Raghu,

I would think that because you want to build a legal product that doesn't require any license, your first task is to investigate your country's regulations regarding RF emissions. There will likely be a limited number of bands available to you, each with its own unique resistrictions. Then in each appropriate band you can search for radio equipment that meets your link requirements.

I also want to throw out another link for you. If 900 MHz FM is an option, Radiotronix makes some low price and versatile 868-870/900-926 MHz multichannel transceivers. The output power is on the order of 4 dBm but I bet you are looking for something closer to 20 dBm. If your budget is tight and time is abundant you could fabricate your own transceiver and amplifier by using the transceiver, an antenna switch (e.g. ADG918), and a broadband amplifier.

- Lee