S11 requirement for communication device antenna

Does anyone know S11 or VSWR requirement for broadband antenna applied for 4G handset?

I have read many papers that all require S11 below -6dB (VSWR < 3) in the interested frequency bands but I don't know why we can take -6dB as the margin. Could any one explain for me or show me a specification or regulation about VSWR < 3:1 for antenna.

There is no "regulation". Antennas are better if they are better matched to line impedance. In 4G and other handsets, the proximity to human body, hands, etc., affects this matching. Certain compromise is sought for. For instance, new transmitter MMICs integrate a "tuning" section and a reflectometer detector, by which the controller optimizes the antenna under given conditions, to achieve the highest transmit power, and coverable distance to a base station.

Thanks, Jiripolivka
However, S11 -6dB is considered as margin for S11 when designing internal antenna. I really wonder why we've got this figure.
For instance, when I read below papers, all of them conform to this specification (VSWR 3:1)
http://www.mediafire.com/file/m88a1y...ile_Device.pdf
http://www.mediafire.com/file/cza9z3...DCN_US-PCS.pdf
http://www.mediafire.com/file/bx1p5m...AX_WLAN(1).pdf
http://www.mediafire.com/file/233gbb...ne_antenna.pdf

There is no general VSWR limit, not even for internal antennas. A few years ago was it common that internal antennas only requirement was as low VSWR as possible at 50 Ohm. High VSWR makes transmitter less effective as a part of total TX power is reflected back to transmitter. In worst case can it destroy the transmitter and cause EMI related problems but not less important, ineffective transmitter costs more battery power for a certain radiated level.
Today is focus more on total system efficiency, including optimizing antenna efficiency relative its impedance for best possible TIS and TRP.
Required antenna impedance is then seldom equal to 50 Ohm antenna impedance, making VSWR at 50 Ohm of less value.
4G is not different in these aspects except that it in a handset that contains two antennas are also a certain amount of isolation between the antennas wanted.
It is up to the company that specifies total system performance to define antenna requirement, they can set any specifications.
The telecommunications company can also set their own quality requirements. A very ineffective cellphone with poor RX or not reaching max TX radiation level can be denied to be used as it not will result in that actual basestation density will give full area coverage.
HAC and SAR can also be a part of antenna/system specification as well as how antenna should behave when phone is held i different positions.
A 50 Ohm resistor is not a effective antenna but it have low VSWR in a 50 Ohm system => VSWR alone is not always telling if it is a acceptable antenna.
I have done a extreme example at http://www.antune.net/demo/bluetooth/
It is an antenna which have a VSWR of 1:50 and still is very useable after that a matching network is added.

Thanks E Kafeman