16-11-2012, 03:32 PM
High Throughput of WiMAX MIMO-OFDM Including Adaptive Modulation and Coding
High Throughput of WiMAX MIMO-OFDM.pdf (Size: 894.92 KB / Downloads: 48)
INTRODUCTION
The first WiMAX systems were based on the IEEE 802.16-2004 standard [1]. This targeted fixed broadband wireless applications via the installation of Customer Premises Equipment (CPE). In December, 2005 the IEEE completed the 802.16e-2005 [2] amendment, which added new features to support mobile applications. The resulting standard is commonly referred to as mobile WiMAX.
An ever crowded radio spectrum implies that future demands must be met using more data throughput wireless technologies. Since system bandwidth is limited and user demand continues to grow, spectral efficiency is vital. One way to improve link capacity, and potentially increase spectral efficiency, is the application of MIMO. It is well reported in the literature that MIMO physical (PHY) layer techniques have the potential to significantly increase bandwidth efficiency in a rich scattering environment [3]. Orthogonal Frequency Division Multiplexing (OFDM) is a well-established technique for achieving low-cost broadband wireless connectivity, and has been chosen as the air interface for a range of new standards, including IEEE802.16d/e.
THE WIMAX PHY DESCRIPTION
The IEEE 802.16 standard was firstly designed to address communications with direct visibility in the frequency band from 10 to 66 GHz. Due to the fact that non-line-of-sight transmissions are difficult when communicating at high frequencies, the amendment 802.16a was specified for working in a lower frequency band, between 2 and 11 GHz. The IEEE 802.16d specification is a variation of the fixed standard (IEEE 802.16a) with the main advantage of optimizing the power consumption of the mobile devices. The last revision of this specification is better known as IEEE 802.16-2004 [1].
On the other hand, the IEEE 802.16e standard is an amendment to the 802.16-2004 base specification with the aim of targeting the mobile market by adding portability.
OFDM Parameters in WiMAX
As mentioned previously, the fixed and mobile versions of WiMAX have slightly different implementations of the OFDM physical layer. Fixed WiMAX, which is based on IEEE 802.16-2004, uses a 256 FFT-based OFDM physical layer. Mobile WiMAX, which is based on the IEEE 802.16e-20052 standard, uses a scalable OFDMA-based physical layer. In the case of mobile WiMAX, the FFT sizes can vary from 128 bits to 2048 bits.
Table I shows the OFDM-related parameters for both the OFDM-PHY and the OFDMA-PHY. The parameters are shown here for only a limited set of profiles that are likely to be deployed and do not constitute an exhaustive set of possible values.
SIMULATION RESULTS
In this section SISO and MIMO BER and Throughput results are presented using the Fixed WiMAX simulator. The Simulation model was implemented in Matlab® 7. The PHY parameters used in simulation are given in Table I, with boldfaced values correspond to the OFDMA-PHY parameters. A carrier frequency of 2GHz is considered. For Spatial Multiplexing, an MMSE receiver is used to remove the inter-stream interference on a per sub-carrier basis. The link throughput is calculated from the PER as given by equation (3) in section IV. On the downlink, we consider that no sharing of OFDMA symbol. In this case of single-user MIMO, the multiple streams are intended for the same receiver; we have considered transmission formats with only a single stream for a single user as a basic OFDM in fixed WiMAX.