Cross-Polarization Interference Cancelation (XPIC)Performance in Presence of Non-linear Effects

Hugo Proenca1, Nuno Borges Carvalho1

1Instituto de TelecomunicacoesUniversidade de Aveiro

3810-193 Aveiro, PortugalTel: +351 234377900 Fax: +351 234377901 e-mail: hproenca@ua.pt, nbcarvalho@ua.pt

Abstract—Microwave technology, used in main pointsof mobile backhaul, has now evolved to take advantageof the introduction of Internet Protocol (IP) into thebackhaul network to drive higher capacities, increasedfrequency efficiency, increased flexibility, and optimalcost by combining a number of features to enabletrue gigabit transport speeds. One of those evolutionsregards to the frequency reuse schemas like Co-ChannelDual Polarization (CCDP), which uses two parallel com-munication channels over the same link with orthogonalpolarizations. Due to the nature of these systems, itis inevitable that cross-polar interference occur (e.g.rainfall, multipath, equipments imperfections...), whereCross-Polarization Interference Cancellation (XPIC)technology is adopt as a countermeasure against thatinterference. However, this cancelation block, normallylocated at baseband level, process signals that can beaffected by non-linear impairments. This paper showsthe performance achieved by two XPIC configurations,based on newly introduce XPIC model, in the presenceof Saleh model non-linear element, placed at transmit-ter, for a 256-quadrature amplitude modulation (QAM)signal. CCDP system, which includes XPIC, is simulatedusing Simulink tool from Mathworks.

Index Terms—CCDP, non-linear effects, Saleh model,Simulink, XPD, XPIC

I. SUMMARY

The use of CCDP is nowadays common, since itcan almost double the link capacity over the phys-ical channel (using orthogonal polarizations). How-ever, cross-polar interference, which can be charac-terized by Cross-Polarization Discrimination (XPD),occurs due to several factors like rainfall, multipathpropagation, equipment imperfections, or antennasmisalignment. XPIC is considered a key techniquefor realizing orthogonal polarized wave multiplexingtransmission, because it processes and combines thesignals from the two receiving paths to recover theoriginal signals. In [1], it is mention that the preferredmethod to perform cross-polar cancelation is usingadaptive channel equalization in combination with aninterference canceler. Several realizations using thisconfiguration can be found in [2] and in [3] andothers.

In this paper, the XPIC performance under non-linear effects will be presented using simulated re-sults. The simulation are done with a model that

Fig. 1: Transmitter-Receiver CCDP system simulator (TopModel)

has been developed for emulate a transmitter-receiverCCDP system, using Simulink tool from Math-works [4]. The XPIC model considered is based onthe principle for adaptive interference canceling pre-sented in [5] and the cross-polarization interferencechannel model used is Rummler’s 2-path with cross-polarization interference, which have been used aschannel model in [2]. The system non-linear elementsare characterized by Saleh model, which exhibits non-linear distortions both in amplitude (amplitude-to-amplitude (AM/AM)) and phase (amplitude-to-phase(AM/PM)) [6].

II. CCDP SIMULATOR

The transmitter-receiver CCDP system simulatorgeneral model is presented in figure 1, where 4 blocks(left to right) can de identified: (1) Transmitter; (2)Channel; (3) Receiver; and (4) bit error rate (BER)measurement. The emulation of the CCDP systemis done at the baseband level, i.e., the signals travelsthrough the channel model as complex numbers andare not modulated using In-Phase and Quadrature(IQ) modulation, and not converted into Intermedi-ate Frequency (IF) or radio frequency (RF), where,simulation of RF stage is also not considered.

The Cross-Polarization Interference channel modelused is Rummler’s 2-path with cross-polarizationinterference, which have been used as channel modelin [7] [2] [8].

