Characteristics of LTE Physical layer

The table below provides some of the specifications of the LTE Physical Layer.

Transmission BW		1.25 MHz	2.5MHz	5MHz	10 MHz	15 MHz	20 MHz
Slot Duration		0.5 ms
Sub Carrier Spacing		15 KHz
Sampling Freq		1.92 MHz	3.84 MHz	7.68 MHz	15.36 MHz	23.04 MHz	30.72 MHz
FFT Size		128	256	512	1024	1536	2048
CP Length usec	Short	5.21x1, 4.69/9 x 6	5.21x1, 4.69/9 x 6	5.21x1, 4.69/9 x 6	5.21x1, 4.69/9 x 6	5.21x1, 4.69/9 x 6	5.21x1, 4.69/9 x 6
	Long	16.67	16.67	16.67	16.67	16.67	16.67

Transmission BW: LTE supports varying bandwidth from 1.25 MHz to 20 MHz, allowing the operators to utilize BW chunks available. The operator can use chunks of 1.25 MHz at different frequencies rather than a 5 mHz(which is required in UMTS).

The Radio Frame of 10ms is divided into 10 sub frames of 1 ms each. Each sub frame has two slots of 0.5ms each.

Sub carrier spacing: It is difference in the center frequency of two adjacent sub carriers.

T-FFT: This is one FFT period and is equal to 66.667 us.

FFT size: This is the size of the FFT employed in the IFFT and FFT.

Sampling Frequency: With the FFT size of 128, this means 128 samples are taken in each FFT period. Therefore, sampling frequency = 128/66.667 us = 1.92 MHz. Note that this is half of 3.84 MHz, which is the chip rate in UMTs). Sampling frequency for other BW sizes can be calculated likewise.

Cyclic Prefix Length: Length of the prefix attached to each OFDM symbol before transmission is 4.69 us or 16.67 us depending on Short/extended prefix in use.

Ts: This is the unit of time in LTE and is equal to 1/(1500x2048) = 0.52086 us.  Every time duration is expressed in terms of Ts.

LTE X2 (inter eNB) Handover

The LTE standards come with well defined interface between eNB(X2), which makes handover between eNB possible.

LTE X2-based Handover

The steps of the handover as in the figure above are described below:

(1) The old eNB upon receiving the appropriate measurement reports from the UE will initiate the Handover procedure by sending a HANDOVER_REQ to the new eNB.This requires intelligent eNB which are aware of the other eNB in the vicinity.

(2) The new eNB sends a HANDOVER_ACK. This will contain a RRC_CONNECTION_RECONFIG message containing the parameters of the new cell for the UE.It will also have the GTP-U TEID for the data connection for DL data to be sent from the old eNB.

(3) The old eNB relays the RRC_CONNECTION_RECONFIG message to the UE.

(4) At this point, the old eNB starts forwarding the DL data destined for the UE to the new eNB using the GTP-U TEID from step 2.

(5) The UE does a Random Access Procedure in the new cell under new eNB to get the Timing Advance and UL grant.

(6) UE sends the RRC_CONNECTION_RECONFIG_COMPLETE message as a handover confirmation to the new eNB.

(7) The new eNB will now ask the MME to switch the GTP-U tunnel for DL data connection from the old eNB to the new eNB by sending the PATH_SWITCH_REQ message which has the new GTP-U TEID.

(8) The MME will in turn send the UPDATE_UP_REQ having the GTP-U TEID from the last step to S-GW.

(9) The UL and DL data starts in the new eNB. The new eNB however has to buffer the UL data till the data connection to the S-GW is switched completely.

(10) (11)The old eNB forwards the last packet to new eNB before the swicth.

(12) The S-GW will confirm the switch by sending UPDATE_UP_RESP to the MME.

(13)  MME in turn confirms the switch to the new eNB with PATH_SWITCH_ACK.

(14) Now that the new eNB has completely taken over the role of the eNB serving the UE, it sends a class 2 one-sided message (UE_RESOURCE_REL) to the old eNB.

(15) The data on new data link continues.

CSoHSPA : CS Voice over HSPA

The new features added in Rel7 for HSPA have made CS voice to be run over HSPA possible. It would have been possible to map CS voice over HSDPA and HSUPA in Rel6, but the resources at both the UE and the network would have been wasted, and it was not commercially viable.

The implementation till Rel 6 was to have a HSPA bearer for PS traffic and establish DCH bearer for CS voice. PS only traffic would be mapped to HSDPA in DL and HSUPA in UL and at the onset of CS call, everything would be moved to DCH. And when the CS call is disconnected, there would be radioBearerReconfiguration to move the PS to HSPA again.




