1xEV-DO_Overview_ZTE.doc

1xEV-DO Overview.Author NameDepartmentContact InformationAnit LohtiaCore RF Engineering(972) 685-41241. IntroductionThe wireless operators want to increase average revenue per user (APRU). One of the most important service to generate incremental revenue is to provide wireless data services. The first major step to provide high speed data service in CDMA networks was introduction of 1xRTT technology. As the data traffic in wireless networks continues to increase, there is a need to increase RF capacity of a carrier. 1xEV-DO is an evolution of 1xRTT to provide high speed data service and improve capacity of a RF carrier. The core network for a 1xEV-DO network is same as the core network for 1xRTT data network. However, there is no interaction with a MSC/MTX in a 1xEV-DO network. A 1xEV-DO network is an end-to-end IP network. Handoffs from one sector to another sector as mobiles move across sectors are important in mobile networks. The handoff mechanisms in a 1xEV-DO network are different depending on the radio node controller (RNC) of the target cell and the network architecture. If the target cell is homed in the serving cells RNC, a handoff is on the physical layer. If the target cell is on a different RNC served by the same Packet Data Serving Node (PDSN), a 1xEV-DO session handoff takes place. If the target cell is on a different RNC served by a different PDSN, a mobile IP handoff takes place. In a 1xEV-DO network overlay on a 1xRTT network, handoff management is also required when hybrid terminals move across the network boundaries. This section addresses the technical attributes of 1xEV-DO technology that make it spectrally more efficient. The network architecture of a 1xEV-DO network and functionality of different network nodes are discussed. The different handoff mechanisms are explained. Finally the process of upgrading a 1xRTT network to 1xEV-DO is discussed.2. Technical Attributes of 1xEV-DOThe data traffic flow is asymmetrical. There is a lot more data flowing from the network end to access terminals than from the access terminals to the network. Therefore RF capacity required in the forward link of a wireless data network is more than RF capacity required for the reverse link. 1xEV-DO technology was developed to exploit this anomaly of asymmetric capacity requirement in data networks.A 1xEV-DO carrier has bandwidth of 1.25 MHz as does a 1xRTT carrier. The reverse link in 1xEV-DO is similar to the reverse link of 1xRTT. However there are significant changes in the forward link. The forward link in 1xEV-DO is a time division multiplexed (TDM) CDMA channel. At a given instant of time in 1xEV-DO forward link, a single user gets all the power of a 1xEV-DO carrier. A 1xEV-DO carrier always transmits at the full power when there is control or data traffic to be transmitted. There is no power control in the forward link in a 1xEV-DO network. There is a scheduler in a basestation that determines which user will get the next timeslot in the forward direction. In 1xEV-DO the power is constant, and the data rate is changed based on RF conditions in the forward link. A basestation cant decide the data rate to be used in the forward link. A mobile determines the optimum data rate based on estimated C/I in the forward direction. The basestation has to transmit at the data rates requested by mobiles. A mobile uses data rate channel (DRC) to tell the data rate the basestation must use in the forward direction.There are nine different data rates in the forward link. A mobile can request a data rate from 38.4 kbps to 2457.6 kbps depending on the RF conditions. The reason these high data rates can be achieved using a 1.25 MHz carrier is that 1xEV-DO uses higher order modulation techniques. There are three different modulation techniques, QPSK, 8-PSK and 16-QAM, used in the forward link. The higher modulation techniques, such as 8-PSK and 16-QAM, can be used when a mobile is in good RF conditions with higher C/I. In the forward link of a 1xEV-DO network, the interference is from the neighboring sectors only. There is no co-channel interference from the other users in the same sector in the forward link because there is only one user transmitting at a given instant of time.In a Rayleigh fading environment, mobiles go through deep fades as shown in Figure 1. In a 1xRTT system, a basestation has to increase the transmit power to mobiles during fades. This increase in the transmit power increases interference and decreases RF capacity of the system. In 1xEV-DO, a basestation scheduler gives negative bias for scheduling time slots to mobiles when they are in a fade. A proportionally fair scheduler gives a time slot to the user with the highest ratio of the data rate requested to the average throughput of the user. When a user goes into a fade, the data rate requested will be lower. This will lower its ratio of the data rate requested to the average data throughput, therefore probability of allocating a time slot for the user during a fade will be less. This results in multi-user diversity gain and increases RF capacity of 1xEV-DO.Figure 1 Two Rayleigh Faded Signals Forward link Sector SwitchAn access terminal receives data only one basestation at a time. There is no soft handoff in the forward link. An access terminal receives the data from the strongest carrier in its active channel set. It monitors the signal strength of all carriers in its active set, and requests a switch in the carrier if another sectors carrier becomes stronger. An access terminal can only request switch to a carrier in its active set. The soft handoffs in the reverse link are similar to 1xRTT reverse link soft handoffs.Reverse Link Data RateThough the reverse link of 1xEV-DO system is similar to 1xRTT, there are subtle differences between the two. In 1xEV-DO, there is no direct control on the data rates to be used by mobiles in the reverse direction. A basestation does not have knowledge of the data rate that a mobile will use for the reverse link. Each packet transmitted by a mobile has rate information embedded in it. This reverse rate indictor tells the rate a mobile is transmitting in the reverse direction. A basestation broadcasts a reverse channel activity bit (RAB) to tell mobiles if the reverse link is loaded. The RAB is set to busy if the reverse link is loaded. If the RAB is set to busy, a mobiles transmit data rate in the reverse link is decreased or kept the same depending upon the outcome of a random number based trial. Each mobile generates a random number. If the random number is higher than a threshold for a particular data rate, the data rate for the mobile is decreased. The probability of decreasing the data rate is higher for higher data rates. Conversely if the RAB is not set to busy, a mobiles transmit data rate in the reverse link is increased or kept the same depending upon the outcome of the random based trial. The probability of increasing the data rate is higher for lower data rates.There is a paradigm shift in 1xEV-DO technology. In 1xRTT technology, the network determines the data rate to be used in the forward link, and can directly control the data rate in the reverse link. However in 1xEV-DO technology, an access terminal determines the data rate to be used in both the forward link as well as the reverse link. A basestation cant force the data rates. The performance of a 1xEV-DO network is highly dependent on the algorithms used in access terminals. A basestation does not flexibility to choose data in the forward link, but it can decide when to send the burst for each mobile. Some of major differences between 1xEV-DO and 1xRTT technologies are listed in Table 1.1xEV-DO1xRTTOnly non real-time data rates up to 2.45 MbpsData rates up to 307 kbps as well voice servicesAdaptive coding and modulationFixed coding and modulationForward link time division multiplexedWalsh code separated forward linkFull power burst pilot in forward linkContinuous pilot in forward linkNo forward link power controlFast forward link power controlFixed power, variable rateFixed rate, variable powerNo soft handoff in forward linkSoft handoff in both linksAccess terminal controls data rateBasestation controls data rateSupports only data services and no interaction with MSCSupport data services as well supports IS-41 services with a MSCTable 1 Comparison of 1xEV-DO & 1xRTT3. Network Architecture of 1xEV-DOA 1xEV-DO carrier provides only data service. The network required for 1xE-DO technology is an end-to-end IP data network. An access terminal talks to a basestation or an access point (AP) over the air. The basestation is connected to a radio node controller (RNC). A RNC is responsible for radio resource management as well as mobility management. A RNC in a 1xEV-DO network has similar functionality as a basestation