Method and communications device for improving frequency band support

1. A communications device for improving frequency band support, comprising:

a radio frequency (RF) signal processing device, processing a plurality of RF signals to generate a plurality of baseband signals;

a baseband signal processing device, processing the baseband signals;

a memory device; and

a processor, determining whether the communications device supports at least one frequency band of a first type of communication service supported by a network when the communications device is camped on a cell using the second type of communication service; and performing a scanning procedure according to a priority when the communications device does not support the at least one frequency band,

wherein the scanning procedure includes scanning all frequency bands of the first type of communication service supported by the communications device;

wherein after the communications device scans all the frequency bands of the first type of communication service supported by the communications device, the processor determines whether there is the at least one frequency band of the first type of communication service supported by the network, and transmits an ATTACH request message to the network to request to use the first type of communication service over the at least one frequency band when finding the at least one frequency band.

2. The communications device for improving frequency band support as claimed in claim 1 , wherein the communications device further receives an ATTACH accept message transmitted from the network, and uses the first type of communication service.

3. The communications device for improving frequency band support as claimed in claim 1 , wherein the communications device further receives an ATTACH failure message transmitted from the network, and stops the scanning procedure.

4. The communications device for improving frequency band support as claimed in claim 1 , wherein the communications device further receives an ATTACH failure message transmitted from the network, and re-performs the scanning procedure.

5. The communications device for improving frequency band support as claimed in claim 1 , further comprising:

a counter, counting a number of cell reselections when the communications device determines that there is no at least one frequency band, and the processor re-performs the scanning procedure when the number is greater than a threshold.

6. The communications device for improving frequency band support as claimed in claim 1 , further comprising:

a timer,

wherein the processor starts the timer when the communications device determines that there is no at least one frequency band, and the processor re-performs the scanning procedure when the timer expires.

7. The communications device for improving frequency band support as claimed in claim 1 , wherein the communications device has a system information block (SIB), and the SIB includes a field corresponding to the first type of communication service.

8. The communications device for improving frequency band support as claimed in claim 1 , before the determining step, the communications device uses a second type of communication service, wherein the first type of communication service has a higher priority than the second type of communication service.

9. The communications device for improving frequency band support as claimed in claim 8 , wherein the first type of communication service is long term evolution (LTE) service, and the second type of communication service is universal mobile telecommunications system (UMTS) service.

10. A method for improving frequency band support, used in a communications device, comprising:

determining whether the communications device supports at least one frequency band of a first type of communication service supported by a network when the communications device is camped on a cell using the second type of communication service; and

performing a scanning procedure according to a priority when the communications device does not support the at least one frequency band,

wherein the scanning procedure includes scanning all frequency bands of the first type of communication service supported by the communications device;

wherein after scanning all the frequency bands of the first type of communication service supported by the communications device, the method further comprises:

determining whether there is the at least one frequency band of the first type of communication service supported by the network; and

transmitting an ATTACH request message to the network to request to use the first type of communication service over the at least one frequency band when finding the at least one frequency band.

11. The method for improving frequency band support as claimed in claim 10 , comprising:

receiving an ATTACH accept message transmitted from the network; and

using the first type of communication service.

12. The method for improving frequency band support as claimed in claim 10 , comprising:

receiving an ATTACH failure message transmitted from the network; and

stopping the scanning procedure.

13. The method for improving frequency band support as claimed in claim 10 , comprising:

receiving an ATTACH failure message transmitted from the network; and

re-performing the scanning procedure.

14. The method for improving frequency band support as claimed in claim 10 , comprising:

counting a number of cell reselections when determining that there is no at least one frequency band; and

re-performing the scanning procedure when the number is greater than a threshold.

15. The method for improving frequency band support as claimed in claim 10 , comprising:

starting a timer when determining that there is no at least one frequency band; and

re-performing the scanning procedure when the timer expires.

16. The method for improving frequency band support as claimed in claim 10 , wherein the communications device has a system information block (SIB), and the SIB includes a field corresponding to the first type of communication service.

17. The method for improving frequency band support as claimed in claim 10 , before the determining step, the communications device uses a second type of communication service, wherein the first type of communication service has a higher priority than the second type of communication service.

18. The method for improving frequency band support as claimed in claim 17 , wherein the first type of communication service is long term evolution (LTE) service, and the second type of communication service is universal mobile telecommunications system (UMTS) service.