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PERFORMANCE OF FREQUENCY OFFSET SYNCHRONIZATION IN A

SINGLEAND MULTI-ANTENNA IEEE 802.16-

2004 SYSTEM

José A. Rivas Cantero M. Julia Fernández-Getino García

Dpto. de Teoría de la Señal y Comunicaciones, Universidad Carlos III de Madrid

3rd COST 289 WorkshopENABLING TECHNOLOGIES FOR B3G SYSTEMS

July 12-13, 2006Aveiro, Portugal

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1. Background•Motivation•OFDM•MIMO-OFDM•IEEE 802.16-2004•STC

2. Frequency offset estimation algorithms•SISO systems•MIMO systems

3. Results

4. Summary and Conclusions

Outline

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Nowadays the combination of Multiple-Input Multiple-Output (MIMO) systems and Orthogonal Frequency Division Multiplexing (OFDM) technologies (MIMO-OFDM) is one of the most attractive techniques to provide broadband communications

IEEE 802.16-2004, also known as IEEE 802.16d, is the standard that describes the air interface for fixed broadband wireless communications. Physical layer based on OFDM modulation.

This standard just proposes a typical SISO system, and leaves as optional the development of a MISO 2x1 system.

This work extends the standard to a MIMO scheme. Several scenarios (SISO, MISO, MIMO) are developed and compared.

1. Background-Motivation

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A critical issue is frequency offset estimation and correction: SISO systemsMIMO systems

In all these schemes several algorithms are compared:Channel estimation algorithms:

Maximum Likelihood Time Frequency (ML-TF)LS estimator (Time domain)

Space Time Coding (Alamouti configuration) is usually employed. Influence in:

Bit Error Rate (BER)Data transfer rate

1. Background-Motivation

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OFDM: Multicarrier modulation which divides the bandwidth in several ortogonal channels.

Suitable for data transmission in wireless channels due to its robustness against multipath fading.Easy implementation by FFT.

1. Background - OFDM

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OFDM block diagram

Time-Frequency scheme

Cyclic prefix: avoids ISI and ICI

1. Background - OFDM

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MIMO: Use of multiple antennas both in the transmitter and in the receiver

Several channels among emitter and receiver.High capacity system.Diversity in a fading environment.

MIMO-OFDM system :

1. Background MIMO- OFDM

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1. Background- IEEE 802.16-2004

Air interface for fixed broadband wireless communications standard

Revision of IEEE Std 802.16-2001.IEEE802.16e. Approved December 2005. WMAN mobile.

NLOS propagation. 2-11 GHzOFDM. FFT 256 points

Data subcarriers (QPSK, 16-QAM, 64-QAM-optional)Pilot subcarriers: Estimation purposes (BPSK)Null subcarriers: DC and guard band.

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Standard specifies preambles both for UL and DL:UL: One OFDM symbol. Only even subcarriers are not null:

DL: Two OFDM symbols. In the second one only even

subcarriers are not null :

One symbol with only even subcarriers different from zero =>Two equal halves in time domain.

1. Background- IEEE 802.16-2004

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PMP => Point – Multipoint structureIn simulations TDD is employed:

DLpreamble

ULpreamble

1. Background- IEEE 802.16-2004

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1. Background- IEEE 802.16-2004

Wimax scenarios

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1. Background - STC

Alamouti scheme

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Implementation of MISO system is optional.2x1 system employing Space-Time Coding.When using more than one transmitter preamble emitted in the DL is not the long preamble (2 OFDM symbols). It is a OFDM symbol where only odd subcarriers are not null.Preambles emitted by both antennas are orthogonal.Schemes studied in this work:

SISO MISO 2X1. STCMIMO 2X2. STCMIMO 2X2. NO STC

* The first one is the standard one.

* The second one is optional.

* The rest ones are new, and are not

implemented in the standard yet.

1. Background

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1) ML Algorithm:Estimation in frequency domain (Subcarrier by subcarrier). Interpolation is needed.Frequency estimator:

2. Frequency offset

2) LS Algorithm:Estimation in Time domain.Expression:

In the simulations two channel estimation algorithms compatible with IEEE 802.16-2004 standard are employed:

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2. Frequency offset

CHANNEL ESTIMATION ALGORITHMS

Very similar performance in terms of BER. NO STC

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2. Frequency offset

Frequency synchronization must be performed in the receiver. No synchronization => orthogonality loss among symbols.

