iip2 requirements in 4g lte handset receivers

IIP2 Requirements in 4G LTEHandset Receivers

INTRODUCTION

n 2nd order non-linearity is a big challenge in modernDCR(Direct-conversion receivers), especiallyin frequency division duplex (FDD) systems such as 3GWCDMA and 4G LTE where transmission and receptionare simultaneous but in different frequency ranges[1].

OVERVIEW OF LTE STANDARD

n LTE uses Orthogonal Frequency Division Multiplex-ing with multiple access scheme for downlink (DL),hence the name OFDMA[1].

n OFDM allocates user just in time domain, but OFDMAallocates user in time and frequency domain[2]


n For uplink (UL), LTE uses Single-Carrier FrequencyDivision Multiple Access (SC-FDMA) to reduce thePeak-to-Average Power Ratio (PAPR) of the userequipment (UE) and improve the linearity of the poweramplier[1]


n OFDM based modulation schemes allow variable BWallocations for different users based on need.

n The signal is composed of a number of resource blocks(RBs), each resource block composed of 12 subcarrierswith 15kHz subcarrier frequency spacing. Therefore oneRB occupies a bandwidth of 180kHz[1-2].


n The number of resource blocks assigned to a certainuser determines the allocated bandwidth. LTEstandarad species 6 bandwidth options listed asbelow[2] :


n The overall LTE key parameters[2] :

IIP2 REQUIREMENT IN FDD-LTE RECEIVERS

n An ideal receiver would have innite IIP2 thatcorresponds to zero 2nd order intermodulationdistortion(IM2)

n In reality, a receiver has nite IIP2 and thereforeproduces IM2 distortion contributing to thereceiver total noise oor in addition to otherimpairments such as thermal noise, reciprocal mixing,and TX noise in the receiver band

n The lower the target IM2 level is, the better thesensitivity and the more challenging the IIP2requirement will be [1]


n = ∙ − −

n Because TX signal is much larger than RX signal, theIM2 is dominated by TX signal [3]


n As shown above, the IM2 level is completely mitigatedwhen TX OFF. It proves again that the IM2 is dominatedby TX signal [1]


n Since we are interested in the receiver chip IIP2requirement, we need to compute the IM2 level at thereceiver chip input

n = ∙ ( − )− −

n Every 1dB change in TX power corresponds to 2dBchange in the IM2, and similarly every extra 1dBduplexer isolation corresponds to 2dB relaxation in theIIP2 requirement [1]

n Thus, we should make the duplexer isolation as largeas possible to reduce the TX power at chip input


n IM2 product is also modulated and spread infrequency even beyond the channel bandwidth [1,5]


n If using traditional two tone CW with identical level asinput, we have [6]:


n So the power level of IM2 at low frequency (f2 – f1) canbe expressed as [3]:


n If the signal is AM modulated, such as TX leakagesignal, the previous 2 tone formula can not be fullyapplied

n The difference is called correction factor(CF) [1,3]


n For higher bandwidths, IIP2 is relatively relaxed [1]

n In other words, the higher the BW, the lower IM2 [1] :

Reference

[1] IIP2 Requirements in 4G LTE Handset Receivers

[2] LTE technology introduction, Rohde & Schwarz

[3] IP2 and IP3 Nonlinearity Specifications for 3G/WCDMA Receivers

[4] LTE System Specifications and their Impact on RF & Base Band Circuits

Application Note, Rohde & Schwarz

[5] RF Wireless Receivers in CMOS a prospective from the University of

Pavia

[6] RF Transceiver Module Design Chapter 3 Nonlinear Effects

iip2 requirements in 4g lte handset receivers

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