UMTS: Alive and Well - 4G Americas

6.1.4 TRANSMIT DIVERSITY EXTENSION FOR NON-MIMO UESThe 2x2 MIMO operation for HSDPA specified in Rel-7 allows transmission of up to two paralleldata streams to a MIMO UE over a single carrier. If and when the HSDPA scheduler in the basestation decides to only transmit a single stream to the UE for any reason (e.g. because the radiochannel temporarily does not support dual-stream transmission), the two transmit antennas in thebase station will be used to improve the downlink coverage by single-stream transmission usingBF.As MIMO is being deployed in more and more networks, single-stream transmission using BFalso towards non-MIMO UEs that reside in MIMO cells becomes an increasingly attractivepossibility. Therefore, this option has been introduced in Rel-9, re-using L1/L2 solutions from Rel-7 MIMO to as large extent as possible. This is referred to as “single-stream MIMO” or “MIMO withsingle-stream restriction.” For a Rel-9 dual-carrier HSDPA user, the usage of single-stream MIMOcan be configured independently per carrier.6.2 LTE ENHANCEMENTS6.2.1 IMS EMERGENCY OVER EPSEmergency bearer services are provided to support IMS emergency sessions. A maindifferentiator of an IMS emergency session is that emergency service is not a subscription serviceand therefore, when the UE has roamed out of its home network, emergency service is providedin the roamed-to network and not the home network.Emergency bearer services are functionalities provided by the serving network when the networkis configured to support emergency services. Emergency bearer services can be supplied tovalidated UEs and depending on local regulation, to UEs that are in limited service state andotherwise not allowed on the network. Receiving emergency services in limited service state doesnot require a subscription. Depending on local regulation and an operator's policy, the MME mayallow or reject an emergency request for network access for UEs in limited service state. Tosupport local regulation, four different behaviours of emergency bearer support have beenidentified as follows:1. Valid UEs only. No limited service state UEs are supported in the network. Only normalUEs that have a valid subscription, and are authenticated and authorized for PS servicein the attached location are allowed.2. Only UEs that are authenticated are allowed. These UEs must have a valid IMSI.These UEs are authenticated and may be in limited service state due to being in alocation that they are restricted from regular service. A UE that cannot be authenticatedwill be rejected.3. IMSI required, authentication optional. These UEs must have an IMSI. Even ifauthentication fails, the UE is granted access and the unauthenticated IMSI retained inthe network for recording purposes.www.3GAmericas.org February 2010 Page 47