OFDM BASED MIMO TESTING FOR RF ICs · 4 [email protected] ABSTRACT Multiple-input–multiple-output (MIMO)-based systems are extremely popular as they offer data rates as

OFDM BASED MIMO TESTING FOR RF ICs

GOPALA KRISHNA.M *1, UMA SANKAR.CH 2, PHANI KUMAR.Y 3, PAVAN KUMAR .V.G4

1, 2 Electronics & Communication Department, RK College of Engineering, Vijayawada, A.P, India

3 Electronics & Communication Department, Amrita Sai Institute of Science & Technology,

Vijayawada, A.P, India

4 Electronics & Communication Department, Sri Sunflower College of Engg. & Tech., Machilipatnam,

A.P, India

[email protected]

[email protected]

[email protected]

4 [email protected]

ABSTRACT

Multiple-input–multiple-output (MIMO)-based systems are extremely popular as they offer data

rates as twice as fast as currently available systems. Their testing becomes more complicated due to the

increased number of RF paths. This increases the overall test cost of these devices both in terms of test

time and instrumentation cost. In this paper, we demonstrate a low cost MIMO test solution which

targets critical specifications that are fundamental to the MIMO system operation, such as gain, IIP3, and

phase imbalances between the RF paths. Our test methodology measures these parameters with a single

test setup that enables the calculation of these performance parameters. Using the proposed test method,

RF MIMO systems can be tested using a mixed signal tester, and on-board circuitry within a reasonable

accuracy. Both simulation and measurement results confirm the high accuracy and repeatability of our

test technique.

KEY WORDS — Multiple, input, Spectrum, Analyzer, MIMO, RF testi ng, OFDM, WLAN.

I. INTRODUCTION

RECENTLY, multiple-input–multiple-output (MIMO) communications systems that contain

multiple RF paths are reported to increase the spectral efficiency and to improve the link reliability

without increasing the bandwidth. However, the increase in the number of RF paths also translates as

increased test time and instrumentation costs. Moreover, additional performance metrics that are

necessary to characterize the MIMO operation, such as mismatch parameters between the RF branches

of the transceiver, need to be tested to meet the stringent specifications of the overall system.

International Journal of Electronics, Communication & Instrumentation Engineering Research and Development (IJECIERD) ISSN: 2249-684X Vol.2, Issue 1 Mar 2012 23-35 © TJPRC Pvt. Ltd.,

Gopala Krishna.M, Eswara Rao.Y, Uma Sankar.Ch & Phani Kumar.Y 24

In this paper, we present a frequency domain multiplexing scheme to combine the information in the

RF domain into a single time domain signal to avoid RF switching and instrument settling times. This

allows us to significantly reduce the test time, as the amount of instrument settling and switching

operations are reduced. In order to obviate the need for high frequency instrumentation, we down-convert

output signals lower frequency signals through fully characterized on-board mixers. The characteristics

of the mixer are de-embedded from the measurements in order to determine true characteristics of the

device under test (DUT).

The applicability of this method is demonstrated through simulations and measurements on a 2x2

MIMO system, in an earlier publication . In this work, the test method is extended to any arbitrary

number of RF branches. In addition, several new test setups that include more than two signal generators

to reduce the number of switching operations are demonstrated. A guidance for selecting the optimum

test setup based on accuracy and test time considerations is provided. Furthermore, a modification of the

frequency domain multiplexing scheme is demonstrated to enable coupling measurements between RF

branches.

This paper is organized as follows. We present an overview of MIMO devices in Section II. In

Section III, we discuss the proposed test method. In Section IV, we demonstrate the viability of the

proposed method and finally conclude the paper in Section V.

II. MIMO OVERVIEW

In the MIMO technology, the channel capacity is increased using several techniques, such as

transmit diversity and spatial multiplexing, through multiple antennas at the transmitter and the receiver

[18]. As MIMO systems increase the channel capacity without increasing the bandwidth or the SNR,

they represent a promising solution to increase the performance of next generation wireless

communication systems [5].

In MIMO systems, the quality of the link, and consequently, the data rate strongly depend on the

channel conditions, such as relative power loss and time delays between the transmit paths that

differentiate one path from the other [2]. While the channel conditions change in the field and create

different link qualities, the internal mismatches between RF branches should not distort the MIMO

operation.

Thus, the mismatch parameters should be accurately characterized.

A. Effects of RF Impairments

MIMO systems have to satisfy all the functional requirements of SISO systems. In addition,

mismatch parameters among the RF branches severely affect the overall system quality. Therefore,

additional performance parameters that are un applicable to SISO systems need to be measured for

MIMO systems. For instance, in SISO systems, a phase rotation in a signal path may not damage the

system operation. However, a phase mismatch among the RF branches of a MIMO system may create

singular channel conditions, which degrade the signal transmission capabilities of the system.

