 shows the general concept of a typical mm-wave fibre-radio system

incorporating WDM. In the downlink direction, optical mm-wave channels, spaced at

an effective WDM separation, are generated in the CO by using WDM optical

sources, and are passed through a suitable multiplexer that aggregates them to a

composite signal. The multiplexed signals are then transported over optical fibre to

the remote nodes (RN), where the individual optical mm-wave signals are

demultiplexed and directed to antenna BSs for mm-wave wireless distribution. In the

uplink direction, mm-wave signals generated at the customer sites are converted

45

Chapter 2: Literature Review

CO

Remote Node

(RN)

BS1

BS2

BSN

BS1

BS2

BSN

CO

Remote Node

(RN)

BS1

BS2

BSN

BS1

BS2

BSN

Fig.2.12: Schematic diagram of typical mm-wave fibre-radio feeder network incorporating WDM

from electrical-to-optical form at BSs and sent to the RN, where the optically

modulated signals are multiplexed before directed to the CO through fibre for further

processing. Such fibre-radio feeder network enables a large number of BSs remotely

share the switching and signal processing hardware located at the CO, in addition to

simplifying the complexity of BSs by enabling passive multiplexing and

demultiplexing functionality at the RNs. Since each of the optical mm-wave channels

are effectively separated from others, they can be independent in protocol, speed, and

direction of communication. As mentioned in Chapter 1, it is envisaged that future

wireless bandwidth will be met by mm-wave WDM fibre-radio systems, where each

of the remote antenna BS will be allocated a WDM optical carrier to transport the

optically modulated mm-wave signals to/from the CO through the fibre optic feeder

network, irrespective of direction of communication. However, using the same

wavelength for both downlink and uplink communication is not any requirement,

since channel offset scheme as well as interleaved downlink and uplink channels can

also be used.

46

Chapter 2: Literature Review

With the maturity of WDM components and system technologies, the effective

WDM channel separations in the conventional optical access and metro domain are

gradually replaced with dense-wavelength-division-multiplexing (DWDM)

separations of 100 GHz, 50 GHz, and 25 GHz. The introduction of DWDM fibre

feeder networks in mm-wave fibre-radio systems may surprisingly increase the

capacity of the systems by supporting huge number of BSs required for future

multiple interactive broadband wireless services. Also, it is important that mm-wave

fibre-radio systems can coexist with other conventional DWDM access and metro

technologies, as it is expected that mm-wave fibre-radio systems will be realised by

utilising the unused capacity of the existing optical infrastructure in the access or

metro domain, instead of deploying separate fibre-radio backbone. However, the

inherent wideband characteristics of mm-wave signals (25-100 GHz) impose spectral

restrictions in realising fibre feeder network with a channel separation ≤ 100 GHz.

Fig. 2.13 shows the optical spectra of OSSB+C modulated N optical mm-wave

channels with a WDM channel separation and a mm-wave carrier frequency of

∆fWDM and ∆fmm-wave respectively, where ∆fmm-wave < ∆fWDM . In order to realise

DWDM fibre feeder networks for mm-wave fibre-radio systems, in most of the

cases, it is necessary to reduce ∆fWDM < ∆fmm-wave, which has been an active area for

∆fmm-wave

∆fWDM

S1

C1

S2

C2

SN

CN

∆fmm-wave

∆fWDM

S1

C1

S2

C2

SN

CN

Fig. 2.13: Optical spectra of the N optical mm-wave channels in a WDM feeder network for mm￾wave fibre-radio systems.

47