Linearity Requirements in LTE-Advanced Mobile Transmitter

LTE-Advanced (LTE-A) is an emerging wireless communication system, which builds on the foundation of Long Term Evolution (LTE), adding many unprecedented arrangements in utilization of radio resources. Most notably LTE-A allows allocating non-contiguous resources in frequency domain, which significantly enhances the flexibility of the multiple access scheme. Contiguous and non-contiguous carrier aggregation, both intra- and inter-band, are essential components of LTE-A. However, these features set very high requirements especially for mobile transmitters, which simultaneously should be cheap, small, linear and power efficient. Linearity in particular is an important aspect because non-contiguous allocation is prone to produce severe intermodulation distortion, which will degrade transmit signal quality and cause interference to users operating on adjacent frequency ranges. Because variety of wireless systems have to coexist and interoperate in the scarce spectrum supply, there are stringent requirements for unwanted spectrum emissions. There is an inherent trade-off between linearity and power efficiency. Therefore it is significantly difficult to fulfill regulatory spectrum emission requirements with current transmitter technology without sacrificing battery life when operating near the maximum output power. As a compromise LTE-A allows relaxations to maximum output power requirement according to the used submodulation, number of used resource blocks and possible coexistence situations. In practice the transmitter power amplifier (PA) input power is reduced which linearises the PA response. However, this forces the PA to operate less efficiently. This simultaneously constrains using larger constellations and/or higher coding rates because of degraded link budget. Therefore maximum power reduction (MPR) should be minimized. In this thesis LTE and LTE-A are introduced and the models and effects of transmitter nonlinearity are discussed. Linearity requirements of mobile LTE-A transmitter are evaluated using both simulations and measurements in different transmission scenarios, including LTE and LTE-A releases from 8 to 12. The results of the analysis can be used in development of intelligent radio resource management algorithms, advanced MPR specifications and digital predistortion systems

Linearity Requirements in LTE-Advanced Mobile Transmitter

LTE-Advanced (LTE-A) is an emerging wireless communication system, which builds on the foundation of Long Term Evolution (LTE), adding many unprecedented arrangements in utilization of radio resources. Most notably LTE-A allows allocating non-contiguous resources in frequency domain, which significantly enhances the flexibility of the multiple access scheme. Contiguous and non-contiguous carrier aggregation, both intra- and inter-band, are essential components of LTE-A. However, these features set very high requirements especially for mobile transmitters, which simultaneously should be cheap, small, linear and power efficient. Linearity in particular is an important aspect because non-contiguous allocation is prone to produce severe intermodulation distortion, which will degrade transmit signal quality and cause interference to users operating on adjacent frequency ranges. Because variety of wireless systems have to coexist and interoperate in the scarce spectrum supply, there are stringent requirements for unwanted spectrum emissions. There is an inherent trade-off between linearity and power efficiency. Therefore it is significantly difficult to fulfill regulatory spectrum emission requirements with current transmitter technology without sacrificing battery life when operating near the maximum output power. As a compromise LTE-A allows relaxations to maximum output power requirement according to the used submodulation, number of used resource blocks and possible coexistence situations. In practice the transmitter power amplifier (PA) input power is reduced which linearises the PA response. However, this forces the PA to operate less efficiently. This simultaneously constrains using larger constellations and/or higher coding rates because of degraded link budget. Therefore maximum power reduction (MPR) should be minimized. In this thesis LTE and LTE-A are introduced and the models and effects of transmitter nonlinearity are discussed. Linearity requirements of mobile LTE-A transmitter are evaluated using both simulations and measurements in different transmission scenarios, including LTE and LTE-A releases from 8 to 12. The results of the analysis can be used in development of intelligent radio resource management algorithms, advanced MPR specifications and digital predistortion systems

Linearity Challenges of LTE-Advanced Mobile Transmitters: Requirements and Potential Solutions

In order to provide higher data rates and to improve radio spectrum utilization, 3GPP has introduced the concept of carrier aggregation (CA) in its Release 10 and onwards, commonly known as LTE-Advanced standard. The CA technology, in particular when applied in a noncontiguous manner, poses serious design and implementation challenges for radio transceivers, mainly due to the allowed flexibility in the transmitted signal characteristics and the nonlinear radio frequency (RF) components in the transmitter (TX) and receiver (RX) chains. As a consequence, substantial nonlinear distortion may occur that not only degrades the transmitted signal quality but can also affect the concurrent operation of the coexisting receiver, when operating in the frequency division duplex (FDD) mode. In this article, the key technical design challenges in terms of linearity requirements for LTE-Advanced mobile terminals are reviewed, and the corresponding self-interference problem related to the potential desensitization of the device's own receiver is highlighted. Then, technical solutions to mitigate the self-interference at the RX band due to a nonlinear power amplifier (PA) in the transmitter chain are reviewed, with specific emphasis on digital self-interference cancellation methods. As demonstrated through simulation and actual RF measurement examples, the cancellation solutions can substantially mitigate the RX desensitization problem, thus relaxing the RF isolation requirements between the TX and RX chains. Such cancellation methods are one potential enabling technique towards the full exploitation of the fragmented RF spectrum and the CA technology in future LTE-Advanced and beyond mobile networks.acceptedVersionPeer reviewe