A method for a joint time-frequency synchronization of User Equipment, UE, in NB-IoT Networks already connected to a serving cell obtaining an optimal time-frequency offset estimation, the method comprising the following stages: - (stage 1) demodulating a received NB-IoT subframe R m containing resource elements in the frequency domain with regard to a pre-computed reference NB-IoT subframe B m containing expected resource elements in the frequency domain, whereas the pre-computed resource elements of B m are derived from information of the serving cell, and storing resource elements of the demodulated NB-IoT subframe W m in matrix W, with m being a natural number equal or greater than 0 and presenting the m-th processed subframe, - (stage 2) computing a metric V m (f,t) defined by V m ( f,t) = ∑ K = 0 N C − 1 ∑ l = 0 N S − 1 W K, l ⋅ p l − f ⋅ q k − t 2 with w k,l being the elements of matrix W of the m-th processed subframe W m and functions q k t = exp 2 πjt k − N c − 1 2 Δf s and p l ( f) = exp(2 πjτ l f) with regard to a region of pre-defined discrete frequency offset values f ∈ F and discrete time offset values t ∈ T, Nc is the number of subcarriers in a subframe, Ns is the number of symbols in a subframe and Δf s is the subcarrier spacing - (stage 3) recursively combining the metric V m (f,t) in a non-coherent way over multiple demodulated NB-IoT subframes W m resulting in a new metric U m (f,t), - (stage 4) recursively computing a reference value S m over multiple demodulated NB-IoT subframes W m, based on a subframe energy of the presently processed demodulated NB-IoT subframe W m, - (stage 5) comparing the non-coherently combined new metric U m (f,t) with the reference value S m, whereas - if a normalized metric value of U m (f,t) / S m does not exceed a threshold K Threshold (m), stages 1 to 5 of the method are recursively repeated up to a pre-defined maximum number of subframes M max have been demodulated with M max being a natural number greater than 0, otherwise - (stage 6) if the normalized metric value of U m (f,t) / S m exceeds the threshold K Threshold (m), arg-max values with regard to the discrete frequency offset values f ∈ F and the discrete time offset values t ∈ T are computed based on the non-coherently combined metric values of U m ( f,t) obtaining an optimal value of the time-frequency offset estimate, provided the number of combined subframes W m has not exceeded M max. - (stage 1) demodulating a received NB-IoT subframe R m containing resource elements in the frequency domain with regard to a pre-computed reference NB-IoT subframe B m containing expected resource elements in the frequency domain, whereas the pre-computed resource elements of B m are derived from information of the serving cell, and storing resource elements of the demodulated NB-IoT subframe W m in matrix W, with m being a natural number equal or greater than 0 and presenting the m-th processed subframe, - (stage 2) computing a metric V m (f,t) defined by V m ( f,t) = ∑ K = 0 N C − 1 ∑ l = 0 N S − 1 W K, l ⋅ p l − f ⋅ q k − t 2 with w k,l being the elements of matrix W of the m-th processed subframe W m and functions q k t = exp 2 πjt k − N c − 1 2 Δf s and p l ( f) = exp(2 πjτ l f) with regard to a region of pre-defined discrete frequency offset values f ∈ F and discrete time offset values t ∈ T, Nc is the number of subcarriers in a subframe, Ns is the number of symbols in a subframe and Δf s is the subcarrier spacing - (stage 3) recursively combining the metric V m (f,t) in a non-coherent way over multiple demodulated NB-IoT subframes W m resulting in a new metric U m (f,t), - (stage 4) recursively computing a reference value S m over multiple demodulated NB-IoT subframes W m, based on a subframe energy of the presently processed demodulated NB-IoT subframe W m, - (stage 5) comparing the non-coherently combined new metric U m (f,t) with the reference value S m, whereas - if a normalized metric value of U m (f,t) / S m does not exceed a threshold K Threshold (m), stages 1 to 5 of the method are recursively repeated up to a pre-defined maximum number of subframes M max have been demodulated with M max being a natural number greater than 0, otherwise - (stage 6) if the normalized metric value of U m (f,t) / S m exceeds the threshold K Threshold (m), arg-max values with regard to the discrete frequency offset values f ∈ F and the discrete time offset values t ∈ T are computed based on the non-coherently combined metric values of U m ( f,t) obtaining an optimal value of the time-frequency offset estimate, provided the number of combined subframes W m has not exceeded M max.