Why this offset appears? Channel effects. Synchronization loss among system elements, especially between emitter and receiver oscillators.

ε represents the normalized frequency offset:

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2. Frequency offset

Preambles composed of two equals halves in time domain => algorithms based on finding them (Correlation). Offset is composed of an integer and a fractional part.

Correction:

¡¡ Frequency offset Change in the phase of the received signal (in time domain) !!

Target: Residual offset as small as possible.

Received signal in time domain

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Using this fractional part estimation and LS channel estimation a joint channel estimation and frequency estimation can be derived. It takes into account the estimation of the integer part of the frequency offset:

1) Estimation and correction of the fractional part of the frequency offset 2) Consider the integer frequency offset hypothesis from (-M,-M+2…, -2,0,2,…,M) where M is the maximum possible even integer offset and obtain the corresponding LS channel estimates by circularly shifting the FFT outputs accordingly.3) Calculate the corresponding LS error for the channel estimates obtained in the previous step 4) Iterate over steps 2 and 3 till all frequency offset hypotheses are considered and choose the one that minimizes the LS error.

2. Frequency offset

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2. Frequency offset

MIMO systems: Based on correlation between signals too!

Adapted from an algorithm proposed for WLAN systems.

First of all we estimate time-domain channel responses between any pair of transmit and receive antenna assuming that the frequency offset has been completely compensated.

We define two different signals:

Signals which really arrive to the antennas (yt ).

From yt first channel estimation is performed: (Hl) . We define the signal which should arrive in case that this estimation were correct: (yt’ ).

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2. Frequency offset

Therefore yt (r,1)’ represents the signal which would arrive to the rth receive antenna in the time instant 1, supposing that the first channel estimation is correct. With 1 or 2 we distinguish between the two equal halves which composes the OFDM symbol in time domain (T(p,1);T(p,2)), t = 0,1,…,127.

To obtain the fractional offset we can measure the phase change

between yt (r,1) yt (r,1)’ * and yt (r,2) yt (r,2)’ * :

Once the fractional offset is found, the correction is performed as in the SISO systems.

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BER OF DIFFERENT SCHEMES:

3. Results

2X2 SystemNo Space Time Coding.

Spatial Multiplexing => Double data transfer rate.

Highest BER.

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BER OF DIFFERENT SCHEMES:In all of them ML-TF channel estimator has been employed. Using a 2X2 scheme data transfer rate is doubled, in case no space time coding is applied. It could be very useful for situations where a big amount of data must be transferred, although BER is higher than in the typical SISO scenario. Last two curves in Figure show the benefits of employing Space Time Coding (Alamouti configuration). When using ST Coding data transfer rate is not doubled, keeping the data rate of the SISO case, although a second antenna has been added in transmission. On the other hand BER of the system decreases significantly. The 2X1 system is leaved as optional in the standard. If a second antenna is added in reception, it can be clearly appreciated how much BER decreases, reaching 10-8 values just with a signal to noise ratio of 20 dB

3. Results

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FREQUENCY OFFSET ESTIMATION EFFECTS:

The maximun aceptable residual offset can be ε = 0.01

3. Results

SISO SYSTEMS

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FREQUENCY OFFSET ESTIMATION ALGORITHMS:

εresidual_SISO ≈ 0.001 y εresidual_MIMO ≈ 0.01.

3. Results

LS channel estimation (ε = 0.3).

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Extension to the IEEE 802.16-2004 standard.Addition of a second antenna in the receiver.Several scenarios, combining SISO, MISO and MIMO configurations. Use/ Not use of Space Time Coding.

Frequency offset must be taken into account. With presented algorithms residual error is almost null. In MIMO systems this offset in perceptible in terms of the MSE of the channel estimation, but can be considered as offset free in terms of BER.

Depending on the requeriments of the systems in terms of BER, data transfer rate, physical space to add more antennas to the system and cost, one of the schemes studied in this paper may be chosen to implement a next generation fixed broadband wireless access downlink system based on IEEE 802.16-2004 standard.

4.Summary and Conclusions