Jan Tubbax

Compensation of IQ imbalance and phase noise in OFDM systems

Nowadays, a lot of effort is spent on developing inexpensive orthogonal frequency-division multiplexing (OFDM) receivers. Especially, zero intermediate frequency (zero-IF) receivers are very appealing, because they avoid costly IF filters. However, zero-IF front-ends also introduce significant additional front-end distortion, such as IQ imbalance. Moreover, zero-IF does not solve the phase noise problem. Unfortunately, OFDM is very sensitive to the receiver nonidealities IQ imbalance and phase noise. Therefore, we developed a new estimation/compensation scheme to jointly combat the IQ imbalance and phase noise at baseband. In this letter, we describe the algorithms and present the performance results. Our compensation scheme eliminates the IQ imbalance based on one OFDM symbol and performs well in the presence of phase noise. The compensation scheme has a fast convergence and a small residual degradation: even for large IQ imbalance, the overall system performance for an OFDM-wireless local area network (WLAN) case study is within 0.6 dB of the optimal case. As such, our approach greatly relaxes the mismatch specifications and thus enables low-cost zero-IF receivers.

Joint compensation of IQ imbalance and frequency offset in OFDM systems

Zero-IF receivers are gaining interest because they enable low-cost WLAN OFDM terminals. However, zero-IF receivers introduce IQ imbalance which may have a huge impact on performance. Rather than increasing component cost to decrease the IQ imbalance, an alternative is to tolerate the IQ imbalance and compensate it digitally. Current solutions converge too slowly for bursty WLAN communication. Moreover, the tremendous impact of a frequency offset on the IQ estimation/compensation problem is not considered. We analyze joint IQ-CFO estimation/compensation and propose a low-cost, highly effective compensation scheme. For large IQ imbalance (/spl epsi/=10%, /spl Delta//spl phi/=10/spl deg/) and large frequency offset, our solution results in an average remaining degradation below 0.5 dB compared to the reference case without IQ imbalance or frequency offset. It therefore enables the design of low-cost, low-complexity WLAN OFDM receivers.

OFDM versus Single Carrier with Cyclic Prefix: a system-based comparison

In recent years, wireless indoor networks have received a lot of scientific and industrial attention. Most systems rely on the use of orthogonal frequency division multiplexing (OFDM) because of its capability to elegantly cope with multipath interference. However, while OFDM provides a nice solution for the digital modem, its front-end requirements should be investigated as well. To that goal, we have set up a simulation environment which comprises both the digital modem and the most important front-end nonidealities. We show that for the same data rate, bandwidth and transmit power constraints, Single-Carrier with Cyclic Prefix (SC-CP) allows the design of a more power-efficient modem than OFDM and is therefore a better candidate for portable wireless terminals.

Compensation of transmitter IQ imbalance for OFDM systems

Zero-IF transceivers are gaining interest because of their potential to enable low-cost OFDM terminals. However, the zero-IF architecture introduces IQ imbalance which may have a huge impact on the performance. Rather than increasing component cost to decrease the IQ imbalance, an alternative is to tolerate the IQ imbalance and compensate it digitally. Current solutions require extra analog hardware at the transmitter. We analyze transmit IQ imbalance estimation and propose a low-cost, highly effective estimation scheme, which is fully digital and located at the receiver. Performance analysis shows that this scheme can provide up to 4 dB gain while meeting the IEEE 802.11a constellation accuracy specification and more if larger IQ imbalance is present in the transmitter. It therefore enables the design of low-cost, low-complexity OFDM modems.

OFDM versus single carrier: a realistic multi-antenna comparison

There is an ongoing discussion in the broadband wireless world about the respective benefits of orthogonal frequency division multiplexing (OFDM) and single carrier with frequency domain equalization (SC-FD). SC-FD allows for more relaxed front-end requirements, of which the power amplifier efficiency is very important for battery-driven terminals. OFDM, on the other hand, can yield improved BER performance at low complexity. Both schemes have extensions to multiple antennas to enhance the spectral efficiency and/or the link reliability. Moreover, both schemes have nonlinear versions using decision feedback equalization (DFE) to further improve performance of the linear equalizers. In this paper, we compare these high-performance OFDM and SC-FD schemes using multiple antennas and DFE, while also accounting for the power amplifier efficiency. To make a realistic comparison, we also consider most important digital imperfections, such as channel and noise estimation, transmit and receive filtering, clipping and quantization, as well as link layer impact. Our analysis shows that for frequency-selective channels the relative performance impact of the power amplifier is negligible compared to the frequency diversity impact. The higher frequency diversity exploitation of SC-FD allows it to outperform OFDM in most cases. Therefore, SC-FD is a suitable candidate for broadband wireless communication.

