IP Flow Mobility and Seamless Offload(IFOM)

Unlike LIPA or SIPTO that are dependent on upstream network nodes to provide the optimization of routing different types of traffic, IFOM relies on the handset to achieve this functionality. It explicitly calls for the use of simultaneous connections to both macro network, e.g., LTE, UMTS and WiFi. Therefore, IFOM, unlike LIPA and SIPTO, is truly a release 10-onward only technology and it is not applicable for user terminals pre-Release 10. IFOM is being specified via 3GPP TS 23.261 [1]. Following diagram shows the interconnectivity model for IFOM capable UE.

IFOM uses an Internet Engineering Task Force (IETF) Request For Comments (RFC), Dual Stack Mobile IPv6 (DSMIPv6) (RFC-5555) [2].

Since IFOM is based on DSMIPv6, it is independent of the macro network flavor. It can be used for a green-field LTE deployment as well as a legacy GPRS packet core.

Earlier on we looked at the mobile network industry attempts of integration between packet core and WLAN networks. Common characteristic of those efforts was the limitation of the UE, its ability to use one radio interface at a time. Therefore, in earlier interworking scenarios UE was forced to use/select one radio network and make a selection to move to an alternative radio for all its traffic. Today many smartphones, data cards with connection managers already have this capability, i.e., when the UE detects the presence of an alternative access network such as a home WiFi AP, it terminates the radio bearers on the macro network and initiates a WiFi connection. Since WiFi access network and packet core integration is not commonly implemented, user typically loses her active data session and re-establishes another one.

Similarly access to some operator provided services may not be achieved over WiFi. Considering this limitation both iPhone IOS and Android enabled smartphones to have simultaneous radio access but limited this functionality to sending MMS over the macro network while being connected to WiFi only.

IFOM provides simultaneous attachment to two alternate access networks. This allows fine granularity of IP Flow mobility between access networks. Using IFOM, it will be possible to select particular flows per UE and bind them to one of two different tunnels between the UE and the DSMIPv6 Home Agent (HA) that can be implemented within a P-GW or GGSN. DSMIPv6 requires a dual-stack (IPv4 or IPv6) capable UE. It is independent of the access network that can be IPv4 or IPv6.

参考：3G and 4G wireless blog

Compare OFDMA, OFDM and CDMA

The advantages and disadvantages of OFDM will be detailed in the table below:

OFDM advantages

• OFDMA can easily adapt to severe channel conditions without the need for complex channel equalisation algorithms being employed
• It is robust when combatting narrow-band co-channel interference. As only some of the channels will be affected, not all data is lost and error coding can combat this.
• Intersymbol interference, ISI is less of a problem with OFDM because low data rates are carried by each carrier.
• Provides high levels of spectral efficiency.
• Relatively insensitive to timing errors
• Allows single frequency networks to be used - particularly important for broadcasters where this facility gives a significant improvement in spectral usage.

OFDM disadvantages

• OFDM is sensitive to Doppler shift - frequency errors offset the receiver and if not corrected the orthogonality between the carriers is degraded.
• Sensitive to frequency timing issues.
• Possesses a high peak to average power ratio - this requires the use of linear power amplifiers which are less efficient than non-linear ones and this results in higher battery consumption.
• The cyclic prefix used causes a lowering of the overall spectral efficiency.

OFDMA advantages

• With spectrum becoming more fragmented, especially for systems such as LTE and LTE advanced, fact that OFDMA provides flexibility of deployment across a variety of frequency bands with little need for modification is of paramount importance.
• When used in a cellular system, it is possible to minimise interference from neighbouring cells by using different carrier permutations between the two cells.
• Again when used with a cellular system, interference within the cell are averaged by using allocation with cyclic permutations.
• A single frequency network can be used to provide excellent coverage and good frequency re-use.
• Offers frequency diversity by spreading the carriers all over the used spectrum

OFDMA disadvantages
It has a relatively high sensitivity to frequency offsets as this degrades the orthogonality between the carriers

• It is sensitive to phase noise on the oscillators as this degrade the orthogonaility between the carriers
• Requires complex electronics to run the software - DSP including FFT algorithms needed for the forward error correction. This is always active regardless of data rate, although when no data is being transmitted the system can hibernate. However power consumption can be an issue.
• If only a few carriers are assigned to each user the resistance to selective fading will be degraded or lost.
• When used in a cellular system, co-channel interference from neighbouring cells is more complicated to combat than with CDMA as allocation of carriers needs to be coordinated between the two.
• The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control

OFDM CDMA comparision

OFDM advantages

• OFDM can combat multipath interference with greater robustness and less complexity. Equalisation can be undertaken on a carrier by carrier basis.
• OFDMA can achieve higher spectral efficiency with MIMO than CDMA using a RAKE receiver.
• Cell breathing does not occur as additional users connect to the base station.
• Can be used to provide a single frequency network.
• It is relatively easy to aggregate spectrum.
• It can be scaled according to the requirements relatively easily

CDMA advantages

• Not as complicated to implement as OFDM based systems
• As CDMA has a wide bandwidth, it is difficult to equalise the overall spectrum - significant levels of processing would be needed for this as it consists of a continuous signal and not discrete carriers.
• Not as easy to aggregate spectrum as for OFDM

参考：http://www.radio-electronics.com/info/rf-technology-design/ofdm/ofdma-cdma.php

Tutorial[11] - LTE security

LTE security is an issue that is of paramount importance. It is necessary to ensure that LTE security measures provide the level of security required without impacting the user as this could drive users away.

Nevertheless with the level of sophistication of security attacks growing, it is necessary to ensure that LTE security allows users to operate freely and without fear of attack from hackers. Additionally the network must also be organised in such a way that it is secure against a variety of attacks.

LTE security basics

When developing the LTE security elements there were several main requirements that were borne in mind:

• LTE security hand to provide at least the same level of security that was provided by 3G services.
• The LTE security measures should not affect user convenience.
• The LTE security measures taken should provide defence from attacks from the Internet.
• The secruity functions provided by LTE should not affect the transition from existing 3G services to LTE.
• The USIM currently used for 3G services should still be used.

To ensure these requirements for LTE security are met, it has been necessary to add further measures into all areas of the system from the UE through to the core network.

The main changes that have been required to implement the required level of LTE security are summarized below:

• A new hierarchical key system has been introduced in which keys can be changed for different purposes.
• The LTE security functions for the Non-Access Stratum, NAS, and Access Stratum, AS have been separated. The NAS functions are those functions for which the processing is accomplished between the core network and the mobile terminal or UE. The AS functions encompass the communications between the network edge, i.e., the Evolved Node B, eNB and the UE.
• The concept of forward security has been introduced for LTE security.
• LTE security functions have been introduced between the existing 3G network and the LTE network.

LTE USIM

One of the key elements within the security of GSM, UMTS and now LTE was the concept of the subscriber identity module, SIM. This card carried the identity of the subscriber in an encrypted fashion and this could allow the subscriber to keep their identity while transferring or upgrading phones.

With the transition from 2G - GSM to 3G - UMTS, the idea of the SIM was upgraded and a USIM - UMTS Subscriber Identity Module, was used. This gave more functionality, had a larger memory, etc.

For LTE, only the USIM may be used - the older SIM cards are not compatible and may not be used.