978-1-4244-7412-7/10/$26.00 ©2010 IEEE

Fig. 2: Transmitter block detail

Fig. 3: Memoryless Nonlinearity block common nonlinear-ity model, from [4]

A. Transmitter

In this system, the transmitter sends a periodicframe to the receiver. This frame is constituted bytwo parts: (1) Preamble; and (2) Data. For emulate aCCDP system, the transmitter uses two independentpaths for horizontal and vertical polarization. Pream-ble part is randomly generated (before simulationstarts) and, for avoid correlation between preambles,correlation factor between Horizontal and Verticalpreamble is less than 20%. Also, preambles arealways modulated using quadrature phase shift keying(QPSK), and data modulation is configurable (256-QAM was used). The transmitter blocks are repre-sented in figure 2. After constructing the transmitframe, a raised cosine pulse filter is applied, and thenthis signal traverses the non-linear component.

1) Transmit non-linear element: For emulate anon-linear amplifier, a non-linear element (Memo-ryless Nonlinearity block) was placed after the raisedcosine block. The block used in this simulation tocharacterize the non-linearity uses a simple memory-less Saleh model as follows (figure 3): (1) Multiplythe signal by an input gain factor; (2) Split the com-plex signal into its magnitude and angle components;(3) Apply an AM/AM conversion to the magnitudeof the signal to produce the magnitude of the outputsignal; (4) Apply an AM/PM conversion to the phaseof the signal, and adds the result to the angle ofthe input signal to produce the phase shift in theoutput signal; and (5) Combine the new magnitudeand angle components into a new complex signal andmultiply the result by the output gain factor, whichis controlled by the Linear gain parameter.

In the Saleh model method, nonlinear models ofamplifiers are based on a simple two-parameter (αand β) formula for each of the functions of the

Table I: Saleh parameters used [6]

Function α βA(r) 1.9638 0.9945Φ(r) 2.5293 2.8168

-300 -200 -100 0 100 200 300 400

-70

-60

-50

-40

-30

-20

-10

0

Frequency (kHz)

Mag

nitu

de-s

quar

ed, d

B

SalehLinear

Fig. 4: Spectrum for a 256-QAM data signal used in sim-ulator (without and with Saleh model)

amplitude-phase model. Equation 1 refers to AM/AMfunction and equation 2 refers to AM/PM func-tion [6]. Several experimental amplitude-phase andquadrature experimental data, collected in literature,were fitted to the purposed formulas. For this work,one of that estimations, described in [6], will be used,using the Saleh parameters values that are presentedin table I. In figure 4 the spectrum response of thesenon-linear blocks to a 256-QAM signal is presented.

A(r) =αa.r

1 + βa.r2(1)

Φ(r) =αφ.r

2

1 + βφ.r2(2)

Where:

• αa and βa are AM/AM parameters• αφ and βφ are AM/PM parameters

B. Channel

A CCDP transmission system characterization re-quires an appropriate channel model on microwaveLine-of-Sight (LOS) links. The channel modelingadopted for conventional digital radios systems is cus-tomarily based upon the Rummler’s 2-path model,described in [7]. To this 2-path model, which is usedas co-channel model, a cross-polarization interferencepath is added, it is characterized by flat attenuation,delay, and phase (parameters also used to define theecho path - multipath). Channel model used in thiswork for multipath fading and cross-polarization in-terference was also used in [7] [2] [8]. Like in [8] [9],an Additive white Gaussian noise (AWGN) was also

Fig. 5: Channel block details

Fig. 6: Receiver block details

added after this model. Other imprecisions could alsobe added, for instance a small continuous increasingphase offset could exist between transmitter and re-ceiver, which is not considered in the Rummeler’smodel and was introduced in [10]. Considering theprevious presented points, the channel model used inthis work is presented in figure 5.

C. Receiver

The receiver block is responsible to process thesignal coming from the impaired channel and recon-struct, as best as possible, the data signal originated inthe transmitter. For that, it uses a channel equalizationand XPIC to cancel the channel cross polarizationeffect. Also, an equalizer will try to compensate thenon-linear effect when it is present. The receiverblock is represented in figure 6. This block can bedivided in two parts: (1) one is the effective receiverblocks; and (2) the signal-to-noise ratio (SNR) mea-surement block.