controller (BSC) in a 1xRTT network. However, a RNC does not interact with a Mobile Switch Center (MSC) as a BSC does in a 1xRTT network. A PDSN is a gateway to public data networks in a 1xEV-DO network. A network reference diagram for 1xEV-DO network is shown in Figure 2.Figure 2 1xEV-DO Network ArchitectureA 1xEV-DO network is an all IP network. In an IP network, the data packets are routed based on IP addresses of the nodes. There is no need for international mobile subscriber identification/ Mobile Identification Number (IMSI/MIN) for routing in a 1xEV-DO network whereas in a cellular/PCS network calls are routed based on IMSI/MIN. Therefore 1xEV-DO access terminals do not have pre-assigned IMSI/MIN. This presents a potential problem because the R-P session for each mobile between a RNC and a PDSN is indexed on an IMSI. To successfully transfer sessions from one RNC/BSC to another RNC/BSC on the same PDSN, the IMSI of the mobile should be the same. Since a mobile does not a IMSI, a RNC assigns a IMSI for the mobile when it starts a R-P session for a mobile with a PDSN. There is an optional interface A12 introduced in 1xEV-DO standards. There can be an optional Access Network Authentication Authorization and Accounting (AN-AAA) server in a 1xEV-DO network. An AN-AAA has two functions. Firstly, it performs terminal authentication. There is no authentication of 1xEV-DO device at MSC level as a 1xEV-DO terminal does not register with a MSC. AN-AAA server authenticates 1xEV-DO terminal devices. This authentication is based on Network Access Identifier (NAI) or in other words it is a userid-password based authentication. Secondly, AN-AAA returns a IMSI value in its authentication accept message to the RNC. The RNC uses this IMSI for R-P session establishment with a PDSN. This method can be used to assign same IMSI for a hybrid terminal in 1xEV-DO network as the one it has in 1xRTT network. This is required for session handoffs between 1xEV-DO network and 1xRTT network.If AN-AAA is not deployed in a 1xEV-DO network, a RNC will have to use some other proprietary method to assign a IMSI to each mobile. It has to ensure that IMSIs are unique across the network. Without AN-AAA, session handoffs between 1xEV-DO and 1xRTT networks are not feasible. A mobile will have to rely on mobile IP to keep the same IP address as it moves across the boundary of the networks. If mobile IP is not deployed, a new IP address will have to be assigned every time a mobile crosses a network boundary.The drawback of deploying an AN-AAA server is that it increases operational cost. An operator has to manage and maintain an AN-AAA database in addition to IP network AAA database at PDSN level. A user is authenticated at the access network as well as IP network level. The decision to deploy AN-AAA is based on benefit of faster handoffs and improved system performance across the network boundaries at the cost of an additional database and AAA server management.4. 1xEV-DO Call FlowWhen a 1xEV-DO access terminal is turned on for the first time, it requests a Unicast Access Terminal Identifier (UATI) from the access network. A RNC assigns a unique UATI to each mobile in its serving area. Every RNC assign it owns UATI to mobiles in its serving. After UATI assignment, a 1xEV-DO session is established between the mobile and the RNC. The session parameters are negotiated during the session establishment process. If A12 interface is deployed, the access terminal authentication is performed. A 1xEV-DO session establishment between a mobile and a RNC is equivalent to a registration in cellular networks.The RNC sends a R-P session registration request to the PDSN for the mobile. Once R-P session is established, the next step is a PPP session establishment between the mobile and the PDSN. The mobile is authenticated at the PDSN, and is assigned an IP address during this process. Once PPP session is established, the mobile is ready to transmit and receive IP data packets. Figure 3 shows the message flow for 1xEV-DO session establishment.Figure 3 1xEV-DO Call Flow5. Handoffs in 1xEV-DO networksThere are three levels of hierarchic in a 1xEV-DO network for mobility management. The first level of mobility is between basestations of same RNCs. The second level of mobility is across the boundary of RNCs served by the same PDSN. The third level of mobility is across PDSNs as shown in Figure 4. Figure 4 