Wireless communication systems using an advanced radio access technology known as Long-Term Evolution, or LTE, are currently under development by members of the 3rd-Generation Partnership Project (3GPP). For downlink transmission (base station to mobile terminal), 3GPP has specified the use of Orthogonal Frequency-Division Multiplexing (OFDM) technology. In OFDM systems such as LTE, the signal is structured so that multiple simultaneously transmitted symbols are generally orthogonal to one another. As is well known to those skilled in the art, this is primarily achieved in two ways. First, the multiple sub-carriers of the OFDM signal are constructed by the transmitter so that they are mutually orthogonal over an OFDM symbol interval. Second, a so-called cyclic prefix, having a duration that exceeds the expected delay spread of the transmission channel, is inserted at the beginning of each OFDM symbol. Upon demodulation, the cyclic prefix is discarded, thus avoiding or at least mitigating inter-symbol interference caused by time dispersion.
Each symbol of the LTE OFDM signal is an information symbol, carrying user data or control channel data, or a reference, or “pilot,” symbol, and is modulated using QPSK, 16QAM, or 64QAM modulation schemes. Thus, upon reception, each symbol is demodulated to two, four, or six “soft” bits. Typically, the value of each soft bit corresponds to the log-likelihood ratio between the probability that the transmitted bit had a value of one and the probability that the originally transmitted bit had a value of zero. This likelihood is usually calculated using an estimated signal-to-noise ratio for this symbol.
All practical standards for data transmission also include a channel coding scheme to improve the reliability of the transmission of information bits. Generally speaking, such a scheme transforms N uncoded information bits into M encoded bits, where M>N. Although the details of various coding schemes vary, erroneous bits can often be corrected provided that the errors are relatively few, given the coding rate N/M and other code constraints. In most modern channel coding schemes, such as the turbo coding techniques specified by 3GPP for LTE, the decoder exploits the reliability information inherent in the soft bit values provided to it.
If the decoder is given correct information on which of the demodulated bits are more reliable than others, i.e., through the soft bit values, the decoding performance of a typical decoder is much better than if all bits are treated equally. Those skilled in the art will appreciate that different bits may have different levels of reliability, or “trustworthiness,” for several reasons. For instance, some received information symbols may be subject to more fading than others; the corresponding demodulated bits are therefore less reliable. Other symbols may be more affected by interference than others, again resulting in less reliable demodulated bits.
In typical OFDM systems, including LTE systems, certain symbols in the time-frequency grid defined by the OFDM signal are designated as reference symbols. These reference symbols have a value that is known to the receiver and are used by the receiver as pilot symbols to characterize the propagation channel between the transmitter and the receiver and to estimate the noise and interference variance of the received signal. These channel estimates and noise-plus-interference estimates are used to calculate an estimated signal-to-interference ratio (SIR) for use in demodulating the information symbols.
One problem with this general approach is that the reference symbols are relatively sparse. In general, the propagation channel information yielded by the reference symbols is generally highly correlated with the propagation channel characteristics experienced by nearby information symbols in the time-frequency grid of the OFDM signal. Thus, channel estimates obtained from the reference symbols may be reliably used in demodulating and decoding neighboring symbols. However, the same may not be true with respect to interference, especially inter-cell interference caused by OFDM signals transmitted from neighboring base stations.
In a reuse-one system such as 3GPP LTE systems, interference from neighboring base stations can be heard by a mobile terminal in a large part of the serving cell. If the interfering downlink signal is less than fully utilized, some of the resource elements of the interfering OFDM symbol are empty, while others contain modulated data. Thus, the interference to the desired OFDM signal varies from one resource element to another. Because the reference symbols used by the wireless receiver to estimate interference power are sparse, this variation from one resource element to another is not measured.
As will be discussed in more detail below, the conventional assumption that all resource elements in the vicinity of a reference symbol are subject to the same levels of interference as the reference symbol can cause poor performance in the decoding of the data symbols carried by those resource elements. Accordingly, improved techniques in OFDM systems for estimating inter-cell interference from neighboring OFDM signals are needed.