BASEBAND RADIO MODEM DESIGN USING GRAPHICS PROCESSING UNITS

Abstract

A modern radio or wireless communications transceiver is programmed via

software and firmware to change its functionalities at the baseband. However, the

actual implementation of the radio circuits relies on dedicated hardware, and the

design and implementation of such devices are time consuming and challenging. Due

to the need for real-time operation, dedicated hardware is preferred in order to meet

stringent requirements on throughput and latency. With increasing need for higher

throughput and shorter latency, while supporting increasing bandwidth across a

fragmented spectrum, dedicated subsystems are developed in order to service individual

frequency bands and specifications. Such a dedicated-hardware-intensive

approach leads to high resource costs, including costs due to multiple instantiations

of mixers, filters, and samplers. Such increases in hardware requirements in turn

increases device size, power consumption, weight, and financial cost.

If it can meet the required real-time constraints, a more flexible and reconfigurable

design approach, such as a software-based solution, is often more desirable

over a dedicated hardware solution. However, significant challenges must be

overcome in order to meet constraints on throughput and latency while servicing

different frequency bands and bandwidths. Graphics processing unit (GPU) technology

provides a promising class of platforms for addressing these challenges. GPUs,

which were originally designed for rendering images and video sequences, have been

adapted as general purpose high-throughput computation engines for a wide variety

of application areas beyond their original target domains. Linear algebra and signal

processing acceleration are examples of such application areas.

In this thesis, we apply GPUs as software-based, baseband radios and demonstrate

novel, software-based implementations of key subsystems in modern wireless

transceivers. In our work, we develop novel implementation techniques that allow

communication system designers to use GPUs as accelerators for baseband processing

functions, including real-time filtering and signal transformations. More

specifically, we apply GPUs to accelerate several computationally-intensive, frontend

radio subsystems, including filtering, signal mixing, sample rate conversion,

and synchronization. These are critical subsystems that must operate in real-time

to reliably receive waveforms.

The contributions of this thesis can be broadly organized into 3 major areas:

(1) channelization, (2) arbitrary resampling, and (3) synchronization.

1. Channelization: a wideband signal is shared between different users and

channels, and a channelizer is used to separate the components of the shared signal

in the different channels. A channelizer is often used as a pre-processing step in

selecting a specific channel-of-interest. A typical channelization process involves signal

conversion, resampling, and filtering to reject adjacent channels. We investigate

GPU acceleration for a particularly efficient form of channelizer called a polyphase

filterbank channelizer, and demonstrate a real-time implementation of our novel

channelizer design.

2. Arbitrary resampling: following a channelization process, a signal is often

resampled to at least twice the data rate in order to further condition the signal.

Since different communication standards require different resampling ratios, it is

desirable for a resampling subsystem to support a variety of different ratios. We

investigate optimized, GPU-based methods for resampling using polyphase filter

structures that are mapped efficiently into GPU hardware. We investigate these

GPU implementation techniques in the context of interpolation (integer-factor increases

in sampling rate), decimation (integer-factor decreases in sampling rate),

and rational resampling. Finally, we demonstrate an efficient implementation of arbitrary

resampling using GPUs. This implementation exploits specialized hardware

units within the GPU to enable efficient and accurate resampling processes involving

arbitrary changes in sample rate.

3. Synchronization: incoming signals in a wireless communications transceiver

must be synchronized in order to recover the transmitted data properly from complex

channel effects such as thermal noise, fading, and multipath propagation. We investigate

timing recovery in GPUs to accelerate the most computationally intensive

part of the synchronization process, and correctly align the incoming data symbols

in the receiver. Furthermore, we implement fully-parallel timing error detection to

accelerate maximum likelihood estimation.