3GPP LTE Channels and MAC Layer

Contents:-

• LTE MAC Layer Functions
• MAC in the LTE Protocol Stack
• LTE Channel Architecture

• Downlink PDCP, RLC and MAC Sublayer Organization
• Uplink PDCP, RLC and MAC Sublayer Organization
• LTE Downlink Channels

• LTE Uplink Channels
• 
• Random Access Procedure
• UL/DL-SCH data transfer

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A document on SC-FDMA from Agilent

Contents:-

• LTE objectives and timing
• OFDM:The choice of nect-generation wireless
• Assessing the advantages and disadvantages of OFDM
• Introducing SC-FDMA
• Comparing SC-FDMA and OFDM
• Multipath resistance with short data symbols?
• Examining real SC-FDMA signal 

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MIMO in LTE Operation and Measurement

This application note is written for people who need an understanding of Multiple Input Multiple Output (MIMO) radio operation as it applies to Long Term Evolution (LTE). MIMO (otherwise known as spatial multiplexing) is one of several multiple antenna techniques that are being implemented in LTE; however, this application note will primarily focus on MIMO implementations.


Contents:-

• Inputs and Outputs
• Single User and Multi-User MIMO
• Single and Multiple User MIMO in the Uplink
• LTE Transmitter and Receiver Design and Test
• Receiver Design and Test
• Transmitter Design and Test
• A Systematic Approach to Verifying Transmitter Quality

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Long Term Evolution (LTE) Video Presentations

Here you can find some of excellent video trainings off LTE which covers following topics.


Contents:- 



1. LTE Introduction

• Motivations for LTE
• LTE market and background
• Requirements
• Evolution path to LTE

2. LTE Parameters and Downlink Modulation

• LTE parameters and frequency bands
• What is OFDMA?
• OFDMA multiple access and downlink frame structure
• OFDMA transmit and receive chains

3. OFDMA and Downlink Frame Structure Details

• Downlink OFDMA time-frequency multiplexing
• LTE Spectrum Flexibility and Bandwidth
• FDD downlink frame structure detailed
• TDD frame structure

4. SC-FDMA and LTE Uplink

• Introduction to SC-FDMA and uplink frame structure
• Uplink SC-FDMA tranmsit and receive chains
• Peak to Average Power Ratio (PAPR) comparison with SC-FDMA and OFDMA

5. Network and Protocol Architecture

• LTE/SAE network architecture
• EPC -Evolved Packet Core
• Base Station control plane and user plane protocol stacks
• EPC protocol stacks

6. Channel Mapping and UE Categories

• Logical and transport channel mapping in downlink and uplink
• LTE UE Categories

7. Initial Cell Search and Cell Selection

• Downlink physical channels and signals
• Cell Search and Selection in LTE
• Primary synchronization signal
• Secondary synchronization signal
• Reference signals

8. System Information

• Downlink reference signal details
• Master Information Block on PBCH
• System Information on DL-SCH

9. Random Access Procedure and EPS Bearer Setup

• Random access preamble transmission to eNodeB
• Random access response from eNodeB
• Resource allocation and contention resolution
• Signaling on PDCCH
• Hybrid ARQ
• RRC Connection Setup and EPS Bearer Setup

10. Uplink Channels and Signals

• Uplink physical channels and signals
• PU-SCH: Physical Uplink Shared Channel
• Uplink assignment signaling on PDCCH
• Uplink frequency hopping
• PUCCH

11. LTE mobility and MIMO Introduction

• Intra MME Handover over the X2 interface
• RRC States
• MIMO Basics
• Transmit diversity
• Spatial multiplexing
• Beamforming

12. Downlink and Uplink MIMO in LTE

• Downlink MIMO modes
• Transmit diversity
• Spatial multiplexing
• Cyclic delay diversity
• Beam forming
• Spatial multiplexing downlink transmitter chain
• Code book based precoding
• Uplink MIMO
• Uplink transmit antenna selection
• Multi-user MIMO

13. eNodeB and UE RF Performance Requirements

• eNodeB modulation quality measurements
• eNodeB performance requirements
• UE performance requirements

14. UE Certification and Field Trials

• LTE terminal testing stages
• LTE terminal certification
• LTE field trial scenarios

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How Random Access procedure works in LTE?

The RACH procedure in LTE is used for four cases: 

• Initial access from disconnected state (RRC_IDLE) or radio failure .
• Handover requiring random access procedure.
• DL or UL data arrival during RRC_CONNECTED after UL PHY has lost synchronization (possibly due to power save operation).
• UL data arrival when are no dedicated scheduling request (PUCCH) channels available.

Timing is critical because the UE can move different distances from the base station, and LTE requires microsecond level precision; the speed-of-light propagation delay alone can cause enough change to cause a collision or a timing problem if it is not maintained. 

There are two forms of the RACH procedure: Contention-based, which can apply to all four events above, and noncontention based, which applies to only handover and DL data arrival. The difference is whether or not there is a possibility for failure using an overlapping RACH preamble.

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what is use of cyclic prefix in LTE OFDM signals?

To limit the influence of ICI on OFDM systems completely by hardware we would have to have transmitters and receivers with under 0.1ppm frequency stability. This would drastically increase the cost and complexity of hardware.Thus quite a big part of the OFDM software in the receiver deals with frequency correction using the cyclic prefix, but also reference or pilot signals sent with the signal.



The cyclic prefix (CP) consists of the last part of the following symbol. The size of the cyclic prefix field depends on the system and can even vary within one system. Cyclic prefixes are used by all modern OFDM systems and their sizes range from 1/4 to 1/32 of a symbol period. Most receiver structure use the cyclic prefix to make an initial estimation of time and frequency synchronization.


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What is special sub frame in LTE?

