Wireless solutions are becoming a major network interface for mobile devices, and now is migrating to fixed equipment in consumer and industrial applications. These wireless solutions utilize cellular, wireless phones, Wi-Fi, and WiMAX® bands. Historically, the method of increasing the bandwidth on these channels involves an increase in the baseband frequency of establishing a connection. Today, however, the amount of transported data has exceeded the speed that can be processed using traditional design techniques on a new open spectrum. Therefore, the new system is transitioning from single input, single output (SISO) system to multiple input, multi-output (MIMO), and multi-frequency / protocol connection options. MIMO logic and specifications have been included in new standards; however, they drive system complexity by requiring multiple RF amplifiers and transceiver systems in a single design.
What is MIMO?
RF communication uses a set of receivers and transmitters to send data from one point to another. In a clear spectrum area, RF communication has been dominated by the SISO system, and the SISO system consists of a set of antennas and RF circuits.
As the data rate and the signal congestion increase, multiple antennas configurations including a plurality of receivers and transmitters are being used. These MIMOs are configured to improve communication performance by extending the total transmit power of the antenna to achieve an array gain of the spectral efficiency and / or to improve the diversity gain of connection reliability. Spectrum efficiency is a measure of a bit per second per second; diversity gain is measured by the fading characteristics of the wireless signal, and the signal is reflected by a carrier medium and an obstacle such as a wall such as a wall. Figure 1 shows a combination of various antennas and transmitter / receiver from SISO to MIMO.
Figure 1: MIMO antenna configuration
In order to achieve these multi-antenna systems, data and RF must be split and modified and passed separately. MIMO technology consists of three core parts: precoding, spatial multiplexing, and diversity coding. The most common method for Wi-Fi, WiMAX, and cellular systems is a method called MIMO-OFDMA (orthogonal frequency division multiple access). OFDMA supports spatial multiplexing and diversity coding.
The precoding code can be described as a form of multi-stream beamforming. Typically, it is considered to be a collection of all spatial processing occurred at the transmitter. In a single layer beamform, the same signal is broadcast from each transmit antenna, which has an appropriate phase and gain weighting such that the signal power maximizes when the signal power reaches the receiver input. If the receiver has a plurality of antennas, the transmit beamforming cannot simultaneously maximize the signal level at all receiving antennas, thus executing precoding of data transmitted into a plurality of streams.
Spatial Multiplexing uses the MIMO antenna and uses a high speed signal, and then divides it into a plurality of low-speed signal streams. Then, each of these streams is transmitted from different transmit antennas in the same frequency channel. When these signals reach a plurality of receiver antennas, each receiver antenna has different spatial features, and the receiver can separate these streams into parallel channels. The maximum number of spatial streams that can be used is defined by the minimum number of antenna groups in the transmitter or receiver path.
Diversity coding methods apply to the single stream of the transmission, not a multi-stream scheme. The signal is encoded using techniques called empty coding. From each send antenna broadcast signal from each of the orthogonal encoding. Diversity Codes use independent fading characteristics in multiple antenna paths to enhance signal diversity.
Integrated solution
The MIMO solution adds considerable amount of additional system overhead and complexity for the design of currently no wireless interface. For the first addition of the connected product, an integrated solution that supports the entire MIMO functionality including the transceiver is available.
An example of these integrated systems is MAX2839 of Maxim Integrated Products. The SOC is a complete direct conversion, a zero multi-frequency RF transceiver for 2 GHz 802.16e MIMO mobile WiMAX applications. The device contains a transmitter with two receivers. To achieve high performance, the isolation between the receivers is greater than 40 dB.
Figure 2 shows a block diagram of the MAX2839 of Maxim. The MAX2839 integrates all the circuitry required to implement RF transceiver, RF to baseband reception path and baseband to the RF transmission path; it includes a VCO, frequency synthesizer, crystal oscillator, and baseband / control interface. The local oscillator I / Q orthogonal phase error can be digitally corrected at a step of approximately 0.125 degrees in the chip. Broadband RF radio solutions require only RF bandpass filters, crystals, RF switches, PAs, antennas, and some passive devices.
Figure 2: MAX2839 MIMO wireless broadband RF transceiver (provided by Maxim Integrated Products)
This design provides a programmable single-chip filter for the receiver and transmitter, without an external SAW filter, suitable for all 2 GHz and 802.16e configuration files integrated into the design. This design supports up to 2048 FFT OFDM, and implements programmable channel filters with 3.5 to 20 MHz RF channel bandwidth. These RF functions are controlled by low power closing mode, allowing the system to be used in mobile applications to extend battery life. The chip can run RF on a power source of 2.7 to 3.6 volts.