Joint compensation of IQ imbalance, frequency offset and phase noise in OFDM receivers†

Zero-IF receivers are getting a lot of attention because of their potential to enable low-cost OFDM terminals. However, zero-IF receivers also introduce IQ imbalance which can have a huge impact on the performance. Rather than increasing component cost to decrease the IQ imbalance, an alternative is to tolerate the IQ imbalance and compensate for it digitally. Current solutions either require additional analog hardware or are based on digital algorithms that converge too slowly for bursty communication. Moreover, the impact of a frequency offset and phase noise on the IQ imbalance estimation/compensation problem is not considered. In this paper, we analyze the joint IQ imbalance/frequency offset/phase noise estimation and propose a low-cost, highly effective, all-digital mitigation scheme. For large IQ imbalance large frequency offsets and in the presence of phase noise our solution still results in an average implementation loss below 0.5 dB. It, therefore, enables the design of low-cost, lowcomplexity OFDM receivers. Copyright

Joint Compensation of IQ Imbalance, Frequency Offset and Phase Noise

Zero-IF receivers are gaining interest because of their potential to enable low-cost OFDM terminals. However, Zero-IF receivers introduce IQ imbalance which may have a huge impact on the performance. Rather than increasing component cost to decrease the IQ imbalance, an alternative is to tolerate the IQ imbalance and compensate it digitally. Current solutions require extra analog hardware or converge too slowly for bursty communication. Moreover, the impact of a frequency offset and phase noise on the IQ estimation/compensation problem is not considered. In this paper, we analyze the joint IQ imbalance-frequency offset-phase noise estimation and propose a low-cost, highly effective, all-digital compensation scheme. For large IQ imbalance ( ℯ= 10%, ΔΦ= 10°), large frequency offsets and in the presence of phase noise, our solution results in an average implementation loss below 1 dB. It therefore enables the design of low-cost, low-complexity OFDM receivers.

A single-carrier/OFDM comparison for broadband wireless communication

OFDM is a popular modulation scheme for broadband wireless communication. It elegantly handles multipath and has low complexity. Recently, single-carrier with frequency-domain processing (SC-FD) is gaining attention as a possible competitor for OFDM. It is based on the same principles and, therefore, it has similar multipath handling capabilities and low complexity. Moreover, SC-FD promises several other attractive advantages, such as power amplifier efficiency and inherent frequency diversity exploitation. Current performance comparisons do not consider these effects combined. We make the comparison in a realistic scenario considering both the power amplifier efficiency and frequency diversity. Our results show that, in a multipath environment, SC-FD can outperform OFDM by 4 dB, largely due to the better frequency diversity exploitation. Therefore, SC-FD is a good alternative to OFDM for battery-powered terminals, and is therefore especially suited for uplink communication.

OFDM vs. single-carrier: a multi-antenna comparison

In this paper, we compare high-performance OFDM and single-carrier with frequency-domain equalization (SC-FD) schemes using multiple-antennas and decision-feedback equalization (DFE), while also accounting for the power amplifier efficiency. To make a realistic comparison, we also consider most important digital imperfections such as channel and noise estimation, transmit and receive filtering, clipping and quantization as well as link layer impact. Our analysis shows that for frequency-selective channels, the relative performance impact of the power amplifier is negligible compared to the frequency diversity impact. The higher frequency diversity exploitation of SC-FD allows it to transmit more efficiently than OFDM in most cases. Therefore, SC-FD is a suitable candidate for broadband wireless communication, especially for battery-powered up-link transmission.

Living with a real radio

In this chapter, an overview of the interaction between the mixed-signal front-end and the OFDM digital modem was presented. This interaction must be well understood to analyze the impact of a mixed-signal front-end on the link bit error rate performance and to set at system level the front-end specifications optimizing the design margins and the associated risks.