LTE design requirements

There have been many discussions and articles written about LTE in the past one year or so. As LTE is pushing forward so is the design and implementation phase for different companies involved in LTE development. Following are the requirements for LTE which engineers should take into account while designing the LTE system.

Capabilities:
The targets for downlink and uplink peak data-rate requirements are 100 Mbit/s and 50 Mbit/s, respectively, when operating in 20 MHz spectrum allocation. For narrower spectrum allocations, the peak data rates are scaled accordingly. Thus, the requirements can be expressed as 5 bit/s/Hz for the downlink and 2.5 bit/s/Hz for the uplink. Obviously, for the case of TDD, uplink and downlink transmission cannot, by definition, occur simultaneously. Thus the peak data rate requirement cannot be met simultaneously. For FDD, on the other hand, the LTE specifications should allow for simultaneous reception and transmission at the peak data rates specified above. LTE should support at least 200 mobile terminals in the active state when operating in 5 MHz. In wider allocations than 5 MHz, at least 400 terminals should be supported.

System performance:
The LTE system performance design targets address user throughput, spectrum efficiency, mobility, coverage, and further enhanced MBMS.

The LTE user throughput requirement is specified at two points: at the average and at the fifth percentile of the user distribution (where 95 percent of the users have better performance). A spectrum efficiency target has also been specified, where in this context, spectrum efficiency is defined as the system throughput per cell in bit/s/ MHz /cell.

In terms of mobility LTE should be able to provide good rates even when the user is moving at 500km/h

Deployment-related aspects:
The deployment-related requirements include deployment scenarios, spectrum flexibility, spectrum deployment, and coexistence and interworking with other 3GPP radio access technologies such as GSM and WCDMA /HSPA.

The requirement on the deployment scenario includes both the case when the LTE system is deployed as a stand-alone system and the case when it is deployed together with WCDMA/HSPA and/or GSM. Thus, this requirement is not in practice limiting the design criteria.

The coexistence and interworking with other 3GPP systems and their respective requirements set the requirement on mobility between LTE and GSM, and between LTE and WCDMA/HSPA for mobile terminals supporting those technologies. Table below lists the interruption requirements, that is, longest acceptable interruption in the radio link when moving between the different radio-access.

Spectrum flexibility and deployment:
The basis for the requirements on spectrum flexibility is the requirement for LTE to be deployed in existing IMT-2000 frequency bands, which implies coexistence with the systems that are already deployed in those bands, including WCDMA/HSPA and GSM. A related part of the LTE requirements in terms of spectrum flexibility is the possibility to deploy LTE -based radio access in both paired and unpaired spectrum allocations that is LTE should support both Frequency Division Duplex (FDD), and Time Division Duplex (TDD).

Architecture and migration:
A few guiding principles for the LTE RAN architecture design as stated by 3GPP are:
A single LTE RAN architecture should be agreed.

The LTE RAN architecture should be packet based, although real-time and conversational class traffic should be supported.

The LTE RAN architecture should minimize the presence of ‘single points of failure’ without additional cost for backhaul.

The LTE RAN architecture should simplify and minimize the introduced number of interfaces.
Radio Network Layer (RNL) and Transport Network Layer (TNL) interaction should not be precluded if in the interest of improved system performance.

The LTE RAN architecture should support an end-to-end QoS. The TNL should provide the appropriate QoS requested by the RNL.

QoS mechanism(s) should take into account the various types of traffic that exists to provide efficient bandwidth utilization: Control-Plane traffic, User-Plane traffic, O & M traffic, etc.
The LTE RAN should be designed in such a way to minimize the delay variation (jitter) for traffic needing low jitter, for example, TCP/IP.

Radio resource management:
The radio resource management requirements are divided into enhanced support for end-to-end QoS, efficient support for transmission of higher layers, and support of load sharing and policy management across different radio access technologies.

The enhanced support for end-to-end QoS requires an ‘improved matching of service, application and protocol requirements (including higher layer signalling) to RAN resources and radio characteristics. ’ The efficient support for transmission of higher layers requires that the LTE RAN should ‘provide mechanisms to support efficient transmission and operation of higher layer protocols over the radio interface, such as IP header compression.’ The support of load sharing and policy management across different radio access technologies requires consideration of reselection mechanisms to direct mobile terminals toward appropriate radio access technologies in all types of states as well as that support for end-to-end QoS during handover between radio access technologies.

Complexity:
The LTE complexity requirements address the complexity of the overall system as well as the complexity of the mobile terminal. Essentially, these requirements imply that the number of options should be minimized with no redundant mandatory features. This also leads to a minimized number of necessary test cases.

General aspects:
The section covering general requirements on LTE address the cost-and service related aspects. Obviously, it is desirable to minimize the cost while maintaining the desired performance for all envisioned services. Specific to the cost, the backhaul and operation and maintenance is addressed.