IEEE NJ Coast Seminar, 12/01/2008Walid Ahmed et. al. Performance Analysis of a Novel Real- Time Closed-Loop Technique for Non- Linearity Correction of Power Amplifiers Walid K. M. Ahmed, Senior Member IEEE Qing Li, Member IEEE Ajit Reddy, Senior Member IEEE This work has been done while the authors were with Tyco Electronics

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Outline General Closed-Loop Block Diagram. Motivation: The EDGE/GSM Challenge! Predictive Algorithm. DTx System Model and PA Closed- Loop Correction. Results.

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Closed-Loop Approach: General Generic Pictorial Illustration of a Block Diagram for the Proposed Technique Target/ Reference Signal Non-Linear Device Detection/ Measurement Module Pre-Distortion Module

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge Highly efficient PA implies severe non-linearity. Hence, PA characteristics need to be corrected for, or linearized, in baseband, which involves pre- distortion of the baseband signal prior to modulating the PA. Otherwise, acceptable spectral and waveform quality performance can not be achieved for the transmitted signal. The severe PA non-linearity is also expected to change versus many parameters, such as temperature, PA power level setting (PA bias), PA load (VSWR), PA part-to-part tolerance and aging. Open-loop (factory calibration) of such a non-linear PA behavior is difficult since it is not possible to store such a large number of PA correction table versus so many parameters. On-chip non-real-time closed-loop solutions, e.g., calibrate the PA before every call or when turning the phone on and use the generated tables for the entire transmission duration, may not work if the PA is expected to change its AMAM/AMPM behavior during transmission due to, for example, load and VSWR changes.

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge Hence, frequent calibration is needed, which implies the need for an adaptive closed-loop solution. Two challenges come with such an approach: When to calibrate? Standards usually do not allocate off-air time for such an operation to be done! How to maintain signal quality while calibrating and transmitting at the same time?

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge In EDGE/GSM, for example, the switching-transient requirements are very stringent. It is not possible to let the PA run free of correction (i.e., without linearization) at any point in time. It is also not possible to have inaccurate correction tables applied to the PA. Moreover, a high-efficient PA will have a high level of non-linearity that will change from one EDGE/GSM burst to the next due to (at least) load/VSWR changes. Hence, pre- stored (or pre-loaded) correction tables will not suffice. The question then becomes: How and when do we calibrate the highly-non-linear PA in an EDGE/GSM application EVERY BURST?!!

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge The answer to the question of WHEN is attained by calibrating the PA on the EDGE/GSM burst ramp-up. In order to do that, the power ramp-up profile must be designed in a special way to ensure all amplitude levels are covered, yet a smooth rising ramp is achieved in order not to violate the tough EDGE/GSM switching-transients requirements. A simple, novel, robust and arbitrarily tunable ramp-up design technique has been proposed by Ahmed (see Walid K. M. Ahmed, Method and Apparatus for Signal Power Ramp-Up in a Communication Transmitter. Patent pending, U.S. Patent Serial # 11/385,212 ).

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge Augmented Symbols for Ramping up Tail Symbols of a Normal Burst

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge The answer to the question of HOW is the focus of this presentation.

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge...The chicken and egg problem: A major challenge in calibrating on the ramp is that while collecting the PA response data, we are also transmitting! Hence, we must have correction data available to ensure quality transmission…but didnt I just say we dont even have correction yet since we are still calibrating!! Moreover, the attempt at performing such an on-the-fly correction/linearization of the PA while ramping up will suffer from memory effects, that is, the collected measurements of the PA AM/AM and AM/PM non-linearity lag behind the current bias point of the PA due to the inevitable forward and feedback loop path delay. Hence, resulting-in inaccurate correction (or inversion of the PA non-linearity), which clearly leads to un- acceptable performance that fails the stringent requirements of the EDGE/GSM specifications. Accordingly, one must use a mechanism that can break into the future in some sense and predict what the next non-linearity point on the PA curve would look like based on previous on-the-fly measurements! A novel algorithm has been proposed by Ahmed and Li (see Walid K. M. Ahmed and Qing Li, Method and Apparatus for a Non-Linear Feedback Control System. Patent pending )