OFDM based MIMO Testing for RF ICs 25

Figure 1 (A) Signals at different frequencies. (B) Signals at the same frequency

B. MIMO Test Challenges

The MIMO test problem can be reduced to a SISO test problem by sequentially testing each signal

path. However, there are various drawbacks to this approach.

First of all, the overall test time for each product, which includes signal generation and capturing,

switching, and instrument settling times, increases as the number of RF paths increase. This may

unreasonably elongate the test time. Heuristically, the test time for one NxN MIMO system is at least N

times that of a SISO system. This time may become longer with the addition of the RF switching times

and instrument settling times which are typically in the range of 10-80 ms per switching.

In addition, since only one path is active while others are idle in sequential testing, it may not

capture the true MIMO behavior, in which all the paths operate simultaneously.

A low cost alternative is frequency domain multiplexing, which will be discussed in detail in the

next section. The frequency domain multiplexing technique allows characterization of all branches in a

MIMO system using a composite output signal, overcoming most of these challenges.

C. High Volume Manufacturing Considerations

Application of a low cost test method for high volume testing (HVM) may reveal several challenges.

First of all, the custom test equipment may not reach the calibration and repeatability standards of

commercially available ATEs. However, in an HVM environment, ATEs need to interface with a variety

of DUTs, such as packaged, on wafer, and diced. The interfaces between the DUT and the ATE, which

are frequently called as load-boards, are nonideal. The impairments in the signal path needs to be

calibrated in order to obtain the true response of the DUT [8]. While the ATEs are internally calibrated to

perfection, the load boards need calibration and they may dominate the overall measurement error.

Therefore, both low cost test methods and ATE vendors need to consider calibration techniques to

minimize the measurement error.While customized test solutions seemingly require more development


time, custom test solutions that employ a low cost custom ATE may significantly reduce the overall test

time [14], thereby the time-to-market.

III. TEST METHODOLOGY

The following three major challenges associated with MIMO system need to be studied in order to

provide a low cost test methodology:

• high cost of signal generation and analysis in the RF domain;

• the need for multiple test set-ups, which increases the test time due to prolonged instrument

settling and switching times;

• the need for coherent instrumentation for phase mismatch measurements.

In order to obviate the need for RF analyzers, low cost test approaches, such as the use of down-

conversion mixers on the load-board to translate the RF signals to lower frequencies, can be employed. It

should be noted that auxiliary devices such as up/down-conversion mixers, combiners/splitters, and even

the load board signal traces need to be characterized in order to increase the accuracy of the

measurements. Since the characterization data can be reused during product testing, one time

characterization can be attained at a relatively lower cost.

Figure 2 : Test setup

A. Proposed Test Setup

A frequency domain multiplexing scheme is employed in the test setup, as illustrated in Fig. 2. Since

the measurement of IIP3 parameter requires two tone test stimuli, two-tone test signals with strategically

selected frequencies are applied to the MIMO system. After combining the outputs of the RF branches

using a passive combiner, the composite signal is down-converted using a fully characterized on-board

mixer. As shown in Fig. 2, the two tone waveform at w1 and w2 frequencies is applied to the first branch

( TxInA) of the system, and the two tone input stimuli with w2 and w3 frequencies is applied to the second

branch (TxInB ). as long as the signal frequencies are adjusted such that the intermodulation products from

paths A and B do not overlap with the fundamental tones (w1 , w2 , w3), the output signal components at

the w1 and w3 frequency locations are solely dependent on the characteristics of their respective

branches.

B. Gain and IIP3 Measurements


The path gain of each branch can be calculated by measuring the output power at the w1 and w3

frequency locations. However, note that calculated gain values are slightly different than the calculated

gains when a single tone stimuli is applied to the each branch individually. The deviation between the

single tone gain and two-tone gains are negligible when the amplitude of the input signal is smaller than

the 1 dB compression point of the DUT.

The IIP3 of each branch can be calculated by using (1), where the PiA, PiB are the input powers to the

first and second branches, and Pw is the output power measured at the frequency location. The IIP3

measurements are valid when the power of the intermodulation products are above the noise floor, and

the input power is below the 1 dB compression point.

(1)

C. Phase Mismatch Measurement Method

The definition of phase shift in a frequency translation system is ambiguous, since the input and

output signals are at different frequencies. An alternative is to define the phase shift as the time delay of

the output signal.

For synchronous sampling, small synchronization errors between signal analyzers may introduce

large phase errors.

