What is your suggestions if i need to design transmission link (fiber optic) with length more than 500 km ? (without repeater )

ROPA(Remote Optically pumped Amplifier) system is best suited for this kind of link design, A passive Regeneration unit and lots of CDC is must in case of non-coherent signals. Otherwise in coherent transponders case insertion loss of CDC (chromatic dispersion compensation) can also be neglected and with the help of forward and backward ramans at source/sink sites it can be designed according to no of wavelenghts required by operator 

see the formula to calculate the the  Maximum optical link length is

ð¿ððð¥ = 10 /ð¼( log ( ðð ðð ))

ðð = ðððµℎð/ 2 

 Where ð¿ððð¥ = Maximum optical link length ðð = input optical power ðð = Minimum no. of photon B = Bit rate hν = Energy of photon ðð = Received power

WITHOUT REPEATER IT IS VERY DIFFICULT  TO DESIGN LINK WITH SUCH A LONG SPAN BECAUSE FIRST OF ALL WE WILL HAVE TO GO FOR  OPTICAL FIBER  WITH VERY LESS ATTENUATION PER KM AND IF WE FIND SUCH A OFC EVEN THAN  NOISE  LEVEL WILL BECOME VERY HIGH WITH SUCH LONG SPAN AND SIGNAL AT RECEIVER END WILL BE DIFFICULT TO RETRIEVE

we can use mbifier card in in between

In Huawei network we use RGU for long haul transmission.

Coherent technology and FEC over RAMAN and EDFA amp span.

I will use RAMAN Amplifier in both stations for uplink and downlink  

without repeater is really not possible. If we are able to manufacture the OFC with 5 to 10dB loss means it is possible. Even submarine cables using repeater in the sea, and Feeding power along with the cables.

Designing an Ultra Long Haul Backbone DWDM network which can multiplex up to80 -100 10G channels can cover long distance transmission application in high capacity core optical networks. These networks can be of continental scale, and have current maximum link capacities in the vicinity of1Tbit/sec, consisting of roughly80-100 optical channels at10 Gb/s data rates.

Tramsmission link with optical fiber of more than500Kms is not possible. The optical fiber loss is approximatily0.33 dBm/Km. Taking a transmission link of500 Kms result in165dBm. Generally an Optical transmission system work with transmission link having loss less than30dBm. An optical signal get distorted with disperson and other losses as the length of optical fibre increases. So, without repeater it is not possible.