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge An Example of EDGE time-burst/time-mask/ST graph

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Motivation: The EDGE/GSM Challenge ST performance without AM/PM correction during ramp-up

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Predictive Algorithm PA Output ( y ) Actually measured PA Input ( x ) PA Output Actually measured Required Point PA Input ( ) yfx 1 - = Begin Prediction Path Delay Elapsed? Path Delay Elapsed? Extrapolate Corrected PA Input Extrapolate Corrected PA Input Apply PA Input Uncorrected, or from Preloads Apply PA Input Uncorrected, or from Preloads Prediction Time Over? Prediction Time Over? Stop and Use Measured Data as Correction LUT for Modulation Stop and Use Measured Data as Correction LUT for Modulation Simplified Pictorial Illustration of the Non-Linear Predictive Algorithm

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Predictive Algorithm X i-1 XiXi dX = X i – X i-1 dy = y i – y i-1 y i-1 yiyi Xi Yi dy/dx If for any x i in set X, there is a y i in set Y following the function y i = f(x i ) (i = 1, 2, …n), then we can find Y i = f(X i ) = f(x i ) + f (x i )*(X i – x i ) + 1/(2!) f (x i )*(X i – x i )2 + … If (X i – x i ) is small enough, we can simplify it to linear (1st order) extrapolating: Y i = f(X i ) f(x i ) + f (x i ) * (X i – x i ) = y i + dy/dx * (X i – x i ) ynyn xixi yiyi y i = f(x i ) x1x1 x2x2 xnxn y1y1 y2y2 Xi Yi

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed DTx System Model A differential-phase down- converter is used, i.e., RF input and RF output of the PA are mixed to save an extra un-modulated LO and to obtain AMAM while zoom into AMPM (see Walid K. M. Ahmed and Dale Douglas, Multi- Mode Selectable Modulation Architecture Calibration and Power Control Apparatus, System, and Method for Radio Frequency Power Amplifier. Patent pending, U.S. Patent Serial # 11/347,455 ).

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed PA Closed-Loop Correction

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results Amplitude Predictor Phase Predictor Numerical noise is filtered by a 3 rd order Bessel filter

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results DTx EDGE/GSM PA Characteristic Examples:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results EDGE/GSM Switched Transient and ACPR Specification Center Frequency (kHz) Bandwidth (kHz) ACPR Limit (dBm) ST Limit (dBm)

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results GSM Low Band Performance:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results GSM High Band Performance:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results EDGE Low Band Performance:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results EDGE High Band Performance:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results GSM Low Band Performance:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Performance Results EDGE Low Band Performance:

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Conclusions A novel predictive real-time closed-loop non-linearity correction technique has been proposed and tested. Despite the severe non-linearity of the PA, the predictive algorithm has been able to achieve healthy performance margins compared to the stringent limits set by the EDGE/GSM standard. The results indicate that the predictor based closed loop approach is very promising compared to other linearization techniques. It offers a simple, yet robust, efficient and intelligent solution that is not- only independent of modulation/transmission protocol constraints, but is also flexible enough to handle a wide range of PA non-linearity levels, in order to make the PA correction reliable and robust against variations due to temperature, process, power and aging. It should be noted that this technique can be applied to any PA modulation architecture (e.g., polar or IQ).

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed Conclusions, continued Also, this technique can be applied during actual modulation/data- transmission periods (i.e., not necessarily on a ramp-up time such as in EDGE/GSM) if the performance specifications allow for that. This is possible in interference-averaging modulation schemes such as CDMA technologies, where switching transients are not of critical importance. Hence, we can afford longer prediction periods in order to allow for sometime during modulation such that the signal covers all possible modulation signal levels (to collect a complete correction table). Clearly, in such a scenario, we can design the predictor to predict in both directions (i.e., forward and backwards prediction). This technique can also be enhanced to perform PA compression prediction. Hence, in addition to correcting for the PA non-linearity on- the-fly, the algorithm can also predict when the PA gets into compression and prevents such an event to avoid future signal clipping if the PA is driven hard into compression (see Walid K. M. Ahmed, Algorithm, Method and Apparatus for Real-Time Adaptive Compression-Control in Power Amplifiers. Patent Pending ).