Mark A. Beach

Controlling Active Load–Pull in a Dual-Input Inverse Load Modulated Doherty Architecture

Mathematical analysis of Doherty amplifiers have assumed many simplifications. Most notably, the peaking amplifier does not contribute power into the load and the peaking stage has an observed impedance of infinity. This paper will show that these simplifications impair the performance of a single-input Doherty amplifier and that phase tuning for compensation is needed to improve the overall system performance. The dual-input Doherty amplifier is capable of overcoming the limitations of power-dependent phase imbalance and phase compensation lines at the input of the peaking stage; however, the characterization of such an architecture is not straightforward. A new measurement technique is proposed to measure dc current, dc voltage, and output power levels to allow unique characterization of a dual-input Doherty amplifier. Phase compensation lines at the input of the peaking amplifier will be shown to be not required, as long as correct offset lines are calculated for both the carrier and peaking stage and that the λ/4 transmission-line length is not necessarily required for active load-pull. Results of a dual-input inverse load modulated Doherty amplifier are presented where the peaking stage delivers 10 dB less of maximum output power than the carrier, while still maintaining Doherty behavior. The peaking stage can therefore be implemented with a smaller device than the carrier.

Overlapped segment piece-wise polynomial pre-distortion for the linearisation of power amplifiers in the presence of high PAPR OFDM signals

A modified piece-wise polynomial pre-distortion is proposed, investigated and compared against classic memoryless polynomial pre-distortion. The proposed enhanced piece-wise polynomial pre-distortion uses the overlap between segments to ensure continuity in the total pre-distortion function. The implementation of overlap allows the use of a simple error estimation algorithm to minimise hand over error. The classic and proposed piece-wise pre-distortion performances are assessed and compared for several sets of polynomial coefficients. The proposed method is shown to consistently outperform classical polynomial pre-distortion in terms of required coefficients and linearity improvement. The method is applied for the linearisation of an envelope tracking Class-J PA at 1.7GHz, under a 1.4MHz LTE signal with a 14.4dB PAPR.

Characterisation of ultra-wideband antenna arrays with spacings following a geometric progression

An ultra-wideband (UWB) antenna array is designed to satisfy given constraints in a (large) contiguous frequency range. Space limitations are tight in many applications and it is often infeasible to design the antenna array with preferred element spacings of λ/2 at the centre frequency. In this study, the authors compare uniform linear UWB antenna arrays and non-uniform reduced-aperture UWB antenna arrays with more compact element spacings following a geometric progression. The authors analyse and discuss performance degradations with respect to irregularly spaced monopoles. The manufactured non-uniform linear UWB antenna arrays are characterised by measurements in an anechoic chamber. The results are correlated with the arrayfactor and compared with measured data of uniform linearly spaced monopoles in the time and frequency domains. It is found that a compact non-uniform linear UWB array with element spacings following a geometric progression can be designed such that its main lobe performs very close to that of a uniform linear array, with an antenna array size reduction of 23.6% for a seven-element non-uniform linear UWB array.

Eigen-Coherence and Link Performance of Closed-Loop 4G Wireless in Measured Outdoor MIMO Channels

Employing feedback in multiple-input-multiple-output (MIMO) systems offers the potential to significantly increase data rates over open-loop schemes. However, the impact the propagation channel has on the behavior and performance of the scheme must be considered and, where appropriate, the signaling designed to suit. In this paper, measured outdoor MIMO propagation data is used to evaluate initially the distributions and statistics of the coherence of the eigenmodes of the channel, for a wide range of mobility scenarios and user devices. Then, the impact on link performance of the differing coherences in time and frequency is evaluated in terms of the feedback delay and spectral spacing of pilots in a practical feedback scheme used in mobile WiMAX. It is found that not only mobility and physical environment, but also the directivity of the antenna array affect the eigen-coherence and also, therefore, the performance of a feedback scheme. Bit-error rate and information capacity are both shown to be sensitive to increased feedback delay and pilot spacing, however, the closed-loop system always outperforms its open-loop counterpart.

Short range ultra-wideband multiple input multiple output channel measurements

In this paper we analyze measurements of short range ultra-wideband (UWB) multiple input multiple output (MIMO) transmission. 2 × 2 near field UWB measurements were undertaken in an anechoic chamber using a Medav UWB MIMO channel sounder. Channel characteristics were recorded for transmitter to receiver separations of 0.1 m to 0.7 m. Two independent channels were achieved, and channel capacity is twice as high as capacity of a single channel.

Inverse active load-pull in an inverse Doherty amplifier

A dedicated inverse Doherty amplifier is presented in this paper using commercially available 10W GaN HEMT devices. Results show that an emulated 1W peaking amplifier and a 10W carrier amplifier can be used to create a highly efficient amplification architecture. Ideal inverse active load-pull is introduced theoretically with experimental results showing good agreement with the theory. A generic characterization scan of the inverse Doherty amplifier yields state of the art drain and power added efficiency in the output power back-off. A drain and power added efficiency of 60% and 50% are obtained at 10dB output power back-off respectively with a maximum output power of 41dBm at 2.1GHz.

Low complexity distributed spectrum access algorithm for cognitive radio

The selection of optimum frequency channels by cognitive radio devices is considered challenging, particularly when such devices have to mutually co-exist and also take account of existing users of the spectrum. In addition to the problem of accurate spectrum sensing, there are additional considerations that result because of the dynamic capabilities of the terminals. Particular problems include that of allowing terminals to discover each other when they may be tuned to different frequencies, operating without a common control channel. This work has developed a spectrum access algorithm by which cognitive radio terminals monitor the available spectrum, discover each others presence and collaborate to select the optimum communications channel. This is achieved while avoiding interference to incumbent users. In contrast to previous work in this area, the proposed algorithm is of low complexity, requiring the terminals neither to dedicate part of their operational time to spectrum sensing, nor to periodically switch to a control channel. It is thus suitable for implementation in simple terminals. A simulation tool, which models the interaction and behaviour of the terminals, has been produced and used to test the performance of the algorithm in a demanding environment. It was shown that the algorithm permits a high level of data throughput, with protection for incumbent spectrum users.