1. Field of the Invention
Embodiments of the present invention generally relate to wireless digital communication systems that use multiple sub-carriers, and more particularly to systems and methods to estimate a Doppler frequency and a time-varying channel in a mobile wireless communications network.
2. Description of the Related Art
Advanced multimedia services continue to drive requirements for increasing data rates and higher performance in wireless systems. A multipath environment with time delay spread and a moving mobile environment with Doppler frequency spread provide challenges to high performance reception of high data rate signals. Digital communication systems that use multiple sub-carriers in parallel are becoming increasingly prevalent in order to offer good performance under varying noise conditions. For example the IEEE 802.11 wireless standards as well as the European digital audio broadcasting (DAB) and international digital video broadcasting handheld (DVB-H) standards employ a communication method known as Orthogonal Frequency Division Multiplexing (OFDM) to address multipath and other transmission impairments.
In an OFDM multiple sub-carrier system, a higher rate data signal may be divided among multiple narrowband sub-carriers that are orthogonal to one another in the frequency domain. The higher rate data signal may be transmitted as a set of parallel lower rate data signals each carried on a separate sub-carrier. In a wireless system, multipath may cause multiple versions of a transmitted data signal to arrive at a receiver with different delays, thereby resulting in inter-symbol interference created by received energy from different data signals transmitted at different times arriving at the receiver simultaneously. Each lower rate sub-carrier's symbol in an OFDM or DMT system may occupy a longer symbol period than in a higher rate single carrier system, and thus dispersion caused by multipath may be substantially contained within the longer symbol period, thereby reducing inter-symbol interference. Thus, OFDM may offer an effective technique to counter interference caused by multi-path fading.
Coherent detection, which may account for absolute phase and amplitude of the transmitted signal as used in quadrature amplitude modulation (QAM), provides better performance than non-coherent detection, which may only account for relative phase of the transmitted signal such as in differential phase shift keying (DPSK). Coherent detection may require knowledge of the changes induced on the transmitted signal by the intervening communication channel. As such, accurate channel estimation may be used to enable coherent detection.
When a multiple sub-carrier system transmits a set of symbols in parallel orthogonally, intervening transmission impairments may affect the orthogonality of the received sub-carrier symbols. To determine the effect of the transmission channel and impairments on receiver performance, the multiple sub-carrier system may send pre-determined transmit symbols, also known as “pilot” symbols, on a number of sub-carriers to estimate the channel. Specific sub-carriers to be used for pilot symbols may be fixed or may vary over time. For example, in an 802.11 system, four of the 52 orthogonal sub-carriers are dedicated as continuous “pilot” subcarriers; while in a DVB-H digital TV system, a number of different sub-carriers are used to transmit pilot symbols at regular intervals and transmit data symbols at other times.
Using an observed received symbol on a “pilot” sub-carrier with knowledge of an associated transmitted symbol, one may estimate a channel transfer characteristic at that sub-carrier frequency and during that OFDM symbol period. For channels that vary slowly in time, one may use selected sub-channel estimates at a sub-carrier frequency during a set of OFDM symbols to estimate the sub-channel at the same sub-carrier frequency for other OFDM symbols adjacent in time. Similarly for channels that vary slowly in frequency, one may use sub-channel estimates at a set of sub-carrier frequencies in a given OFDM symbol to estimate the sub-channel transfer characteristics at other sub-carrier frequencies in that same OFDM symbol. Forming a set of estimates based on a sample of estimates nearby in time or frequency is known as one-dimensional interpolation in time or frequency respectively. Forming an estimate of a sub-channel's transfer characteristic using sample estimates from a set of sub-channel estimates from different OFDM symbols and different sub-carrier frequencies is known as two-dimensional interpolation in both time and frequency.
Wiener filtering is a common form of interpolation used for channel estimation that minimizes the mean-square-error (MSE) of the estimates formed. A minimum MSE estimation Wiener filter may use knowledge of a correlation of the time-varying communication channel's frequency response in both the time domain and the frequency domain to estimate the channel's transfer characteristics. As the detailed statistics for a mobile wireless channel may be unknown, a Wiener filter may be designed using an approximation of the channel based on estimates of key variables such as the maximum echo delay, which may represent a spread in time, and a maximum Doppler frequency, which may represent the spread in frequency induced by the channel respectively.
While prior art methods exist to approximate a channel for Wiener filtering based on a maximum Doppler frequency and a maximum echo delay, these methods may not work well in at least two scenarios: (1) when the maximum Doppler frequency is small, i.e. the mobile is moving slowly or nearly stationary, and (2) when the maximum Doppler frequency is large, i.e. the mobile is moving quickly, and the maximum echo delay spread is also large. Some current techniques may use large amounts of memory and significant time to estimate a low maximum Doppler frequency. Known methods to interpolate channel estimates using a Wiener filter based on a relatively large echo delay for a mobile wireless receiver with a high maximum Doppler frequency may introduce high levels of aliased noise into the results thereby lowering performance. Thus, new methods to estimate the maximum Doppler frequency and to interpolate channel estimates are needed.