Specifications (3GPP TS 25.105 v3.10.0, ‘BS Radio Transmission and Reception (TDD)’, hereinafter referred to as [1]) by the 3GPP (3rd Generation Partnership Project) set out the performance of TDD (Time Division Duplex) Node B (base station in a 3GPP system) equipment. These specifications cover the ‘Adjacent Channel Leakage Ratio’ (ACLR) for Node Bs specified for equipment that is co-sited with other TDD or FDD (Frequency Division Duplex) Node Bs operating on adjacent channels.
For co-siting purposes, stringent specifications on the transmit spectral purity of UMTS TDD Node Bs call for a single channel RF filter to be fitted after the power amplifier (PA). The specification of the RF filter is also extremely stringent and a very high Q passive filter is required in order to achieve the required stop-band. By adopting an RF filter with such a steep roll-off factor, it is unavoidable that the filter has an effect on the the transmit accuracy; in fact, the inclusion of this filter may cause the Node B to fail the transmit accuracy requirement.
Passive RF equalisation is not desirable due to the corresponding increase in the complexity of the RF filter and the fact the amplitude equalisation is not possible without increasing the insertion loss across the pass band. Analogue baseband equalisation is not desirable as the equaliser response needs to be matched to the RF filter to achieve optimum transmit accuracy and tuning baseband filters to match the RF filter, which in practice would have a significant impact on the production of the Node B.
A need therefore exists for digital pre-equaliser for RF filter wherein the abovementioned disadvantage(s) may be alleviated.