MIMO and Smart Antennas for Mobile Broadband ... - 4G Americas

At 700 MHz a 2x2 configuration (two transmitters and two receivers) is most common. A 2x4configuration is preferred at higher frequencies like 2600 MHz, 2300 MHz, 1500 MHz, 1800 MHz, 1900MHz, and 2100 MHz. As described earlier, multiple RRHs can be combined to form other configurationssuch as 4x4 and 4x8. At 2300 MHz, an 8x8 installation for TDD-LTE is expected to be widely deployed.Air Standards/Multi-StandardCurrently RRHs are widely deployed in all major air standards such as GSM, CDMA2000, UMTS,WiMAX, and LTE. Most RRHs will be capable of supporting multiple air standards in one unit. Commonmulti standard implementations include LTE+UMTS, LTE+CDMA2000, or LTE+UMTS+GSM. AnLTE+CDMA2000+UMTS configuration is expected to be less common. RRHs shall be softwareconfigurable and capable of supporting any carrier combination with these multiple standards.Interface StandardThe optical high-speed serial interface between the baseband processing unit and the RRH/TRDUtypically use the Common Public Radio Interface (CPRI) standard or the Open Base station ArchitectureInitiative (OBSAI) standard. Current RRHs support data rates up to 6.144 Gbps. It is expected that thenext generation of RRHs will require line rates up to 12.2288 Gbps or higher.This data rate on the high-speed serial interface reflects the maximum bandwidth of the spectrumtransmitted and received by the Remote Radio Head/Unit as well as the number of branches, and thenumber of units that may be daisy chained with one optical high speed interface.FDD vs. TDDMost RRHs deployed support FDD (Frequency Division Duplexing). However, RRHs also support TDD(Time Division Duplexing) for WiMAX (licensed and unlicensed bands) and LTE (2300 MHz).EfficiencyThe efficiency of an RRH is defined as the RF output power delivered divided by the DC input power. Atmacro power levels, the power amplifier is the largest contributor to the overall power consumption of theRRH. This is not to say that the power consumption in the other subsystems is negligible. The powerconsumption in the digital subsystem can be significant (20-30 W) and this is especially critical for microRRHs.6.6 CABLE TRADEOFFS FOR REMOTE RADIO HEADSA perennial challenge in placing antennas at appropriate heights has to do with the costly, bulky, andlossy coaxial cables. There are often many tens of meters of cable connecting the antennas and the radiotransmitters, which are often located in climate-controlled enclosures on the ground near a tower base orin a central location either on the roof of a building or on a lower floor.So-called “hard line” coaxial feeder cable is typically 7/8” to 1-5/8” or even thicker is used for long straightruns, such as up a tower, with more flexible and thinner jumper cables making shorter connections to theantenna itself on one end and to a lightning arrester and the base station on the other end. These cablesalong with their connectors, lighting arresters, and sometimes combiners, often contribute 2.5 to 3.5 dB ofRF power losses, requiring an amplifier twice as powerful (and consuming twice as DC power) as strictly4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved96