An mobility management entity (MME) handles security procedures, sessions

An LTE network comprises two main entities known as the evolved universal terrestrial radio access network (EUTRAN) and the evolved packet core (EPC). The E-UTRAN includes the eNB which interact among themselves via an x2 interface and with the UEs over the air over the Uu interface. In addition, E-UTRAN communicates with the EPC via the S1 (i.e. S1MME/S1U) interface and is also in charge of modulation/demodulation, Channel coding/decoding, radio resource control, radio mobility management, and detection and correction of errors in the transmitted data. On the other hand, the EPC comprise the following entities: (a) the mobility management entity (MME) handles security procedures, sessions between UE and network, and location management. It connects to the eNB via the S1MME interface; (b) the home subscriber server (HSS) connects to the MME via the S6a interface. It hosts the home location register and the authentication center; (c) the serving gateway (SGW) serves as local mobility manager anchor within the E-UTRAN or mobility with other 3GPP technologies. It connects to the eNB, MME, and the serving GPRS support node (SGSN) through the S1U, S11 and S4 interfaces, respectively; (d) the packet data network gateway (PDN) routes the traf?c to the internet via the SGi interface. It also connects to the SGW , the policy charging and rules function server (PCRF) and the evolved packet data gateway (ePDG) via interfaces S5/S8, S7 (Gx) and S2b, respectively. (e) the PCRF handles the service policy, charging information and QoS parameter information for each user session. (f)?nally, the ePDG interconnects the EPC to untrusted non-3GPP networks that need a secured access 26.B. LTE channelsThe data to be transmitted over an LTE network is classi?ed into user plane and control plane data. The former is actual data intended for the user while the latter is the necessary data for a successful delivery of the user plane data. Ef?cient classi?cation of these data is required to identify its type and purpose. There are three types of channels in LTE known the logical, transport and physical channels. These channels are respectively implemented by the radio link control (RLC), the medium access control (MAC) and the physical layer (PHY). The RLC performs the logical categorization of the control plane data into ?ve logical channels as follows: (a) the paging control channel (PCCH) which carries the paging message; (b) the broadcast control channel (BCCH) which contains the system critical information such as bandwidth, reference signal power, antenna con?guration…; (c) the common control channel (CCCH) which is used by the UE to acquire control information when no radio connection is established; (d) the downlink control channel (DCCH) is used to enable the UE and the eNB to exchange control information on a dedicated bearer resource; (e) the multicast control channel (MCCH) carries control information of a group of UEs in a cell for multicast/broadcast services. The RLC also classi?es the user7plane data in two logical channel as: (a) the dedicated traf?c channel (DTCH) which carries the user plane data in both uplink and downlink directions, and (b) the multicast traf?c channel (MTCH) which is a unidirectional channel used to transmit the user data from the eNB to multiple UEs. The RLC passes the categorized data to the MAC layer where the transport channels are assigned. In the downlink, there are four transport channels. The PCCH and BCCH are mapped to the paging channel (PCH) and the broadcast channel (BCH) while the CCCH, DCCH, MCCH and DTCH are multiplexed into the downlink shared channel (DL-SCH). Since the BCCH carries various types of signaling data, the system information is split into: (a) the master information block (MIB) which carrier system critical information to acquire a cell is carried by BCH, and (b) the system information block (SIB) which contains dynamic system information to insure a reliable radio connection in the uplink is mapped to DL-SCH. As a result, decoding the SIB is important for the user. Yet, since its location on the physical resource grid is unknown, the user is required to decode MIB ?rst to get the system critical information and the scheduling information for SIB. In the uplink, the transport channels used are similar to those in the downlink. The logical channels such as CCCH, DCCH and DTCH as mapped to the uplink shared channel (ULSCH). Since the user does not send paging, broadcast/multicast messages in the uplink, these channels do not exist in the uplink. However, there is one transport channel known as random access channel (RACH) which is used at the initial stage when the UE is not synchronized with the eNB in the uplink. It is the only channel which does not have a corresponding logical channel. In addition to multiplexing and demultiplexing the logical channels, the MAC scheduler also handles: (a) hybrid automatic repeat request (HARQ) in case the received data fails the cyclic redundancy check (CRC) or cannot be decoded, (b) random access process, and (c) QoS class identi?er (QCI) function through logical channel prioritization. This where the MAC layer decides the amount of data from each logical channel to be included in the MAC packet data unit (PDU)/transport block (TB). The TB size is based on the uplink resource grant request message. Since the TB size is ?xed (e.g. 10KB) ?lling it in the order of the priority of the logical channels means that higher priority data would possibly ?ll all the TB leaving the lower priority data out. This is known as channel starvation. To avoid this, one priority bitrate (PBR) is assigned to each logical channel. The TB is then ?lled based on the PBR. (d) decision regarding the appropriate modulation and coding scheme (MCS) is also made. The MAC scheduler receives the channel state information (CSI) in form of channel quality indicator (CQI) based on which it decides the MCS to be used for the communication. The data in the DL-SCH contains the user plane traf?c which needs to be delivered irrespective of the channel quality. Hence, higher MCS such as 64QAM (quadrature amplitude modulation) is used when the channel is good. Otherwise lower MCS such as quadrature phase shift keying (QPSK) is used. Since the latter is the most robustmodulation scheme used in LTE, it is used for signaling traf?c. The MAC scheduler also handles the three following functions implemented in the physical layer such as deciding the number of terminals to transmit, determining the set of resource blocks for the UEs in the downlink, and the selecting the TB size. The transport channels such as PCH and DL-SCH are mapped to the physical downlink shared channel(PDSCH). On the other hand, BCH and MCH are mapped to the physical uplink shared channel (PU-SCH) while the physical RACH is used for RACH. When a UE receives physical data from the eNB it needs to locate the DL-SCH and determine the MCS to used. Hence, level one (L1) and level two (L2) control signals are used. The physical downlink control channel carries the downlink control information (DCI) about the user data, the SIB and the uplink power control information and HARQ control information. The physical control format indicator channel (PCFICH) carries the control format indicator (CFI) which tells the number of OFDM symbols used for PDCCH. Next, the physical hybrid ARQ indicator channel (PHICH) carried the hybrid ARQ indicator (HI) which comprises the HARQ and the ACK/NACH messages to check for successful data delivery. In the uplink, the physical uplink control channel (PUCCH) carried the uplink control information (UCI) for the user traf?c and is mapped to UL-SCH. On the other hand, the physical uplink channel (PUCCH) carries the CQI, ACK/NACH and the scheduling request messages to the eNB. At the physical layer, each TB is subject to thorough digital signal processing prior to being sent over the air. Large TBs (e.g. > 10KB) are fragmented into smaller block with a CRC attached to each. Next, channel coding is performed using a Turbo coding technique which convert each bit of data into 3bits. The resulting data stream undergoes rate matching and scrambling before being mapped to the correct modulation as chosen by the MAC scheduler. After precoding, the bits are ?nally mapped to the REs and endergo the orthogonal frequency division multiplexing OFDM process before being transmitted over the Uu interface.C. LTE data transmissionThe data transmission in LTE is driven by OFDM techniques. It is performed over parallel subcarriers of 15KHz, each subdivided into time slots as blocks of one symbol of 66.7┬Ás also known as resource elements (RE). Transmission of data over these REs requires modulating the data using an appropriate modulation scheme. Such scheme depends on the physical channels mapped on the LTE resource grid. Suppose data, such as video generated at a given bitrate, is to be transmitted using M ? aryQAM. The data is split into M parallel streams of log2M bits. The assignment is performed based on the M ?aryQAM constellation where the amplitude and phase of the subcarrier is adjusted with respect to those of the data stream before the data can be placed on the REs. Therefore the modulated data is product of the complex modulation symbol by the corresponding subcarrier frequency. For instance, in a 20MHz bandwidth, there are 100 resource blocks (RB) where each contains 12 subcarriers.8The Inverse Fast Fourier Transform (IFFT) is used at the transmitter to convert the parallel frequency domaine signals into samples of composite time domain signals. The IFFT samples must be taken above the Nyquist rate. In the 20MHz bandwidth, 2MHz is reserved from guard band. So, the highest frequency components range from 9MHz to -9MHz. Thus, the sampling rate must be at least 18Mega Sample per second. Since the transmitted data may experience degradation due to intersymbol interference (ISI) and delay spread as a result of multipath propagation, even though the effect can be reduced by guard periods, the sudden ?uctuation in the time domain during the transition from symbols to guard periods can also disrupt the orthogonality between subcarriers. Therefore, a cyclic pre?x (CP) is added to each transmission cycle by taking IFFT samples from the end of a period and placing them at the beginning. With that said, the samples signal is converted into a wave form by using a digital to analog (A/D) converter and modulated at the desired radio frequency. OFDM is used in LTE in the downlink, where a each symbol is mapped to one subcarrier. But doing this also results into a high peak to average power ratio (PAPR) as a result of the IFFT summation of multiple parallel subcarriers. Since the UE has a limited power capability, single carrier frequency division multiple access (SC-FDMA) is used in LTE in the uplink, where the symbol duration per subcarrier is shorter than in OFDMA given that each symbol is split across all the subcarriers in each RB.