Figure 3 : Vector representation of time domain waveforms

These problems are solved by combining the outputs of each branch and using the law of cosines in

vector form to calculate the phase mismatches among the RF branches. When two same frequency time

domain waveforms are added or subtracted, the resulting waveform can be analytically expressed in

terms of the amplitudes and the phase mismatch between the added (or subtracted) waveforms. Fig. 3

illustrates the subtraction operation, which can be performed with a passive combiner/splitter, of A and B

waveforms, which are sinusoids at the same frequency with a phase mismatch of фA – фB.

The amplitude of the resulting waveform can be expressed in terms of A and B as follows:


In order to calculate the phase mismatch, only the amplitudes of the A, B, and C vectors need to be

measured. This eliminates the need for complex synchronization of signal generators and analyzers. The

amplitude measurement can be achieved without coherence between the signal generator and the

analyzer. Conceivably, this translates as lower cost test equipment.

Since the voltages are subtracted in the combiner, the phase mismatch between the RF branches can

be calculated by substituting in the place of A, C and B respectively in (2). For

convenience, (2) is revisited as follows:

D. Low-cost Two-Tone signal generation

The most convenient method to generate two-tone stimuli at low frequencies is using direct digital

synthesis and digital-to-analog converters (DAC). In our case, the test methodology requires two

synchronized DACs and a memory unit that keeps the digital patters.

Figure 4 : Low cost two-tone signal generation

Another low cost alternative is to use single tone signal generators and combiners. Since only the

signals at the w2frequency location need to be coherent, three non-coherent single tone signal generators

can be used to generate the required stimuli, as depicted in Fig. 4. Note that, the signal at the frequency

location w2 is divided and its output is combined with another signal. The resulting signals can be

represented as follows:


where фi is the phase of the signal generator , A is the amplitudes of the signals, yTxA(t) is the two-tone

signal synthesized for the first branch of the MIMO system, and yTxB(t) is the two-tone signal synthesized

for the second branch of the MIMO system.

E. Extension to an N x N MIMO System

The test setup introduced in the preceding section can be extended for any MIMO system with more

than two transmit and receive paths. However, there are several implementation choices and a

cost/benefit analysis is needed in order to decide what combination of serial/parallel testing is

advantageous.

Even the ideal test solution for an N x N by system may seem to be employing N signal generators,

in reality, such a system may not have the best performance in terms of test accuracy, test time, and

overall cost. In order to obtain the phase difference among all paths of an N x N system, (N2) phase

difference measurements need to be conducted.

Figure 6 : Two alternative 4 x 4 MIMO measurement test setups using signal switches. (A) Using

two signal generators (B) Using two signal generators

Fig. 5. Placement of tones for 4 x 4 system: solid spikes are used in gain and IIP3 calculations,

dashed spikes are used in phase difference calculations.


Fig. 5 shows a potential frequency domain multiplexing scheme for a 4 x 4 MIMO system.

Table I explains the purpose of the numbered frequency locations. This scheme requires only N

input tones for an N x N system to characterize the required performance parameters as opposed to 2(N-

1) tones.

A high number of spectral components for each path results in an increased number of inter-

modulation products. These tones clutter the spectrum and even overlap with information bearing

signals. This problem can be prevented through significantly reducing the power of the spectral

components used in phase measurements (tones 11–14 in Fig. 5). The power of intermodulation products

arising from these additional tones will be much smaller compared to the fundamental. This improves the

measurement accuracy.

However, using N signal generators to test an N x N system may be infeasible due to the need for a

high number of signal generators. Alternative test setups can be designed based on the number of

available signal generators and using signal switches.

For example, as shown in Fig. 6, two distinct test setups can be constructed for a 4 x 4 MIMO

system using switches.

The first measurement setup [shown in Fig. 6(a)] employs two signal generators and several signal

forwarding switches. By controlling the poles of the switches, the performance parameters of all paths

and their phase differences can be measured. The number of switching operations required for this case is

6 (1-2, 1-3, 1-4, 2-3, 2-4, 3-4).


While the resulting number of switching is more than the number of switching that would be spent

using a sequential approach, the phase difference measurement can be attained without requiring a

coherent instrumentation. Therefore, using two instruments can be still beneficial in terms of the fixed-

capital cost of the test setup.

F. Coupling Measurements

As MIMO devices contain multiple radios operating at the same band, inevitably, there will be a

coupling between the RF branches. A severe coupling may distort the operation of the circuit. The

frequency domain multiplexing scheme described earlier can be slightly modified to measure the

coupling between branches of a MIMO device. However, since a signal combiner is used at the output of

the previously demonstrated test setup, the coupling between branches may not be measured as the origin

of each tone is untraceable. In order to measure the coupling between two RF branches, more than one

signal analyzers are needed.

The frequency domain multiplexing scheme can be slightly modified to measure the coupling

between two branches of a MIMO device, as shown in Fig. 7.

In this setup, there are two signal analyzers and two signal generators. The coupling terms will

appear in both outputs. Since the power of leakage terms usually a lot smaller compared to fundamental

terms, the amplitudes of the fundamental terms may not be degraded while some additional spectral

components appear at the outputs of the branches. Based on the power of these additional tones, the

amount of coupling between two branches of the device can be calculated.

IV. EXPERIMENTAL RESULTS

The test methodology is validated through simulations and measurements on a prototype MIMO

device. The goal in conducting the simulations is to demonstrate the robustness of this test methodology


under process variations. In addition, the feasibility of this test methodology is demonstrated on a

prototype device, which consists of a MIMO transceiver built from off-the shelf integrated circuits, and

bench instruments for signal generation and analysis.

Figure 7 : Measurement setup for coupling between RF branches

Figure 8 : MIMO system model used in MATLAB simulations

A. Simulation Results

The block diagram of the MIMO transmitter that is used for simulations is illustrated in Fig. 8. The

transmit paths contain up-conversion mixers and power amplifiers (PA), which are modeled trough a

polynomial equation outlined below to emulate the nonlinear behaviors (Gain compression and IIP3), and

phase shift [15].

Where αi is the coefficient of the i th order term, ф is the injected phase, x(t) is the input signal, and

y(t) is the output signal. The gain and phase imbalances are injected through changing the polynomial

coefficients and the ф parameter.

The first case illustrates a circuit instance which has a very small phase mismatch between the RF

branches, and a matched gain and IIP3. Since our combiner subtracts the outputs of the branches, when

the phase mismatch is small, the amplitude of waveform at the w2 frequency location that is observed at

the output will be very low. In this case, 1o degree phase imbalance is injected between the paths to

determine the accuracy of the phase measurements.


The remaining cases identify circuits with increasing mismatch parameters between the transmit

branches.

Table III depicts the maximum Gain, IIP3 and phase errors and standard deviations obtained in 50

simulations. The results indicate that the gain, IIP3, and the phase mismatch between the RF branches can

be accurately determined even if the mismatch parameters are very large. This enables the accurate

characterization of the target performance parameters for both marginal and grossly defective circuits.

While the robustness of the test methodology under process variations demonstrated only on a

transmitter, similar results can be obtained for a receiver, as demonstrated in the following section.

B. Measurement Results

In order to validate the theory, experiments are conducted on a MIMO device that is built using off-

the-shelf integrated circuits. Monolithic transmitter and receiver integrated circuits are used to build the

transceiver circuit. The baseband signal generators of Agilent E4438C signal generator are used to

generate low-frequency test signals and its RF frequency generator is employed to supply local oscillator

signals to the up and down-conversion mixers. In order to inject phase differences among RF

branches, different length cables are used to build a differential measurement system, which is illustrated

in Fig. 9. Cables with different lengths have different time delays, which eventually reflect as a phase

mismatch. Three different cable lengths are used: 1”, 18”, and 36” cables with time delays of 3 ps, 1.7 ns,

and 3.5 ns, which shift the phase of the signal at 6 MHz by 0.01, 3.7, and 7.6 degrees, respectively.

Figure 9 : Differential measurement system


C. Extension to a 4 X 4 system

In this section, the simulation results are given for a 4x4 MIMO system using several test setups.

Table VI shows the measurement accuracies with respect to each test setup. Signal Generators shows the

number of signal generators used in each test setup, ADC Resolution shows the number of bits in each

signal generator, Gain Error shows the mean error in Gain measurements, IIP3 Error shows the mean

error in IIP3 measurements, and Phase Error shows the mean error in Phase measurements.

It can be observed that the measurement setup that uses only two signal generators is the most

accurate measurement setup. A detailed test cost-benefit analysis should be conducted in order to select

the best test setup [6].

V. CONCLUSIONS

A novel low cost MIMO test technique that enables measurement of critical performance

parameters, such as gain, IIP3, and the phase mismatch among the RF branches is presented. The test

methodology determines these parameters using a single test setup, thereby eliminating instrument

settling and switching times.

Based on our measurement results, the proposed method accurately determines the critical

performance parameters of the MIMO system while requiring only baseband signal generators, mixers,

and combiners.

The extensibility of the proposed method to any arbitrary MIMO configuration is also demonstrated

through simulations. Based on a cost-benefit analysis, the optimum test set can be determined using the

proposed test technique.

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Documents

OFDM BASED MIMO TESTING FOR RF ICs · 4 [email protected] ABSTRACT Multiple-input–multiple-output (MIMO)-based systems are extremely popular as they offer data rates as