Reference Mobility ModelThere are following different types of handoffs in a 1xEV-DO network and a 1xEV-DO 1xRTT hybrid network: Intra RNC Handoff Inter RNC and Intra PDSN Handoff Inter RNC and Inter PDSN Handoff 1xEV-DO to 1xRTT Handoff IxRTT to 1xEV-DO Handoff6. Intra RNC HandoffAs the signal strength of a target cell exceeds a handoff trigger threshold, an active mobile sends a route update message requesting a handoff. If the target cell is in the area of the serving RNC, the RNC will add the new sector in the active set of the mobile. In the reverse link the mobile will have in soft handoff. However, in the forward direction the mobile receives data from only one sector. The mobile monitors signal strength of the forward links of all sectors in its active set. However, it requests data in the forward direction from only the sector with the strongest signal strength.7. Inter RNC and Intra PDSN handoffAs the signal strength of a target cell exceeds a handoff trigger threshold, an active mobile may request a handoff. If the target cell is not in the area of the serving RNC, the request is ignored. There is no inter RNC soft handoff in a 1xEV-DO network. As the mobile continues to move towards the target cell, the RF connection is dropped. The mobile camps on the target cell, and it starts a new 1xEV-DO session on the target RNC. The target RNC can request the session information from the source RNC if A13 interface is deployed between the source and the target RNCs. If A13 is not deployed, the target RNC goes through 1xEV-DO session establishment process with the access terminal.The target RNC requests a new R-P session for the mobile from the PDSN. The R-P sessions are anchored on IMSI. When the PDSN receives the request for a new R-P session, it realizes that a R-P session exists for the mobile. It clears the old R-P session, and establishes a new R-P session as illustrated in Figure 5. Figure 5 Inter RNC, Intra PDSN HandoffThe PPP session between the PDSN and the mobile is not changed during this R-P session handoff. The mobile continues to use the same IP address.8. Inter RNC and Inter PDSN handoffIf the target RNC is not connected to the source PDSN, a new PPP session has to be established between the mobile and the new PDSN. If mobile IP is deployed and the PDSN acts a foreign agent, the mobile will get register a new care-of-address of its new foreign agent/PDSN. With mobile IP, the mobile can continue to use the same IP address as it moves across the PDSNs. The change in PDSNs is transparent to the end application. However, the process of mobile IP handoff is much longer than the process of R-P session handoff in an intra PDSN handoff. If mobile IP is not deployed, the mobile will get a new IP address during inter PDSN handoffs. The mobile will have to restart it active IP applications.9. 1xEV-DO to 1xRTT HandoffAs long as a 1xEV-DO carrier is available, a hybrid terminal will continue to camp on the 1xEV-DO carrier for the data service. 1xEV-DO to 1xRTT handoff takes place when a mobiles moves out of 1xEV-DO coverage. This handoff takes place in only dormant mode. There is no active or soft handoff between 1xEV-DO and 1xRTT carriers.Figure 6 1xEV-DO to 1xRTT HandoffWhen a hybrid terminal moves out of 1xEV-DO coverage area and 1xRTT coverage is available, it registers with the 1xRTT network for data service. The BSC sends a request to a PDSN to establish a new R-P session for the mobile. If the R-P session request is sent to the same PDSN where the mobile had its PPP session for 1xEV-DO, and its IMSI is the same as its IMSI in 1xEV-DO network, the PDSN will continue to use the same PPP session with the hybrid terminal as shown in Error! Reference source not found. Otherwise, a new PPP session is established between the PDSN and the mobile. The mobile will get a new IP address if mobile IP is not deployed. If mobile IP is deployed, it will perform mobile IP registration and continue to use the same IP address.Hybrid terminals monitor 1xRTT forward link control channel for voice pages both in dormant and active states. They are able to monitor two control channels in the dormant mode because the control channels are in slotted mode operation for both 1xEV-DO and 1xRTT networks. In 1xEV-DO active state hybrid terminals can monitor 1xRTT paging channel because the forward link in the active mode is time division multiplexed. If there is a page f