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Method for combined IQ imbalance and symbol timing error estimation and correction in OFDM-WLAN systems

IP.com Disclosure Number: IPCOM000126125D
Publication Date: 2005-Jul-01
Document File: 8 page(s) / 309K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for combined inverse quantization (IQ) imbalance and symbol timing error estimation and correction in orthogonal frequency division multiplexing (OFDM) wireless local area network (WLAN) systems. Benefits include improved functionality, improved performance, and improved design flexibility. Transreceiver based on Orthogonal Frequency Division Multiplexing (OFDM) are highly sensitive to the front end non-idealities. The In-phase/Quadrature (IQ) imbalance is one of the prominent RF front end non-idealities that can cause an increase in the symbol error rate in the OFDM receiver. If symbol timing error is present then a carrier dependent error would be introduced in to the IQ imbalance, and the current conventional IQ imbalance correction does not address this problem. This would make it difficult for accurate estimation and correction of IQ imbalance problem. This disclosure describes an implementation that would combine the joint estimation of symbol timing error and IQ imbalance and their correction. The outcome is better performance without much of computational burden.

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Method for combined IQ imbalance and symbol timing error estimation and correction in OFDM-WLAN systems

Background

Wireless Local Area Network (WLAN) Transreceiver based on Orthogonal Frequency Division Multiplexing (OFDM) are highly sensitive to the front end non-idealities. The In-phase/Quadrature (IQ) imbalance is one of the prominent RF front end non-idealities that can cause an increase in the symbol error rate in the OFDM receiver. If symbol timing error is present then a carrier dependent error would be introduced in to the IQ imbalance, and the current conventional IQ imbalance correction does not address this problem. This would make it difficult for accurate estimation and correction of IQ imbalance problem. This disclosure describes an implementation that would combine the joint estimation of symbol timing error and IQ imbalance and their correction. The outcome is better performance without much of computational burden.

Effect of IQ Imbalance in Presence of Symbol Timing Error

The OFDM-WLAN standards incorporate provisions for estimation of channel and receiver impairments by pre-pending the packet with a known preamble for every burst of transmission. The IEEE802.11a, for example, provides ten repeated short sequences, t1, t2 ... t10. A short OFDM training symbol consists of 12 sub-carriers, which are modulated by the elements of the sequence S given as

S = { 0, 0, 1+j, 0, 0, 0, -1-j, 0, 0, 0, 1+j, 0, 0, 0, -1-j, 0, 0, 0, -1-j, 0, 0, 0, 1+j, 0, 0, 0,                  ßNegative Frequencies

0,                                                                                                                                                                                             ß DC

0, 0, 0, -1-j, 0, 0, 0, -1-j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0 }                ß Positive Frequencies

These sequences can be effectively used to determine the IQ imbalance.

IQ imbalance in the receiver can be characterized by two parameters, the amplitude imbalance, and the phase mismatch. The IQ imbalance for a signal is represented in time domain as,

,                                                                                                                      --- (1)

Where  and.                                                         --- (1a)

If no IQ imbalance is present, then  and, and the above equation (1) reduces to. The FFT of the equation (1) results

                                                                                                                 --- (2)

Equation (2) implies that the gain and phase mismatches in the receiver mixture will cause the scaling of thesub-carrier by a complex factor. In addition, a spurious component will be present which is equal to the complex conjugate of the symbol at  sub-carrier multiplied by another complex term. The...