WiMAX Forum RF Network Engineer Certification Training Material Chapter 3, Antennas for WiMAX

Posted by communications at 7:37 PM

After the first and second chapter of my self learning, here we go the third chapter :) titled Antennas for WiMAX. Just like traditional Wireless System, there are 3 kinds of antenna used for transceiving Radio Access Signal i.e., Omnidirectional, Sectoral, and Point to Point. The use of those antenna depends on it’s application.

1. Omnidirectional, broadcast 360 degree suitable for large area rural environment.

2. Sectoral, broadcast on intended area: 60, 90, and 120 degree. This kind of antenna is suitable for urban environment which has lage population on a certain area.

3. Point to Point Antenna is used for point to point communication, i.e., Microwave link, backbone etc.

(pictures are filched from wimax.com). If you need to read WiMAX Application material, please visit this my previous post, here

After we have discussed about those traditional antennas, now we are going to discuss advanced antenna technology in WiMAX.

1. Antenna Diversity,

Using two or more receiving antennas on a wireless device to eliminate multipath signal distortion. Typically, the signal from the antenna with the least noise (best SNR) is chosen, and the other antenna is ignored.

There are three techniques in diversity scheme:

1. Space Time Coding (Alamouti Code), is a method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas. STCs rely on transmitting multiple, redundantcopies of a data stream to the receiver in the hope that at least some of them may survive the physical pathbetween transmission and reception in a good enough state to allow reliable decoding. (resource: http://en.wikipedia.org/wiki/Space–time_code) Space time codes may be split into two main types:

2. Antenna Switching, is a simple approach for capturing diversity gains. The purpose is not to combine signals from the multiple antennas available but simply to select the single antenna with the best channel gain at any given time. It is applicable to both DL and UL transmission.
3. Maximum Ratio Combining (MRC), is a processing technique that estimates channel characteristics for multiple antennas and then apply weights to each antenna to maximize signal to noise ratio for the summed signal. MRC achieves diversity and array gain but does not involve active interference mitigation or spatial multiplexing in any way.
Beside Antenna Diversity, there are other technique in WiMAX Antenna, i.e., MIMO and SAS. We are going to discuss it below.
MIMO (Multiple Input Multiple Output)
Multiple Tx/Rx chains and antennas in the base stations is now a well-established technique. Technological advances in high scale integration are making multiple Tx/Rx chains and antennas also economically viable for the mobile and stationary subscriber stations. The WiMAX Forum Mobility Task Group (MTG) defined profile specifies two MIMO versions called Matrix A MIMO and Matrix B MIMO. The IEEE 802.16 standard defines other MIMO classes, for example Matrix C MIMO, which may be adopted by the WiMAX Forum in future profiles. In this post we are going to disucuss Matrix A MIMO and Matrix B MIMO.
Matrix A MIMO
Matrix A MIMO implements the rate 1 Space-Time Coding scheme (commonly known as the Alamouti Code). This technique captures diversity gains by sending a single data stream in two parts out of two antennas, interleaved with transformed/conjugated versions of the same information, so that the receiver has
higher probability of successfully extracting the desired signal. Matrix A achieves a spatial diversity order of two, but does not set out to achieve combining, interference mitigation, or spatial multiplexing gains.
Matrix A MIMO delivers higher link robustness, reducing fade margin by 5 to 6 dB, with little or no degradation as subscriber mobility increases. The impact on end-user data rate is small; reduced fade margin may allow the use of marginally higher order of modulation, but it is not comparable to the 2x throughput gain achieved by comparable Matrix B MIMO through spatial multiplexing. Matrix A MIMO is useful in networks with light loading and relatively high subscriber mobility.
We will now look at how Matrix A MIMO works in a little more detail.
Consider a Matrix A MIMO system, consisting of 2 Transmit and 2 Receive antennas as depicted below.
The signal received by one of the antennas at the receiver is a mixture of the signals transmitted from both of the transmit antennas. The receive signals can be expressed by the following simplified equations:
Rx1(f) = (H1,1(f) x Tx1) + (H2,1(f) x Tx2)
Rx2(f) = (H1,2(f) x Tx1) + (H2,2(f) x Tx2)
The receiver sees a combination of the transmissions from the two transmit antennas and needs to recover the actual transmitted signals. MIMO systems achieve this by using coding schemes that define which signal should be transmitted and when in order to make it possible to recover the original signal. These coding schemes are called ‘Space-Time’ codes because they define a code across both space (antenna
separation) and time (symbols).
Matrix A MIMO is a Space-Time Block Code, so called because the code operates over a block of data. Block codes require less processing power to decode than convolutional codes.
The following matrix defines how the code works:
X is the output of the encoder and S1 and S2 are the input symbols into the encoder. ‘*’ denotes a complex conjugate of the symbol. The rows of the matrix represent the transmit antennas and the columns represent time. Each element of the matrix indicates which symbol is to be transmitted from which antenna and when.
Picture below will guide us on how this matrix works.
On the left hand side the binary bits enter a modulator, which converts binary bits into “symbols” according to the modulation scheme. These complex symbols are then fed into the Encoder, which maps the symbols onto the transmit antennas according to the matrix above.
The code works with a pair of symbols at a time and it takes two time periods to transmit the two symbols. Therefore it has the same rate as the data stream that enters the encoder but the error performance of the system is improved due to the coded information transmitted by the system.
In systems with high SNR performance, the improvement in the error rate achieved as a result of using Space-Time codes could be traded for higher capacity by using a higher order modulation than would otherwise be the case, resulting in marginal increases in throughput.
Matrix B MIMO
For channels with a rich multipath environment it is possible to increase the data rate by transmitting separate information streams on each antenna in the DL direction. Using sophisticated receiver technology, the different streams can be separated and decoded. For example, using 2 transmit and 2 receive Tx/Rx chains (and the associated antennas), up to twice the capacity of a single antenna system can be
achieved. This is particularly useful in urban deployments where long reach is less important than high data rate at the end user device. In WiMAX, spatial multiplexing on the downlink is made possible using the Matrix B MIMO.
The following matrix defines how the code works:
X is the output of the encoder and S1, S2 are the input symbols into the encoder. The row of the matrix represent the transmit antennas; there is no time element because Matrix B MIMO operates over a single time interval. Each element of the matrix indicates which symbol is to be transmitted from which antenna. In this
system 2 symbols are transmitted in a 1-symbol time duration thus providing a twofold capacity increase.
The picture below will exlpain us about the matrix operational on the tx module.
The theoretical upper band of capacity increases achieved by Matrix B MIMO is roughly proportional to the number of Tx/Rx chains used. A 4×4 system will have up to 4 times the capacity of a single antenna system.
Therefore, the capacity gain delivered by Matrix B MIMO linearly depends on the number of Tx and RX antennas and can be expressed as:
X = min (Tx Antennas, Rx Antennas)
However, the number of Tx/Rx chains and antennas that can be used in the mobile subscriber (MS) devices are likely to be the limiting factor for the foreseeable future.
Smart Antenna Systems (SAS)
SAS combine antenna arrays with sophisticated signal processing to enhance SNR for higher throughput and link robustness while simultaneously reducing interference. Beamforming is an example of SAS. When receiving a signal, beamforming can increase the gain in the direction of wanted signals and decrease the gain in the direction of interference and noise. When transmitting a signal, beamforming can increase the gain in the direction the signal is to be sent and direct nulls at users that would otherwise be iterfered with.
SAS typically delivers a +10 to 15 dB link budget improvement relative to a single antenna architecture. In the mobile WiMAX application, its active interference management can push the achievable net spectral efficiency into the 4-5bps/Hz range. SAS alone on the BS side provides operators wnith significant range benefits in the initial stages of network operation, and as their subscriber base grows and the
network becomes interference limited, SAS can provide significant capacity benefits, especially when used in combination with spatial multiplexing techniques. SAS interference mitigation benefits start to diminish as subscriber mobility increases.
For complete reading, you may read this Smart Antenna System whitepaper.
Resource:
www.wimax.com

 

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