In order to make the system work as a transceiver, it requires an antenna array. A series of multiple surface mount antennas, such as the W3108 of Pulse Electronics (see Figure 3), will be a typical solution. These are tuned to specific frequency allocation, such as 2.4 GHz bands for processing Bluetooth, 802.11b / g, zigbee®, 2.4 GHz WLAN, and Wi-Fi. For MIMO configuration, each of the receivers and transmitters in the system requires one of these antennas. They provide a gain of 1.5 DBI, and the input impedance is 50 ohms.
Figure 3: W3108 surface mount antenna of Pulse Electronics (provided by Pulse Electronics)
RF power amplifiers Although these MIMO configurations provide higher data throughput, they are not cost and complexity. Each individual antenna path - a power amplifier to drive it and pick up the RF signal on the transmitting side and receiving side - the power amplifier is required. There may be many of these amplifiers for systems having multiple signal paths not only on the same channel frequency band but support multiple frequency bands. A typical mobile device that supports 802.11g / n wireless bands with Bluetooth® and cellular bands may have up to six PAs in the system. There is a 2.4 GHz G band, two MIMOs for 5 GHz N band, one for 2.4 GHz Bluetooth, two MIMOs for cellular or WiMAX bands. These power amplifiers are typically a single amplifier in a surface mount package due to this signal diversity and associated channel isolation requirements, so they can be distributed around the design of the footprint.
Choosing the right amplifier and its associated passive devices requires the product and process technology provided by different suppliers. Product training modules such as ADI and NXP Semiconductorsoutline provide key details distinguished from their products. These amplifiers apply to the application of KHz to 20 GHz spectrum.
These power amplifiers are supported by simple passive devices, and the primary filter is integrated in the standalone module or integrate with other devices. Figure 4 shows a typical application of antenna driver, as an application of NXP BGA2771 MMIC RF amplifier. These single antenna configurations tend to have a short-distance MIMO application, and when minimizing power is constrained. These single-amplifier devices provide the system with approximately 21 dB insert gain. For higher gain applications, these devices can be cascaded before the drive antenna. In addition to the final level of power output, a single amplifier can also be used for IF and after mixing broadband applications.
Figure 4: NXP BGA2771 MMIC RF amplifier application (provided by NXP Semiconductors)
There may be high noise or excessive fading between the transmitter and the receiver, typically using a multi-stage amplifier. Examples of multi-stage amplifiers have SST12LP08 and SKYWORKS SOLUTIONS for Microchip Technology. SST12LP08 and SKYWORKS SOLUTIONS. The Microchip amplifier is a two-stage device that provides approximately 30 dB gains in the 2.4 GHz band (see Figure 5). In order to achieve temperature stability and high efficiency, the device has a temperature and load unoccupied chip power detector, providing a separate power source for each amplifier and a bias generator. The final result of this combination is that the function 802.11g OFDM adjacent channel power ratio (ACPR) is as high as 23.5 dBm, and the gain change in the range of -40 ° C to + 85 ° C is approximately -2.5 dB.
Figure 5: SST12LP08 high gain amplifier block diagram of Microchip (provided by Microchip Technology)
Similar to the Microchip section, the SkyWorks amplifier (see Figure 6) is also a multi-level design with a power detector. This design uses three phases instead of two phases, and provides 28 dB gain at 2.4 GHz band. For 802.11g applications, the device supports 3.3 and 5.0 volt operations and 54 Mbps OFDM. Multi-level design allows the device to provide an output power closed-loop monitor, so that the driver can minimally reduce the mismatch of multiple channels.
Figure 6: SKYWORKS SE2609L power amplifier power detector (provided by Skyworks Solutions)
Power standard
One of the key to successfully implementing MIMO design is to minimize noise and isolate the signal coupling of the power supply. The ground loop and common grounding of the antenna connection and the amplifier / mixer module can reduce the inter-channel isolation, thereby increasing the signal decline. It is important to use separate grounding for each device and use a separate ground layer for adjacent MIMO signal streams. In the case where the RF is running with the channel frequency of the operation clock that is close to the digital logic that drives the wireless connection, the digital device should refer to a separate ground plane.
For power side, it is also important to keep signal isolation by using a separate regulator or more output power management controller. These devices, such as MAX3663 power management controllers for the Maxim of a single battery system, can support a large number of independent power paths in these MIMO design. Figure 7 shows a block diagram of the MAX8663. It is designed for smartphones, PDAs, Internet devices, and other portable devices, integrated with two synchronous buck regulators, a boost regulator and four low pressure difference linear regulators that can drive two to seven LEDs ( LDO). The device also has a linear charger for single-cell lithium ion battery. To help manage system functions, it integrates a thermal limit circuit for battery charging and peak load to prevent overheating. In low power mode, it can boot the remaining power from the external power supply to the battery charging function.
Figure 7: MAX3663 Multi-Output Power Controller (provided by Maxim Integrated Products)
Multiple single-device adjustment output is the main advantage of MIMO applications, providing consistent and reliable power for all data streams. This helps balancing flow to flow and adjacent channels, because a path does not overwhelming and exceeding other streams.
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