Transceivers Power Pico Base Stations

July 23, 2009
These highly integrated transceivers support 3G and 4G microcell and picocell systems operating at 2.3 to 2.7 GHz and 3.3 to 3.8 GHz, including WiMAX and LTE systems.

Smartphones such as the Apple iPhone are driving demand for more broadband, datacentric technologies and the need for strategically positioned micro- and pico-sized cellular base stations. The AD9356 and AD9357 integrated transceivers from Analog Devices are each designed to provide two receive and transmit channels in a compact footprint while also reducing power consumption in broadband systems, such as WiMAX and Long Term Evolution (LTE) cellular networks. The AD9356 transceiver operates from 2.3 to 2.7 GHz while the AD9357 transceiver works from 3.3 to 3.8 GHz.

Modern picocell- and microcell-based wireless communications networks rely on a significant number of discrete components, and often exceed power budgets and size constraints. A typical picocell base station RF transceiver, excluding the power amplifier (PA), consists of six to eight active components, while a microcell base station can easily double this component count. But using the AD9356 and AD9357 integrated transceivers (Fig. 1), the component count for a tworeceive by two-transmit transceiver can be reduced to a single device while reducing power consumption by at least 50 percent.

Furthermore, by using a single transceiver across multiple base station platforms (picocell and microcell), the hardware design cycle is greatly simplified. The configurability of these new transceivers allows designers to develop and maintain software that can be adapted to multiple platforms. This flexibility, when combined with industry-leading RF performance, significantly speeds time-to-market and translates into greater than 50-percent bill of materials (BOM) savings.

Although cost, power, and size are significant factors, the transceiver must still meet the technical challenges required by these new 4G systems. For the receiver, one of the biggest challenges is dynamic range. The receiver must be able to demodulate signals from a user who is physically close to the base station as well from a different user considerably more distant from the base station. This challenge is commonly referred to as the "near/ far problem."

The AD9356 and AD9357 integrated transceivers address the near/far problem by using several different techniques. First, the transceivers incorporate 12-b analog-to-digital converters (ADCs) to provide the transceiver with generous instantaneous dynamic range. As important, the transceivers also feature an advanced automatic- gain-control (AGC) algorithm.

The flexible implementation within the AD9356 and AD9357 transceivers allows for the use of the AGC in autonomous mode or for control in manual mode using real-time signals from the transceiver. This provides the system integrator with more control of the system gain. It is with the combination of AGC and high-dynamic-range ADCs that the transceivers can resolve the near/far problem without the cost of a high-performance, high-power discrete- circuit solution. (More details on how an AGC can be used in integrated transceivers for 4G base stations can be found on the Analog Devices' web site.)

Beyond the receiver, possibly the biggest innovation for the AD9356 and AD9357 transceivers is the improvement in transmit noise floor. In previous generations of the firm's integrated transceivers, like the AD9354 and AD9355, the external PA power output levels were limited to +33 and +27 dBm, respectively, while meeting the FCC and ETSI spectral mask requirements. The new AD9356 RF transceiver can achieve spectral mask levels of -130 dBc/Hz offset 8 MHz from the carrier from 2.3 to 2.7 GHz (Fig. 2). The transceiver can support levels to +40 dBm per transmitter at the antenna ports.

The AD9357 transceiver can achieve spectral mask performance of approximately -144 dBc/Hz offset 70 MHz (Fig. 3) from the carrier from 3.3 to 3.8 GHz. It can support external PA at levels to +33 dBm per transmitter and still comply with the ETSI requirements. In order to support higher PA power output, a simple low-cost external lowpass filter can be added. Supporting this higherpower PA with integrated transceivers will allow system integrators to extend the range of their systems for enhanced coverage, while maintaining low-cost deployments. More information on transmitter noise performance for integrated transceivers is available on the Analog Devices web site.

In addition to providing best in class receiver and transmitter performance, multiple AD9356 and AD9357 transceivers can be cascaded or synchronized to support four receivers (Rx) by four transmitters (Tx) or even implementations with eight receivers by eight transmitters. These types of implementations can further extend the transmit and receive range of the base station as well as support beamforming techniques for improved coverage and quality of service. The AD9356 and AD9357 transceivers have the capability to easily synchronize both baseband and RF paths between multiple chips. The baseband sections of the multiple chips, which include the receive ADCs, transmit digital-to-analog converters (DACs), and data input and output (DATA I/O) connections, are synchronized by sharing a common reference clock for the phase lock loop (PLL). In addition, a common synchronization pulse must be sent to the multiple chips from the digital baseband chip. Once the synchronization pulse is sent, the baseband sections of the multiple chips will be synchronous.

To phase-synchronize multiple RF paths, the phase of the different paths must be measured. One approach to accomplish this is by using a dedicated receiver to measure the phase of each transmit signal. The phase is modified by applying phase shifts digitally to the baseband data. The AD9356 and AD9357 transceivers provide a dedicated monitor path that can be used for measuring the phase and output power of the transmit path. By using this onboard transmit monitor, the cost and overhead related to a dedicated receive path for monitoring multiple transmit phases can be reduced.

Another feature of the AD9356 and AD9357 integrated transceivers is the capability to use a common external local oscillator (LO) for the multiple chips in an M x N multiple-input, multiple- output (MIMO) communications system. The external LO is required to run at 2x the carrier frequency for the AD9356 and 4/3x the carrier frequency for the AD9357. The advantage of using the external LO is to improve transmit signal-to-noise ratio (SNR) by using a lower-phase-noise RF synthesizer as well as using a common LO, which results in correlated phase noise for the N transmitters. Currently, the AD9356 and AD9357 transceivers, with their internal LO sources, achieve error-vector- magnitude (EVM) performance of -39 dB across their respective frequency bands, considered best in class. By substituting a low-phase-noise external LO, EVM performance of -45 dB is possible.

Analog Devices, Inc., 3 Technology Way, Norwood, MA 02062; (781) 329- 4700, Internet:

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