methods to enhance the impedance bandwdith of the microstriop antennas

Basically the microstrip antenna is a high-Q resonant device. Usually they are inherently narrowband. To increse the impedance bandwdith, the simplest way is to reduce the Q by increasing the thickness and/or lowering the dielectric constant of the substrate.  Therefore, SPA-suspended plate antennas are good options but the size/volume increases slightly.
more information can be found in papers published in IEEE T-AP during 2002-2008 or book entitled Broadband Plate Antennas: Design and Applications.

Good

Thanks. miniaturization and broadbanding technologies are the most important design issues in antenna development. It should be taken serious consideration. 

还以为有什么新意，还是老话重提。有些失望

however, th echallenges are still there. for example, it is very difficult to design an antenna covering 2.3-2.7GHz and 4.9-5.875GHz simultaneously with consistent gain at boresight and a small form factor. Can you help in this? In the market, it is very attractive.

有啥话你不能一气儿说完?！

it is better for you to focus on the discussion, please.

in addition, broad bandwidth can indicate many different performance besides impedance matching, like gain, polarization, etc.

PIFA is a important design in miniaturization of antennas. Now, the new demand for small antennas in handsets is coming while LTE will be used very soon！

知己知彼方能进步！
Book: antennas for portable devices

Could you support some links for the books you mentioned above?

here you are
http://www.amazon.com/Antennas-Portable-Devices-Ning-Chen/dp/0470030739