The new features in UTRAN that have made this possible are:

Continuous Packet Connectivity: Allows the UE to be in CELL_DCH state for most of the time. UE can be configured with DRX cycles in this state and the battery resources conserved. HS-SCCH less operation on HS-DSCH channel saves DL codes. The HS-DSCH is switched to operation with HS-SCCH channel when the required data rate increases. With CPC, everything is mapped to HSPA, there is no DPCCH or DPDCH channel in operation in the DL.

enhanced Fractional-DPCH: This feature allows upto 7 UEs to be multiplexed on one DPCCH channel in the Downlink(for the calculation read enhanced F-DPCH). This F-DPCH channel carries TPC commands for the UE and is required along with HSDPA assignment in the downlink. Therefore, to have for example 35 active simultaneous voice calls, 5 such codes are required.

Non-Scheduled transmission on HSUPA: voice calls required guaranteed data rate(depending on codec, like 12.2 kbps). So, non scheduled grant will be used to transmit the voice packets in Uplink. Network allocates non-scheduled grant to the UE and makes provision in the UL resources (power transmitted by the UE) for the combined non scheduled grants for all the UEs in a cell. To read more about Non scheduled transmission go the MAC Scheduling paper.

Guaranteed QoS by the MAC-(e)hs Scheduler: MAC scheduler will have to be more robust in scheduling the voice packets on a priority basis and guarantee the codec rate in downlink. MAC Scheduling paper.

The case for HSPA+

GSM has been around from as long as the mobile phone has been in use.  It was well suited for voice only and there are still new GSM networks being put in places where the predominant traffic is voice traffic. EDGE was introduced as a GSM evolution to cater to data traffic. Later HSDPA and HSUPA revolutionised the data traffic by allowing high speeds in Uplink and Downlink.
But as the need for higher data rates is increasing, LTE is providing a viable option to the operators. Even higher data rates are being researched upon in LTE Advanced.
Among all this HSPA is again being revolutionised with the HSPA+ technology in 3GPP Rel 7 and beyond.
HSPA+ has promised 4G speeds and has new features that look great to both the network operators and the UE manufacturers.

With only a few LTE enable UEs available in selected markets, LTE has still a long way to go before there can be widespread deployment. Therefore HSPA will be around for quite some time and HSPA+ will match it with LTE for the data rates. This means HSPA+ and LTE will coexist for a long time until cheap LTE UEs are in the market and network vendors have their networks ready for LTE.

HSPA+ Features

	New Feature	Benefits
Rel 7	Higher Order Modulation in DL – 64 QAM	DL speed - 21 mbps
	Higher Order Modulation in UL – 16 QAM	UL speed – 11.5 mbps
	MIMO + 16 QAM	DL speed with 2x2 + 16 QAM - 42
	CPC	-       Huge battery saving in CELL_DCH -       reduced call setup time -       always ON UE
	eFACH	HSDPA data possible in CELL_FACH
	Enhanced F-DPCH	-       DL codes saved for signaling -       Increased cell capacity in the cell
	E-DPCCH power boosting	Allows network to configure UE with parameters to boost the E-DPCCH w.r.t. DPCCH when the data rate threshold is reached.
Rel 8	MIMO + 64 QAM	DL speed with 64 QAM – 28
	DL Dual Cell	DL speed with 64 QAM – 42
	eFACH	HSDPA + HSUPA data possible in CELL_FACH
	CSoHSPA	-       Made possible by enhanced F-DPCH, CPC -       CS traffic mapped to HSDPA/HSUPA -       cell capacity increased tremendously -       network has to manage only HSPA traffic
Rel 9	DL Dual Cell + MIMO	DL speed with (2x2 MIMO + 64 QAM) in each cell – 84 mbps
	UL Dual Cell	Dual cell with 16 QAM – 23 mbps

Comparison of LTE Security and UMTS Security

LTE has inherited most of the Security architecture from UMTS, some enhancements have also been made. While there are lot of places to find the overview of the LTE Security architecture, a side-by-side comparison with UMTS will be helpful for peopple coming from UMTS background.

