This invention relates to adaptive iterative detection schemes.
To improve performance, coverage, and efficiency, modern wireless communication systems utilize digital signaling techniques. However, as these communication systems become more widespread, the problem of achieving clear reception in a noisy transmission channel becomes important. Cellular phone systems are one example of wireless communication systems and they also serve to illustrate the problems associated with such systems. For example, a typical disturbance phenomenon in a cellular phone system is known as inter-symbol interference. This interference may occur from fading, reflections, delays, or other disturbances from the cellular phone system, which typically involve mobility between the transmitter and receiver.
In digital signaling, it should be possible to extract the “correct” symbol despite the presence of interference from other symbols using error control coding. However, this task is often complicated by poor knowledge of the characteristics of the transmission channel itself. For example, it is difficult to know just how cellular phone signals are being reflected, delayed, faded, and otherwise interfered with, as the cellular phone is moved around. Therefore, poor knowledge of the channel leads to uncertainty about integrity and accuracy of the received data. Accordingly, there are many known methods and systems that attempt to estimate the parameters of a transmission channel in order to better reconstruct a transmitted signal. However, in many practical situations, perfect channel estimation is not available at the receiver. Consequently an adaptive iterative receiver may deal with the unknown, and possibly time varying parameters.
Recognizing the above-described difficulties, a new class of codes, referred to as turbo codes, has been introduced. This class of turbo codes has made signaling at power efficiencies close to the theoretical limits possible. The features of a turbo code include parallel code concatenation, non-uniform interleaving, and iterative decoding. Because turbo codes may substantially improve energy efficiency, they are attractive for use over channels that are power and/or interference limited. Furthermore, turbo codes also offer good performance over fading channels. A turbo decoder may be used to decode the turbo code. The turbo decoder may include two soft-input/soft-output (SISO) decoding modules that work together in an iterative fashion.
The SISO decoding module is the basic building block for established iterative detection (ID) techniques for a system having a network of finite state machines, or more generally, subsystems. However, performing iterative detection for systems having parametric uncertainty may be complex and difficult. Previously proposed SISO techniques are either based on an oversimplified channel model, or have complexity that grows exponentially with the observation length (or the smoothing lag).