Explore the Effect of MIMO Integration in WiMAX Systems

By: BY JINBIAO XU AND AFSHIN AMINI, AGILENT EESOF
( 1 May 2007 )

The IEEE 802.16e-2005 Wireless Metropolitan Area Network (MAN) orthogonal frequency division multiple access (OFDMA) mode (mobile WiMAX) is based on the concept of scalable OFDMA (S-OFDMA). S-OFDMA supports a wide range of bandwidth set to flexibly address the need for various spectrum allocation and usage model requirements. Moreover, the Mobile WiMAX specification defines multiple-input multiple-output (MIMO) option, which is a key feature in mobile WiMAX. Smart antenna technologies typically involve complex vector or matrix operations on signals due to multiple antennas. OFDMA allows smart antenna operations to be performed on vector-flat sub-carriers. Complex equalizers are not required to compensate for frequency selective fading. OFDMA therefore, is very well suited to support smart antenna technologies. In fact, MIMOOFDM/OFDMA is envisioned as the foundation for next-generation broadband communication systems. 

 

The Agilent EEsof 802.16e Mobile WiMAX Wireless Library has the added capability for multiantenna simulation, enabling designers to explore the effect of MIMO integration into their WiMAX systems. The mobile WiMAX OFDMA PHY supports a frame-based transmission which includes Downlink (DL) and Uplink (UL) subframe. The mobile WiMAX MIMO transmitter and receiver in Agilentʼs Advanced Design System (ADS) are shown in Figure 1. 

 

In the mobile WiMAX MIMO downlink subframe, the first two parts are preamble and a mandatory partial usage of subchannels (PUSC) zone, which transmits some control messages (such as FCH, DL-MAP and UL-MAP). The zone after the PUSC zone is the STC (SM) zone, whose permutation mode may be PUSC, FUSC or AMC. In this zone, transmit diversity (STC) or spatial multiplexing (SM) can be implemented. To enhance channel estimation and tracking in the MIMO receiver, a midamble (a training sequence) may be present at the first symbol in the STC (SM) zone. The bit stream is distributed into the transmit antennas according to the following transmission format matrix (assuming 2 transmit antennas). 

 

Assuming the MIMO system is with M transmit antennas and N receiver antennas, the STC zone is taken to perform FFT transformation, each received subcarrier is as follows: 

 

r=Hs + w 

 

where H is the M_N channel matrix, and s =[s1, s2, . . . ,sm]T is the M-dimensional transmit signal vector, w is the N-dimensional vector of zero-mean noise with the variance of _2. The channel matrix H can be estimated by the pilots by the Wiener filtering. 

 

The MIMO decoder can be divided into linear and non-linear decoding techniques. The simplest MIMO decoder is the zero-forcing (ZF) decoder, which inverts the channel matrix: 

 

sî = (H*H)-1 H*r = H+r 

 

However, this ZF decoder introduces noise at lower SNRs (Signal-to-Noise Ratios). A better decoder, MMSE (Minimum Mean Squared Error), is employed to minimize the Mean Square Error: 

 

sî = (H*H + M I /_)-1H*r 

 

where _ represents the SNR at each receive antenna, M is the number of transmit antenna. In the Mobile WiMAX MIMO Wireless Library for use with ADS, both ZF and MMSE were implemented.

 

The key features of the library include:

(1) Top-level downlink and uplink MIMO fullycoded signal sources (support Matrix A and B), the detailed structure can be shown by pushing down into these top level DL/UL sources. For the DL source, the STC with 2 transmit antennas and SM with 2 transmit antennas were supported. For the UL source, the SM with 2 transmit antennas and collaborative SM with 2 transmit antennas were supported.

(2) Top-level downlink and uplink MIMO receiver, the detailed receiver structure can be shown by pushing down into these top-level DL/UL receivers. These MIMO receivers include time and frequency synchronization, channel estimation, soft channel decoding corresponding to STC/MIMO transmitter. The 2_1 MISO and 2_2 MIMO are supported for both DL and UL receivers.

(3) ITU channel model and MIMO channel model is provided.

(4) Transmit measurement such as spectrum, constellation, EVM and power.

(5) Receiver measurements such as PER on MIMO channel, sensitivity measurement and adjacent channel rejection measurement. 

 

Designers of Mobile WiMAX systems face the challenges related to increasing design complexity. At the same time, growth in the WiMAX market has led to the presence of more companies in this space. There are certain parameters that dictate the preference of one design over the others. The Mobile WiMAX Wireless Library testbenches provide direct simulation of regulatory conformance and performance versus various impairments to determine tradeoffs of a specific design.

 

 

About the Authors

Jinbiao Xu is a technical lead with the Agilent EEsof EDA Beijing team. He can be reached at jinbiao_xu@agilent.com. Afshin Amini is a product marketing manager for Wireless Libraries in the Agilent Technologies EEsof EDA division. He can be reached at afshin_amini@agilent.com.