Infrastructure radio solution for diversity and MIMO

Infrastructure radio solution for diversity and MIMO

New techniques enable the designer to provide solutions to meet the demand for more information fasteranywhere, anytime

BY ZATIL HAMID
Texas Instruments, Dallas, TX
http://www.ti.com

Designing digital radios always has been a challenge for communications systems. As the demand for higher and higher data rates from triple play (voice, video, and data) applications have made their way into the mainstream, radio design requirements also have greatly increased. Network providers are looking to increase reliability and quality of service (QoS) of mobile communications using diversity, or multiple-input/multiple-output (MIMO). These concepts are not new, but the advances made in analog-to-digital (ADC) and digital-to-analog (DAC) converters, as well as radio frequency (RF) semiconductor design technologies, make implementing these techniques feasible.

Why use diversity and MIMO?

In mobile applications, channel conditions may change very quickly. Multiple objects such as buildings in proximity and large bodies of water, moving objects and even humidity may cause reflections and refractions resulting in multipath signals. This effect is called multipath fading or multipath distortion.

These multipath signals are received by the radio receiver with varying delays, causing differences in phase. Multipath signals that are in-phase with the primary signal at the antenna add to the primary signal strength; while signals that are out-of-phase subtract from the primary signal strength. These drastic and sudden changes in channel conditions affect the signal-to-noise-ratio (SNR) of the signal.

As a result, base transceiver station (BTS) designers must implement systems capability to rapidly redeploy network resources to ensure the mobile user QoS is maintained. When a BTS detects or receives channel estimation information from the mobile indicating that channel conditions are poor, the mobile’s reception already is degrading, affecting the quality of signal.

The network, therefore, must quickly improve channel conditions to avoid dropping the call. Alternatively, good channel conditions should be exploited to provide the highest levels of data rates at maximum spectral efficiency.

The solution

One option is to use diversity. Most BTS radios today employ receive diversity. Diversity receivers enable the BTS to choose and decode the stronger of two separate, spaced apart signals. Transmit diversity with two separate transmitters is used occasionally where terrain conditions require it. This method is relatively simple to implement, but it simply ignores the multipath signals. Thus, it may not be sufficient.

Shown is a radio transceiver configuration using diversity or MIMO.

A second method gaining momentum is MIMO. This refers to transmitter x receiver arrays of n x m where typically n = m, e.g., 2×2, 4×4, etc. This method uses more intensive digital signal processing, but delivers better results. Additionally, newer communications technologies are moving towards using orthogonal frequency division multiplexing (OFDM). By the nature of its closely spaced orthogonal sub-carriers, OFDM is uniquely positioned to exploit multipath signals to increase the channel SNR. As a result, wide signal bandwidth and high data rate technologies such as WiMAX, LTE and UWB, leverage the benefits of MIMO to deliver superior service.

In practical application, the network can respond quickly to the changes in channel conditions. A weak signal triggers the BTS to transmit the same signal to a particular user on multiple signal paths to create a stronger combined multipath signal, thus increasing coverage. However, a strong signal triggers the BTS to transmit more information to the same user, or aggregate the total amount of information to multiple users, maximizing channel capacity.

Silicon solution

Due to the significant benefits of both diversity and MIMO, there is an even greater need for silicon solutions that enable radio system designers to maximize performance while controlling cost. Today, diversity and MIMO radio solutions can be implemented using either high IF heterodyne transceivers or direct conversion transceivers.

High IF heterodyne transceivers

State-of-the-art ADCs such as the quad-channel, 14-bit, 125 Msamples/s ADS6445 are capable of supporting wideband and multicarrier applications. At 170 MHz IF and 125 Msamples/s, the device provides 79 dBc of spurious free dynamic range (SFDR) and 70 dBFs of SNR. This allows an additional RF down conversion stage to be removed, as well as eliminates multiple single-carrier receivers required for a multicarrier implementation.

Meanwhile, high IF transmitters can leverage 1-GSPS DACs which offer LVDS interface to enable the transmission of very wide bandwidths of up to 400 MHz in one device. Integrated digital features such as interpolation filters with spectral inversion capability lend added flexibility to compact solutions.

Direct up and down conversion transceivers

Currently, two major changes are impacting solutions: the increase in signal bandwidth requires faster ADCs and DACs; and complex quadrature or IQ modulators now can transmit baseband signals directly to very high RF frequencies.

However, recent advances in RF design technology have enabled the reception of single carrier signals without requiring IQ correction.

As example, the TRF3703 quadrature modulator. It can output baseband or complex IF signals anywhere from 400-MHz to 4-GHz RF outputs, lending maximum flexibility to frequency placement. Meanwhile, today’s DACs also provide features such as IQ correction as well as gain and offset controls, further improving sideband and carrier suppression.

Finally, direct down converters are getting closer to resolving IQ imbalance (phase and gain), IP2 and dc offset imbalance issues. While dc offset imbalance can be easily avoided using ac coupling, IQ imbalance proves to be a more difficult issue.

However, recent advances in RF design technology have enabled the reception of single carrier signals without requiring IQ correction. The TRF3710 with on-board filters provides an IP2 of 60 dBm and IP3 of 22 dBm (at 42 dB of gain), getting us closer to the ultimate multicarrier wideband direct down converter. ■

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