All researchers in Hardware-constrained communications are listed below.

**Professor Thomas Eriksson***Research area leader:*All researchers in Hardware-constrained communications are listed below.

### About the research area Hardware-constrained communications

When a communication signal is passed through RF transmitter and receiver hardware, various kind of imperfections in the analog hardware will destroy and distort the signal. The phenomenon is sometimes denoted "Dirty RF", and has increased tremendously in importance lately, due to increased demands on capacity and on cost-effective solutions.

Among the most important "Dirty RF" issues are:

Among the most important "Dirty RF" issues are:

- Nonlinear and inefficient power amplifiers
- Phase noise in oscillators
- I/Q imbalance in the modulator and demodulator circuits
- Cross-talk in the transmitter chain of a MIMO transmitter

Of these, we have worked for a long time with the three first issues, and we are currently ramping up our research also in the fourth. We discuss each of these areas briefly below.

#### Nonlinear and inefficient power amplifiers

All power amplifiers are nonlinear to some extent, where the degree varies with the amplifier type and architecture. The nonlinearity is in direct opposition with the efficiency of the amplifier (the consumed power). The amplifier can be made more linear by operating it in backoff (at low output powers), but amplifiers are most efficient when operated close to saturation, with low backoff.

We are currently working with these issues, focusing on Digital Predistortion (DPD) and on signal design for high-efficient amplifier architectures (Doherty, Load Modulation, PWM, delta-sigma etc.).

#### Phase noise in oscillators

An ideal oscillator should oscillate at a single frequency, but in practice all oscillators suffers from phase noise to some degree, leading to an increased bandwidth of the device. The consequence in the communication system is that the signals get phase-shifted in a random way, leading to problems in designing a good receiver.

Our work on phase noise is focused on various ways to analyze and compensate communication systems suffering from phase noise, e.g. by phase-noise modeling, capacity calculations, joint phase noise compensation and detection, phase mitigation in MIMO systems etc.

#### I/Q imbalance in modulator and demodulator circuits

The role of the modulator circuitry is to mix the communication signal with two equal-gain RF signals having 90 degrees phase shift (the I and Q channels). In practice, however, it is difficult to construct analog circuits with exactly 90 degrees phase shift and exactly equal gain. We get I/Q imbalance, which leads to signal distortion.

We are working with compensation of all kinds of I/Q imbalance, including frequency dependent gains, I/Q mixing, nonlinear effects and so on. The work is carried out within GHz Centre.

#### Cross-talk in MIMO transmitters

In a multiple-antenna (MIMO) transmitter, it is impossible to avoid some cross-talk between the transmitter chains. Such cross-talk leads to, if left unhandled, to that the separate amplifiers will see a time-varying load, which in turn leads to signal distortion. In a traditional system this is solved by adding isolator circuits between amplifiers and antennas, but this is costly in terms of component costs and reduced efficiency.

We are currently starting work on compensation MIMO crosstalk using signal processing, avoiding the need for isolators and making efficiency-optimization of the MIMO transmitter possible.

### Research projects

**All power amplifiers are nonlinear to some extent, where the degree varies with the amplifier type and architecture. The nonlinearity is in direct opposition with the efficiency of the amplifier (the consumed power)...**

**>**Nonlinear and inefficient power amplifiers**An ideal oscillator should oscillate at a single frequency, but in practice all oscillators suffers from phase noise to some degree, leading to an increased bandwidth of the device...**

**>**Phase noise in oscillators**The role of the modulator circuitry is to mix the communication signal with two equal-gain RF signals having 90 degrees phase shift (the I and Q channels). In practice, however, it is difficult to construct analog circuits with exactly 90 degrees phase shift and exactly equal gain...**

**>**I/Q imbalance in the modulator and demodulator circuits**In a multiple-antenna (MIMO) transmitter, it is impossible to avoid some cross-talk between the transmitter chains. Such cross-talk leads to, if left unhandled, to that the separate amplifiers will see a time-varying load, which in turn leads to signal distortion...**

**>**Cross-talk in MIMO transmitters