It’s well known that LTE is heavily influenced by the requirements for high peak transmission rate, spectral efficiency, and multiple channel bandwidths. To fulfill these requirements, orthogonal frequency division multiplexing (OFDM) has become one of the key technologies for the physical layer (for more on this subject, please read our previous blog post).
Thanks to its multi-carrier nature, OFDM technology is perfect for data rate optimization for all users in a cell. That feature is the fundamental aspect of OFDMA (a multi-user version of OFDM), which is the art of multiplexing the traffic by allocating specific patterns of sub-carriers in time-frequency space to different users. However, OFDMA has certain drawbacks such as high sensitivity to frequency offset and high peak-to-average power ratio (PAPR). At the base station, the PAPR issues can be overcome by the usage of high compression point power amplifiers and amplifier linearization techniques. On the user equipment (UE) side, these techniques can be expensive to implement. Hence, LTE gets around this by using single carrier FDMA (SC-FDMA) with a cyclic prefix, which reduces PAPR from its single carrier nature (see our blog post on SC-FDMA for more information). That being said, having an OFDM/SC-FMDA modulator/demodulator does not mean that you are ready to attack a real LTE physical layer. The requirements go beyond the modulation techniques.
One of the features that needs to be supported is the use of a variable channel bandwidth. Even the lowest category of LTE user equipment (UE) is required to support all the bandwidths specified for the bands in which the UE is designed to operate, which in general includes 1.4, 3.0, 5.0, 10.0, 15.0 and 20.0 MHz. This implies that transceivers for LTE must be more adaptable than those of previous systems, while also being cleaner in transmission and having better selectivity in reception.
The second important standard feature for LTE is the support of many MIMO schemes. In LTE, it is assumed that even the smallest UE possesses at least two antennas.
This brings us to a third feature: adaptability to better suit different signal-to-interference-plus-noise ratio (SINR). From 4 x 2 MIMO with QAM-64 in high SINR conditions to SISO QPSK in low SINR conditions, together with the ability to vary the instantaneous bandwidth for a specific user, LTE delivers a large variety of modes.
Finally, the LTE signal structure itself brings a challenge on the radio side. The OFDM and SC-FDMA techniques does bring robustness against multipath propagation, but the OFDM synchronization, frequency offset, and phase noise sensitivities are still there(see our blog post on radio impairments).
Nutaq’s Radio420X FMC is one example of a transceiver that provides all the features needed to fulfill these requirements.
Firstly, the selectable bandwidth filter bank supports different bandwidth options and the actual bank of band filters, specific to LTE bands.
Secondly, with the double stack FMC technology from Nutaq, the 2×2 MIMO capability in FDD and TDD mode is enabled and can perfectly fit on any FMC high-pin-count compliant boards. If higher MIMO order is required, additional carriers with their own FMC Radio420X can be added and all the FMC radios can be phase synchronized together from the front panel external clock input
Thirdly, the auto-calibration feature, the shielding, and the excellent RF performance of the LMS6002D RFIC makes the deployment of QAM-64 OFDM physical layers a reality (see our OFDM reference design).
Finally, the ability to control in real time the gains and frequency of the radio’s power amplifiers and local oscillators, respectively, enables the implementation of fast AGC and CFO compensation algorithms which are needed in LTE to maximize SNR and to reduce inter-carrier interference (ICI) to a minimum.
1. Sesia, Stefania, Issam Toufik, and Matthew Baker. 2011. LTE – The UMTS Long Term Evolution: From Theory to Practice, 2nd Edition. West Sussex, U.K.: John Wiley & Sons Ltd.
2. Telesystem Innovations Inc. 2010. LTE in a Nutshell: The Physical Layer. http://www.tsiwireless.com/docs/whitepapers/LTE%20in%20a%20Nutshell%20-%20Physical%20Layer.pdf