03.523GPPLower layers of the GSM Cordless Telephony System (CTS) radio interfaceStage 2TS
The main assumption behind the CTS work item and in particular the CTS radio interface, is that a modified single timeslot state of the art GSM-MS chipset could be used for a home base station, i.e. as a CTS-FP.
The CTS radio interface has been designed to meet a requirement of low generated interference, either from the CTS to existing overlaying PLMNS, either from a CTS to another CTS. This requirement is achieved by the combined usage of the three concepts: beacon concept, AFA concept and TFH concept.
4.1 Beacon concept
A limited number of CTS-MS shall be served by one CTS-FP (see GSM 02.56). Therefore, a broadcast channel continuously transmitted such as the BCCH in GSM is not needed for CTS.
A channel called CTS beacon channel (CTSBCH) is proposed with the following main characteristics: it is transmitted by the CTS-FP every 26 frames in a 52-multiframe pattern, and allows the CTS-MS to synchronise with the CTS-FP. Minimum signalling is also supported by the CTSBCH, so that it is the only logical channel a CTS-FP shall periodically transmit on the CTS radio interface. Every other logical channel is only transmitted "on demand".
4.2 Adaptive Frequency Allocation (AFA) concept
A precise radio frequency planning can not be applied to the CTS-FP/MS pair, as the CTS is intended to be deployed by the end-user. Therefore, a list of frequencies (the GFL) on which it is allowed to operate is given to the CTS. With the AFA, interference measurements will be performed on the frequencies in the GFL to provide a ranking in the AFA table, in order to exclude unacceptably interfered frequencies from the usage in CTS.
4.3 Total Frequency Hopping (TFH) concept
The remaining frequencies are used by the Total Frequency Hopping algorithm in order to reduce the interference of the CTS with the overlaying PLMN and other CTS-FP/MS pairs. With TFH the interference caused by the CTS link is spread across multiple GSM links (interference averaging) and the co-channel interference is due to different users at different locations (interference diversity).
A new hopping algorithm which is especially tailored for use in CTS with improved performance compared to the GSM hopping algorithms shall be used.