To horizontal and vertical polarized signals, aftercoming from the channel block, are passed througha raised cosine pulse filter by using Raised CosineReceive Filter block. This block down-sample thesignal using the square root filter with the sameparameters used in transmitter. After applying thisraised cosine receive filter, both horizontal and ver-tical signals pass through an Equalizer and XPICblock, which performs channel equalization, by de-tecting the preamble and use that as training sequence

Fig. 7: XPIC configurations

(Decision-Feedback Equalizer (DFE) using the Least-Mean-Square (LMS) algorithm for adapt the filterweights), and cross-polarization cancelation. The lastblock of the receiver is the Frame Demodulator,which transform the symbols into integer values thatrepresent the received data.

1) XPIC block: In this work, a XPIC model wasused that used the concept of digital signal processingtheory for cancel undesired signal effects in a sys-tem [11] [5]. Using this concept, two different con-figurations for this XPIC model were tested (figure 7):

1) XPIC before - where the cancelation is placedbefore the equalizer;

2) XPIC after - where the cancelation is placedafter the equalizer;

III. XPIC PERFORMANCE TESTS

Using the CCDP system simulator described be-fore, tests had been performed in order to evaluatethe XPIC performance using a 256-QAM transmitteddata signal, where both XPIC configurations wereconsidered (XPIC before and XPIC after). The val-ues of SNR and BER achieved by the receiver weremeasured for several values of XPD (from 5 to40dBs), with and without XPIC compensation, andwith and without Saleh non-linear element (SNRrepresented in figure 8). Comparing the equalizedSNR achieved with the Saleh non-linear element andwithout it, a degradation of SNR is observed thatincreases with the considered value of XPD. Forexample, in the case of XPD=40dBs the equalizedSNR goes from 31.3dBs (without non-linearity) to18.7dBs (with non-linear effects).

Regarding to XPIC performance, the introductionof Saleh model clearly degrade the cross-polar can-celation effect for both XPIC configurations (in fig-ure 9, the XPIC gain - difference between SNRs)is presented. XPIC before configuration is confirmedas the best XPIC configuration. However, the gainintroduced is clearly degrading when non-linear ef-fects are introduced, but it stills produce better resultsif only a channel equalizer is used. Without non-linear element, the maximum XPIC gain is 11.3dBs(for XPD=20dBs), and the maximum BER gain is102.2 (for XPD=25dBs). In the presence of the non-linear element the maximum XPIC gain is 3.5dBs(for XPD=5dBs), and the maximum BER gain is

10 20 30 405

10

15

20

25

30

35

XPD (dBs)

SN

R (

dBs)

SNR performance under a Rummler channel (256−QAM)

XpicBeforeXpicAfterEqualized

10 20 30 405

10

15

20

25

30

35

XPD (dBs)

SN

R (

dBs)

SNR performance under a Rummler channel with non−linear Saleh model (256−QAM)

XpicBeforeXpicAfterEqualized

Fig. 8: SNR achieved for several XPD values: (left) Linear;(right) Saleh Model

5 10 15 20 25 30 35 40−2

0

2

4

6

8

10

12

XPD (dBs)

XP

IC g

ain

(dB

s)

XPIC gain under a Rummler channel (256−QAM)

Linear XpicBeforeSaleh XpicBeforeLinear XpicAfterSaleh XpicAfter

Fig. 9: XPIC gain for several XPD values

100.12 (for XPD=20dBs). With XPIC after config-uration, XPIC performance achieved is lower thanXPIC before without and with non-linear elements.However, the XPIC gain differences introduce bynon-linear element are not so high.

IV. CONCLUSIONS

With the XPIC simulator, a CCDP system wasemulated and the XPIC performance had been study.XPIC model used was based on concepts of digitalsignal processing theory, where the XPIC block andequalization block are completely separated, and so,no share of information is required and XPIC adaptivefilter do not need to be so complex. Unlike the parallelfashion also used in most of XPIC proposals, themodel used operates in a serial fashion in two possi-ble configurations ( XPIC before and XPIC after).Between these two configurations, XPIC before ispresented as the best XPIC configuration,also in thepresence of non-linear effects, where the gain differ-ence between XPIC configurations are not so differentwhen compared with linear results.

ACKNOWLEDGEMENT

The authors would like to acknowledge the Net-work Systems Development department of PortugalTelecom Inovacao and to Institute of Telecommuni-cation from Aveiro that are involved in PANORAMARadio project, a project that aims to development aPoint-to-Point (PtP) microwave radio equipment.

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