Extending the wireless range of your existing network is possible through the use of a wireless repeater. It may also be that your existing router may just need a firmware upgrade which will enable other features of the hardware, such as an increase in the router’s transmission strength.
An ordinary wireless router can be used as a repeater bridge with a third-party firmware such as DD-WRT. DD-WRT is a Linux-based firmware released under the terms of the GPL for many (WiFi) IEEE 802.11a/b/g/h/n wireless routers based on a Broadcom or Atheros chip reference design. DD-WRT offers many advanced features not found in the OEM firmwares of these devices, or even the firmware available for purchase from Sveasoft. [dd-wrt.com/wiki]
To know if a specific router can be used as a repeater/bridge, a list of supported devices is available on the DD-WRT website.
However, using a repeater bridge for your wireless network has the downside for a client of not having the optimum speed in accessing the internet (if connected via the repeater). A repeater bridge will sacrifice half of the bandwidth available from the primary router for clients wirelessly connected. This is the result of the repeater taking turns talking to two partner devices, and having to relay traffic between them.
Other things to keep in mind to improve your wireless network are positioning your wireless router in a central location and moving it away from obstructions such as walls and metal objects. A router’s antenna can also be replaced with a better one which has a better dBi gain. You can also change the wireless channel used to minimize interferences.