Aug 2010 Expert Advice

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

There are several different approaches to channel estimation training: instantaneous, statistical, pilot tone based, integrated, etc. Each of these trades off things like speed, processing power, complexity, bandwidth usage, or cost as they try to accommodate some of the channel variations.

Even though a mobile channel is constantly changing, many changes are slow enough that a stationary environment is not a bad assumption over the short time periods between estimations. Other variables change too fast for almost any channel estimation technique and other portions of the system must be designed to handle them. In either case, practical designs will only catch a certain fraction of their potential – but in today’s stressed wireless systems, every fraction can be worth it.

It's important to remember that MIMO implementations are much bigger than just the DSP subsystem (where features like channel estimation are usually implemented). Board, chassis and antenna designs create the interface between the device and the channel. Failure to optimize this hardware interface can handicap subsystem designers to the point that no amount of design brilliance can eek out the incremental gains MIMO advertises.

Spatial channel models are typically used for modeling MIMO scenarios since they include the necessary spatial and temporal characteristics of a typical wide-band cellular channel. By using SFE (Spatial Fading Emulation) technique, channel parameters would not change dynamically. It is like testing the phone by considering a snapshot of the scenario the phone will experience when working in the field. Parameters like delay spread, and Doppler spread will be based on the standard channel models used for.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

The challenge is in creating a reference condition that multiple devices can be evaluated against. In the past, this was not as difficult because the device designs didn’t have to find extra system gains from the channel. This is no longer the case with MIMO implementations. The value of MIMO functionality is inseparable from varying channel impairment conditions like multipath. Today there are two approaches for creating changing reference channel environments:

1) An anechoic chamber with 4 to 16 antennas surrounding the device. In this approach, a channel emulation signal is fed into these antennas to create a RF environment that simulates a channel. System complexity aside, this technique has at least three major limitations: first, the ring of antennas only creates a 2D plane of simulated space; second, the limited number of calibrated antennas restricts angular variation; and third, emulated channels and their variables need to be restricted to keep uncertainty and test times down, and to maintain reference commonality across labs.

2) A reverberation chamber which creates a multipath 3D reference environment. In this approach, statistical analysis of many simple measurements quickly summarizes device effectiveness. While this technique is relatively straightforward, its major limitation is that environmental variables cannot be extensively modified to provide highly specific channel simulations

The challenge is in creating a reference condition that multiple devices can be evaluated against. In the past, this was not as difficult because the device designs didn’t have to find extra system gains from the channel. This is no longer the case with MIMO implementations. The value of MIMO functionality is inseparable from varying channel impairment conditions like multipath. Today there are two approaches for creating changing reference channel environments:

1) An anechoic chamber with 4 to 16 antennas surrounding the device. In this approach, a channel emulation signal is fed into these antennas to create a RF environment that simulates a channel. System complexity aside, this technique has at least three major limitations: first, the ring of antennas only creates a 2D plane of simulated space; second, the limited number of calibrated antennas restricts angular variation; and third, emulated channels and their variables need to be restricted to keep uncertainty and test times down, and to maintain reference commonality across labs.

2) A reverberation chamber which creates a multipath 3D reference environment. In this approach, statistical analysis of many simple measurements quickly summarizes device effectiveness. While this technique is relatively straightforward, its major limitation is that environmental variables cannot be extensively modified to provide highly specific channel simulations.

A Spatial Fading Emulator approach, combining an anechoic chamber, several probes and a channel emulator, can emulate any specific environment including a variety of standardized channel models currently used for UE conformance testing based on 3GPP standard. With such a setup, end to end testing of the wireless device, including its antenna configurations, receiver, digital signal processing and software, can be performed in a controlled realistic environment.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

Mutual coupling reduces antenna efficiency. Since small MIMO devices are subject to these effects, the gains in multipath performance need to be able to offset the losses due to mutual coupling. By careful antenna design, this mutual coupling effect can be minimized allowing for measurable MIMO advantages. I should emphasize that the antenna design in this case refers to an entire system design at the device level since chassis, boards, materials, paints, nearby components, etc. can significantly affect antenna efficiencies.

