3GPP Broadband Evolution to IMT-Advanced - 4G Americas

TX antennas and most devices will only have 2 RX antennas, and thus the rank is limitedto 2.AC5 is MU-MIMO, which is not defined for the downlink in Release 8.AC1 and AC7 are single antenna port modes in which AC1 uses a common ReferenceSignal (RS), while AC7 uses a dedicated RS or what is also called a user specific RS. AC1implies a single TX antenna at the base station. AC7 implies an antenna array withantennal elements closely spaced so that a physical or spatial beam can be formedtowards an intended user.LTE is specified for a variety of MIMO configurations. On the downlink, these include 2X2,4X2 (four antennas at the base station), and 4X4. Initial deployment will likely be 2x2.4X4 will be most likely used initially in femtocells. On the uplink, there are two possibleapproaches: single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO). SU-MIMO ismore complex to implement as it requires two parallel radio transmit chains in the mobiledevice, whereas MU-MIMO does not require any additional implementation at the device.The first LTE release thus incorporates MU-MIMO with SU-MIMO deferred for the secondLTE release.Peak data rates are approximately proportional to the number of send and receiveantennas. 4X4 MIMO is thus theoretically capable of twice the data rate of a 2X2 MIMOsystem. The spatial-multiplexing MIMO modes that support the highest throughput rateswill be available in early deployments.For a more detailed discussion of 3GPP antenna technologies, refer to the 3G Americas’white paper “MIMO and Smart Antennas for 3G and 4G Wireless Systems – PracticalAspects and Deployment Considerations,” May 2010.Channel BandwidthsLTE is designed to operate in channel bandwidths from 1.4 MHz to 20 MHz. The greatestefficiency, however, occurs with higher bandwidth. A 3G Americas’ member analysispredicts 40% lower spectral efficiency with 1.4 MHz radio channels and 13% lowerefficiency with 3 MHz channels. 117 The system, however, achieves nearly all of itsefficiency with 5 MHz channels or wider.IPv4/IPv6Release 8 defines support for IPv6 for both LTE and UMTS networks. An Evolved PacketSystem bearer can carry both IPv4 and IPv6 traffic. This enables a UE to communicateboth IPv4 and IPv6 packets (assuming it has a dual stack) while connected through asingle EPS bearer. It is up to the operator, however, whether it assigns IPv4, IPv6, orboth types of addresses to UE.Communicating between IPv6-only devices and IPv4 end-points will require protocolconversionor proxies. For further details, refer to the 3G Americas’ white paper, “IPv6 –Transition Considerations for LTE and Evolved Packet Core,” February 2009.Voice SupportVoice support in LTE will range from no voice, to voice implemented in a circuit-switchedfallback (CSFB) mode to 2G or 3G, to voice implemented over LTE using IMS.As a pure data service, especially for laptops, voice may not be needed. But onceavailable on handheld devices, voice will become important. The easiest implementationwill be CSFB. In CSFB, the LTE network carries circuit-switched signaling over LTE117 3G Americas’ member company analysis 2009.Transition to 4G: 3GPP Broadband Evolution to IMT-Advanced, Rysavy Research/3G Americas, Aug 2010 Page 105