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Advanced Antenna Systems (AAS) and Beamforming: Technology and Advantages

Introduction

Beamforming is a subset of what is known as AAS (Advanced Antenna Systems), or “smart antennas”. AAS typically exploit the properties of multiple antennas operating together. A digital processing algorithm controls and processes the signals sent and received via the multiple antennas. The advantages are improvements in signal strength and/or quality, and more capacity.

AAS technologies are essential to broadband wireless access systems. Any expanding wireless network will have to reuse frequencies, as spectrum is not an unlimited resource. Without the use of smart antennas, co-channel interference will steadily grow and capacity will be severely limited.

With smart antennas, there is a major improvement in avoiding and preventing CCI, increasing overall network efficiency and capacity.

Perhaps the most advantageous technology is beamforming – though most challenging to achieve, because of complex processing power and electronics required. Still, the advantages are clear – much improved range and capacity are most welcome by carriers who deploy broadband wireless systems.

Below is an article on the types of AAS systems culminating in Beamforming, the most advanced and complex current AAS technology.

 

Diversity and MIMO Technologies

Diversity schemes exploit the fact that radio signals will arrive at their destination with a certain shift –in space, time, or frequency - most likely all of those.

  • Space diversity schemes use antennas separated in space, and those techniques are called STC (Space-Time Coding). The signal arrives to multiple antennas, but is distorted spatially and in time. A digital processing algorithm examines the signal received from all the antennas, and attempts to combine them to extract an improved signal. Those techniques do improve the SNR, but they are likely to be only deployed at the base station. For that reason, only work one-way – for signals sent by the terminal, or uplink. Downlink, or signals sent by the base station to the terminal, do not benefit from STC. STC does not improve overall network efficiency – CCI (Co-channel interference) is not avoided.

  • AOA, or (Angle of Arrival), also known as "beam steering" is another spatial technique. AOA uses multiple antennas, but keeps track of the user terminal direction, and uses the antenna element that it assumes has the best position towards the terminal. AOA is a relatively simple, yet not very effective technique.

 

MIMO (Multiple Input, Multiple Output) is used in 802.11n Wi-Fi and some WiMAX implementations. MIMO does help both uplink and downlink to a certain degree. There are two main implementations: MIMO Matrix A and Matrix B.

 

  • MIMO Matrix A uses two or more antennas at both base station and terminal (transmitter and receiver) to transmit the same data stream from multiple antennas. Similarly to STC, MIMO A improves the SNR (Signal to Noise Ratio). The advantage is minor (a few dB) but is relatively simple to design.

  • MIMO Matrix B uses same two or more antennas to transmit different data streams to the same terminal, theoretically doubling the data rate delivered to the user. MIMO B does not improve the coverage, just the data rate and requires ideal conditions (no mobility, very rich multipath, no fading).


WiMAX technologies are very similar to Wi-Fi (both having originated from 802.11), and the 802.11n Wi-Fi uses a technology similar to WiMAX Matrix A and B. Indoor Wi-Fi range and capacity are increased by using multiple antenna streams at the Access Points and at the terminals. Indoors, it works very well; yet not so much in outdoor deployments. For dense, long range networks MIMO is typically not employed due to ineffectiveness, though it is typically relatively cheap and easy to design.

 

Beamforming Technologies

Beamforming is a technology that improves both uplink and downlink SNR performance and overall network capacity. Most other AAS systems are adaptive, but only consider one parameter of the terminal – most likely the distance or angle – to make their calculations.

Beamforming on the other hand, involves a sophisticated algorithm that tracks many more parameters than any other AAS system. Beamforming looks at terminal location, distance, speed, signal/noise level, what QoS level is required, traffic type, etc. This gives beamforming a much bigger advantage in terms of signal improvement.

 

Beamforming operates like a torch, focusing the radio beam.
Fig.1
As opposed to traditional single-element antennas, Beamforming exploits the spatial and physical properties of multiple antenna elements working together.

 

To explain the beamforming process in very simple terms, beamforming works by shaping the beam towards the receiver. Multiple antennas all transmit the same signal, but each is specially distorted in phase. The algorithm considers a “signature”, the unique properties of the terminal relative to the base station, and applies that signature to each transmission.

The multiple transmitted patterns combine in the air by natural coherence of electromagnetic waves, and form a virtual “beam”, a signal targeted to the destination. In the desired direction (where the destined receiver is located) the phases overlap and amplify each other. In the directions where the beam is not supposed to go (that is, anywhere else) the phases will collide and destroy each other.

Theoretically, the more antennas are used in the array, the stronger the beamforming effect - each additional transmitting antenna can double the signal (+3 dB). Practically however, the number of used antennas is a tradeoff between desired beamforming gain and the complexity of implementation.

 

Beamforming simplified illustration
Fig. 2
Beamforming focuses the radio beam from the base station to the terminal.
In unwanted directions, the signals collide and destroy each other.

 

Beamforming has multiple benefits:

  • Higher SNR
    The highly directional transmission improves the link budget, this increasing the range, both open-space and for indoor penetration.

  • Interference avoidance and rejection
    Beamforming overcomes external and internal (CCI) interference by exploiting the spatial properties of the antennas. Since the interference comes from a certain direction, the beamformer can apply a nulling technique – send a “null” towards the interferer, canceling it out.

  • Higher network efficiency
    By significantly reducing the CCI, beamforming can allow much denser deployments than single antenna systems. Thanks to higher link budget, the likelihood of running high-order modulations (64QAM, 16QAM) is much higher even at the edges of the cell. Overall capacity is greatly improved.

 

Beamforming is more complex to make than other multiple antenna techniques, but is a highly beneficial technology. The antenna is the single weakest link in the network, and beamforming solves that problem.

Carriers should consider systems that employ this advanced technology, because of the immense advantages it offers – better range, capacity, efficiency, and ultimately less CapEx and OpEx.

 

Beamforming in Alloyant Broadband Wireless Access systems

Alloyant StreamStar4 Broadband Wireless Access system uses an advanced 8-element Smart Antenna implementation, i.e. the antenna array is made of 8 independent transceivers and antennas controlled by a bank of Digital Signal Processors (DSP).

The 8-element Smart Antenna array can provide a beamforming gain as high as 18 dB (64 times more power). In terms of gain, this is equivalent to having a 120+ watt base station amplifier, but using intelligence instead of raw power.