It increases the correlation between the antennas signals, then the more coupling, the less advantage can be achieved from the presence of multi signals.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

Proximity and alignment are the leading factors for antenna correlation and isolation, while size is clearly the leading factor for efficiency. For all three, the user device itself creates ground plane variations or resonator effects that will influence these measurements. This underscores the need for rapid testing of correlation, isolation and efficiency to allow for dynamic optimization of an antenna design in the device.

We do recognize element locations, spacing between elements, and radiation characteristics of individual elements (radiation pattern, polarization) as the leading factors. These parameters must be jointly considered in order to achieve optimum performance trade off for multiple antenna terminals.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

Multipath fading is due to the radio signal reflecting off of various surfaces on its way to the receiver. These reflections create multiple signal paths as seen by the receiver (multipath) and these variations of the same signal constructively and destructively interfere with each other when they reach the receiver. The destructive interference causes drops in data throughput and, in extreme cases, a dropped connection. Constructive interference can blast the receiver with too much power, leading to signal distortion in the receiver front end.

The biggest environmental concern is the shift to indoor wireless usage. Indoor is rapidly becoming the primary usage environment for wireless devices of all types. The close proximity of walls, fixtures, furniture, etc in a contained space creates a much higher level of multipath -- even to the point where a line-of-sight path to the base station is the exception, rather than the rule.

In real propagation channels, the direct link between the mobile station and the base station coexists with the indirect links caused by the reflection and the diffusion of this ray on the buildings, and different obstacles surrounding the emitters and the receivers. Those different rays are recombined in a constructive or destructive way: this create signal fading at the location of the terminal. Usually, buildings and their related indoor environments are the most common environmental concerns when deploying a cellular network.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

Fading occurs when reflections create multiple versions of a signal which combine, constructively or destructively, at the single reception point. Antenna diversity splits that single reception point into separate reception points. Even though each antenna will receive a multipath signal, if they are independent enough, they will receive it in different ways. Now the receiver can filter and combine these different inputs to its advantage. If you had a separate antenna for every signal path, you would maximise antenna diversity and could, in theory, eliminate the fading effects of multipath. In practice, much of the gains from diversity can be realized with just a few antennas.

Antenna diversity can be implemented by using different mechanism: spatial diversity, frequency diversity, angle of arrival diversity, polarization diversity, multipath diversity, and pattern diversity.

The most common and simplest mechanism is the spatial diversity. It consists of having two antennas separated by physical distance. Due to the phase delays, multipath signals arriving at the antennas differ in fading. This translates in having an improvement in SNR from a diversity antenna system over a single antenna system. This improvement is usually termed “Diversity Gain”.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

Fade margin is the amount of additional system gain or receiver sensitivity that must be maintained to allow for multipath fading effects.

Fading margin gives an indication of how many dBs a received signal level can be decreased in order not to impact receiver performances.

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

In NLOS condition the signal might have undergone scattering, diffraction, polarization changes, and reflection. All of them affect the received signal.

There are NLOS models which could be used to predict the RF signal strengths.

These models provide estimates of the path loss considering distance between TX and RX, antenna gain, antenna heights, used frequencies, and surrounding environment.

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

EUT size constraints largely dictate the antenna configuration design and the antenna type to be used for MIMO implementation. Specifically, the EUT chassis could be the most critical factor when designing a multiple antennas system at low frequency, since the ground plane becomes the main radiator at that specific frequency, and the spatial separation of the antennas in terms of wavelength becomes limited, which directly impacts the correlation, and hence the MIMO performance.

Derek Skousen – Product Marketing Analyst:

MI Technologies

Nicolas GROSS – Applications Director , Mr. Alessandro SCANNAVINI - Field Application Engineer and Meryam ABOU EL ANOUAR - R&D Antenna Engineer:

SATIMO

The easy answer is often "a half wavelength", but the practicalities of design requirements make any easy pronouncements useless – especially since there are more factors involved than separation. For example, the effects of orientation and coupling of other nearby components (both dielectric and conductive) strongly affect correlation and efficiency performance.

When talking about “spatial diversity” a rule-of-thumb is to have the antenna elements spaced 0.5λ. This will guarantee both an antenna correlation lower than 0.5, and low mutual coupling. However, the required element spacing is strictly related to the design and the type of antenna being used.