Coherent OFDM detection requires accurate channel state information (CSI). Mobile radio channels are both time and frequency dispersive, especially at high vehicular speeds, which makes channel estimation a challenging problem in system design. Conventional preamble-based and pilot-aided channel estimation require numerous reference signals, which significantly compromises the system throughput. This paper proposes a novel low complexity iterative turbo channel estimation technique which makes use of preamble, pilots and soft decoded data information in an iterative fashion to improve the system performance over the time and frequency selective fading channel while maintaining the system throughput. The numerical and analytical results show that the proposed technique can approach the performance of systems with perfect CSI with much fewer preamble and pilots symbols compared to existing channel estimation methods.

/pdf/iterative-turbo-channel-estimation-for-ofdm-system-over-4ksu3l7zrd.pdf

Iterative Turbo Channel Estimation for OFDM System over Rapid Dispersive Fading Channel

Techniques for multiplexing pilots in a wireless transmission are described. In one aspect, a transmitter station generates multiple pilot sequences for multiple transmit antennas, with each pilot sequence comprising pilot symbols sent in the time domain on a different set of subcarriers. The transmitter station further generates multiple pilot transmissions for the transmit antennas based on the pilot sequences. In another aspect, a transmitter station generates multiple pilot sequences for multiple transmit antennas based on frequency-domain code division multiplexing (FD-CDM) of a Chu sequence defined by a transmitter-specific value. The transmitter station further generates multiple pilot transmissions for the transmit antennas based on the pilot sequences. In yet another aspect, a transmitter station generates multiple pilot transmissions for multiple transmit antennas based on a first multiplexing scheme and generates multiple data transmissions based on a second multiplexing scheme that is different from the first multiplexing scheme.

Method and apparatus for pilot multiplexing in a wireless communication system

Current OFDM systems assume the channel is not time varying within one OFDM frame, and use channel estimates obtained from preamble or pilots for data symbols within the same frame. This performs poorly in rapid dispersive fading channel with high mobility. This paper proposes a novel iterative turbo channel estimation to improve the performance of OFDM system over fading channel with both time and frequency selectivity. Unlike the conventional channel estimation techniques only using preamble or pilots, the proposed channel estimation technique makes use of preamble, pilots and soft coded data information in a turbo iterative approach. Mean square error analysis and simulation results show that the proposed channel estimation technique can approach the performance of perfect channel state information (CSI) with fewer pilots insertion.

/pdf/iterative-turbo-channel-estimation-for-ofdm-system-over-3k1eftoyty.pdf

Wireless communication systems have evolved over the ages. However, there are some undesirable effects of a wireless communication channel on the signals transmitted through it which are caused by the physical properties of the channel. These in turn always result in attenuation, distortion, delays, and phase shift of the signals arriving at the receiver end of the communication system. Consequently, there is a need to provide perfect and up-to-date estimate of the channel, in order to compensate for these effects, and for accurate signal demodulation, equalization, decoding, and a host of other baseband processing applications at the receiver end of the systems. In this article, a review of the various channel estimation techniques is provided. This review will be of assistance in reaching a compromised decision regarding channel estimation techniques to be employed in any wireless communication systems being developed.

Review of Channel Estimation for Wireless Communication Systems

In this letter, we consider a bi-directional relay network in which two users, U1 and U2, exchange their information via a relay station, RS. Multiple antennas are deployed at both users and at RS. Single carrier cyclic prefix (SCCP) is used to combat intersymbol interference (ISI) in frequency-selective fading channels. The transmission process is divided into two time slots. At the first time slot, both users send their information to RS concurrently. RS then amplifies and broadcasts its received signals at the second time slot. We propose an algorithm to estimate the channel information at end users based on the least square (LS) principle. To further minimize the mean square error (MSE) of the estimate, a method to design the optimal training sequences is also proposed. Simulation results show that the performance achieved by our optimal design is close to that with perfect channel information.

Optimal training sequences for channel estimation in bi-directional relay networks with multiple antennas

This letter proposes a novel method for channel estimation in a single-carrier multiple input-multiple output (MIMO) system with frequency-domain equalization/detection. To this end, we construct novel short MIMO training sequences that have constant envelope in the time domain to preclude the peak-to-average power ratio problem encountered in many systems that utilize the frequency domain for data recovery. Simultaneously, the spectrum in the frequency domain is flat except for a grid of nulls for predefined frequency tones. Armed with these sequences, we provide an algorithm that is optimal in the least squares (LS) sense at a potentially low computational cost. Results show that the algorithm performs identically to other proposed LS techniques. Furthermore, the algorithm is extremely bandwidth efficient in that the total training overhead required to obtain full CSI is just one block.

/pdf/a-channel-estimation-algorithm-for-mimo-scfde-ueyhu5kymy.pdf

A channel estimation algorithm for MIMO-SCFDE

This paper investigates the performance of channel estimation techniques applied to a proposed fourth generation (4G) multi carrier - code division multiple access (MC-CDMA) system. Coherent detection requires a per-sub band estimation of the channel frequency response for each coded orthogonal frequency division multiplexing (COFDM) symbol. This paper compares the performance of a number of pilot symbol assisted modulation (PSAM) methods for achieving such estimates. Performance results are characterised by bit-error-rate (BER), data throughput (DT) and mean-squared error (MSE) for ETSI UTRA specified channel environments. It is shown that in an indoor environment, FFT filtering provides superior data throughput performance, whereas in a high Doppler vehicular environment, a reduced complexity Wiener filter is able to offer the best performance.

/pdf/performance-evaluation-of-channel-estimation-techniques-for-5a5l70g3oa.pdf

Performance evaluation of channel estimation techniques for a proposed '4G' MC-CDMA based system in a time varying channel

In this paper an iterative channel estimation technique for a mobile fourth generation coherent orthogonal frequency division multiplexing (OFDM) system is proposed. Coherent detection dictates that a per-subband estimate of the frequency response of the channel is generated for each OFDM symbol. This is often achieved by inserting pilot symbols amongst the data symbols in the OFDM modulation grid and interpolating accordingly. This paper proposes a lower complexity channel estimation method that exploits coding gain to achieve a channel estimate that yields an overall system performance that is shown to be comparable to the scattered pilot system, but with only one initial training sequence. A tracking algorithm is proposed which improves performance at high mobile velocities.

/pdf/data-derived-iterative-channel-estimation-with-channel-4uliz3il77.pdf

Data-derived iterative channel estimation with channel tracking for a mobile fourth generation wide area OFDM system

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Papers

A channel estimation algorithm for MIMO-SCFDE

Performance evaluation of channel estimation techniques for a proposed '4G' MC-CDMA based system in a time varying channel

Data-derived iterative channel estimation with channel tracking for a mobile fourth generation wide area OFDM system