Security Elements

Authentication and Key Agreement	UMTS based on UMTS-AKA key derivation from the UMTS Authentication Quintuplet(RAND, CK, IK, RES, AUTN) LTE based on EPS-AKA key derivation from the UMTS Authentication Quintuplet(RAND, CK, IK, RES, AUTN) Key Derivation Functions use HMAC-SHA-256 K(ASME) computed from CK, IK, and AUTN, which is used for Integrity and Encryption keys
Integrity	UMTS Integrity protection mandatory for only few RRC messages. Integrity Protection Algorithm UIA1: Kasumi LTE Integrity Protection mandatory for all messages after (and including) Security Mode Command Integrity Protection Algorithm 128-EIA 1: based on SNOW 3G 128-EIA 2: based on AES-128
Encryption	UMTS Encryption Algorithm UEA0: no encryption. UEA1: Kasumi. LTE Encrption done independently at two levels NAS - for EMM and ESM messgaes PDCP-SRB(1 and 2) and DRB (1 .. 11) two SECURITY MODE COMMANDS for two sets of keys {K(NAS-enc), K(NAS-integrity)}   {K(RRC-enc), K(RRC-integrity)} Encryption Algorithm 128-EEA 0: No Encrption 128-EEA 1: based on SNOW 3G 128-EEA 2: based on AES-128

Ec/No description

Abbreviation for received Energy per Chip / power density in the band

The Ec (RSCP)/ No (RSSI - total receive power) is the received energy per chip divided by the power density in the band. No includes the power of specified cell as it indicates total receive power (as a result, Ec/No is deteriorated by increasing 'No') Ec/No (dB) = RSCP(db) - RSSI (db) Since, RSSI > RSCP, therefore Ec/No is -ve.

LTE Security Architecture

This is a paper from NOKIA about LTE Security Architecture

Download Here

LTE Random Access Procedure

Highlights of Random Access Procedure in LTE:

• Contention based access and Non-contention based
• Function of the MAC Layer
• Provide Timing Advance for UL transmission to the UE
• Resource Assignment for transmission of AS message on UL-SCH

Contention Resolution:

To avoid two or more UEs trying to access the RACH channel, the LTE design provides upto 64 different RACH configurations which the UEs in a cell can use. The configuration ID to be used to transmit the RACH Preamble is determined by the higher layer. The algorithm which computes the configuration index to be used in upcoming RACH transmission should be random enough to minimise the chances of collision.

Timing Advance for UL transmission to the UE

The BCH provides the Frame and Slot synchronisation but does not provide the round trip delay. The UE must know how much early it should transmit in UL to compensate the trip delay, so that the transmission arrives at the slot boundary at eNB. This timing advance is provided by the eNB in the Random Access Response.
The UE must do a Random Access when in RRC Connected Mode, the time for synchronisation expires.

Resource Assignment for transmission of AS message on UL-SCH

The Random Access Response message provides the UL-grant for transmission of the AS message. The UE utilises the UL-grant to send a AS message.

Random Access Procedure - Contention based

Figure: Contention based RA Procedure

(1) UE selects a P-RACH configuration ID(out of 64) to be used in P-RACH transmission of the RA Preamble. The RA Preamble is computed and transmitted for the corresponding RA-RNTI.
If there is no response from the network, the UE re attempts with higher transmission power.
Note: the Random Access RNTI(RA-RNTI) is used by the UE to identify the DL-SCH transmission destined for it on the DL-SCH channel.

(2) eNB sends a Radom Access Response on the DL-SCH, using the RA-RNTI sent by the UE earlier. It sends the Timing Advance Info, UL grant for the UE on UL-SCH and a temporary C-RNTI. temp C-RNTI is used by the UE for UL-SCH scheduled tx.

(3) The UE uses the UL grant on UL-SCH to transmit the Access Stratum Message(e.g. RRC Connection Setup Request) along with the temp C-RNTI to identify itself.
The UE can indicate if it requires more resources in the UL using the MAC-Buffer Status Report(BSR).

(4) The eNB sends the Contention Resolution message on the DL-SCH using the temp C-RNTI. This message contains the C-RNTI for the UE. After the UE receives the contention Reslution, it replaces temp-CRNTI with this CRNTI and uses it for all future UL transmissions.

The contention based RA Procedure is used for:

• transition from RRC_IDLE to RRC Connected(RRC Connection Establishment)
• in RRC_CONNECTED when UL data has to be sent and UL Synchronisation state is “non-synchronised”

Note: the UL synchronisation state is changed to non-synchronised after the expiry of the TimeAlignmentTimer. At this point a UL Time Alignment must be done using RA Procedure.


Random Access Procedure - Non Contention based
The non-contention based Random Access procedure is initiated by the network.





Figure: Non-Contention based RA Procedure


(1) The network sends the RA Preamble Assignment Info to the UE on DL-SCH(using the C-RNTI)
(2) UE sends the assgined RA Preamble on the assigned P-RACH configuration ID
(3) Network sends the Random Access Response with Timing Advance Info.


The non-contention based RA Procedure is used by the network for:

• When timing advance is required for UE positioning.
• in RRC_CONNECTED when DL data has to be sent and RA Procedure is required.
• Inter system or intra LTE Handover