As the single frequency block is shared in time domain between UL and DL the transmission in TDD is not continuous. All UL transmission need to be on hold while any downlink resource it is used and the other way around. 


Switching between transmission directions has a small hardware delay (for both UE and NodeB) and needs to be compensated. To control the switching between the UL and DL a guard period GP is allocated which compensates for the maximum propagation delay of interfering components.


Description of the switching mechanism:
In LTE TDD there is maximally one UL->DL and one DL->UL transition per 5ms period (half frame)
UL-> DL transition
Is done for all intra-cell UEs by the process of time alignment. The NodeB instructs each UE to use a specific time offset so that all UEs signals are aligned when they arrive at Node-B. So UL is synchronous very similar to FDD (intra cell case only!)



As shown in above figure there are 7 frame configurations, according to different DL/UL partition.
1. Downlink / Uplink ratio can vary from 1/3 (Frame configuration = 0) to 8/1 (Frame configuration = 1), depending on the service requirements of the carrier
2. Frame always starts with a downlink subframe, used for advertising the frame descriptor, PCFICH and PDCCH. UE hence learns the frame structure in this subframe.
3. 3rd frame is always used for uplink
4. When switching from downlink to uplink, there is need for a special switching subframe. No special subframe is used when switching from uplink to downlink.


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What are the benefits and drawbacks of an OFDMA?

OFDM/OFDMA has several benefits over other transmission schemes:



1. High spectral efficiency: due to the orthogonality between subcarriers it is possible to pack them closely together (15kHz subcarrier spacing)
2. Little interference between subcarriers due to the IFFT/FFT processing (Interference can be introduced by frequency offsets generated by either Doppler or Local Oscillator frequency innacuracies).
3. Robustness in multi-path environments thanks to the cyclic prefix as mentioned before.
4. Straightforward support of the operation in different spectrum allocations with different bandwidths just by varying the number of OFDM subcarriers used for transmission.
5.Simpler receiver design to support high data rate communications. Detecting a rectangular pulse with cyclic prefix requires less hardware. Free capacity can be used then to implement other performance optimization techniques.
6. Easy MIMO techniques implementation


Drawbacks of OFDMA:-


1. The main disadvantage of OFDM/OFDMA is that the signal has a relatively large peak-to-average power ratio (PAPR). This is due to the nature of OFMA where modulated symbols are transmitted in parallel, each one containing a part of the transmission. The power at a certain point in time is the sum of the powers of all the transmitted symbols for a certain connection, which explains that the differences between peak and average powers can be high.

2. This issue reduces the power efficiency of the RF amplifier. Expensive transmission amplifiers are needed, especially on the mobile side, in order to work on a wide range of powers; otherwise the non-linear amplification reduces the orthogonality of the OFDM signal. This is a reason why OFDMA is not optimal for use with mobile or battery-power devices.

3. Other issue of OFDM/OFDMA systems is that tight spacing of subcarriers may lead to loss of orthogonality due to frequency errors. Doppler may cause inter carrier interference (ICI) and the consequent lost of orthogonality. To cope with the problems caused by close subcarrier spacing, LTE has adopted 15 kHz spacing (mobile WiMAX uses 10KHz spacing).

Channel Structure in LTE

The physical layer provides transport channels to the L2. These transport channels differ in their characteristics how data is transmitted and are mapped to different logical channels provided by the MAC layer. Logical channels describe which type of data is conveyed.

LOGICAL CHANNELS:-


The logical channels can be divided into control channels and traffic channels.


Control Channels:-



• Broadcast Control Channel (BCCH): A downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH): A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE.
• Common Control Channel (CCCH): This channel is used by the UEs having no RRC connection with the network. CCCH would be used by the UEs when accessing a new cell or after cell reselection.
• Multicast Control Channel (MCCH): A point-to-multipoint downlink channel used for transmitting MBMS scheduling and control information from the network to the UE, for one or several MTCHs. After establishing an RRC connection this channel is only used by UEs that receive MBMS.
• Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection.


Traffic Channels:-



• Dedicated Traffic Channel (DTCH): A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
• Multicast Traffic Channel (MTCH): A point-to-multipoint downlink channel for transmitting traffic data from the network to the UEs using MBMS.

Some questions and answers on LTE Radio Interface - Part2

1. Which protocol is responsible for Scheduling of user data and HARQ?


MAC


A Medium Access Control (MAC) Hybrid Automatic Repeat reQuest (HARQ) layer with fast feedback provides a means for quickly correcting most errors from the radio channel. To achieve low delay and efficient use of radio resources, the HARQ operates with a native error rate which is sufficient only for services with moderate error rate requirements such as for instance VoIP. Lower error rates are achieved by letting an
outer Automatic Repeat reQuest (ARQ) layer in the eNB handle the HARQ errors.





2. Which protocol is responsible for ciphering of user data?


PDCP


The[B] PDCP protocol maps the EPS bearer onto the E-UTRA Radio Bearer and performs [B]Robust Header Compression (ROHC).NAS messages are protected using the ciphering and integrity protection services provided by the PDCP layer.


The Packet Data Convergence Protocol supports the following functions:


• Header compression and decompression of IP data flows using the ROHC (Robust Header Compression) protocol, at the transmitting and receiving entity, respectively.
• transfer of data (user plane or control plane). This function is used for conveyance of data between users of PDCP services.
• maintenance of PDCP sequence numbers for radio bearers for radio bearers mapped on RLC acknowledged mode.
• in-sequence delivery of upper layer PDUs at Handover 
• duplicate elimination of lower layer SDUs at Handover for radio bearers mapped on RLC acknowledged mode
•[B] ciphering and deciphering of user plane data and control plane data[/B]
• integrity protection of control plane data
• timer based discard


3. How does the frequency domain structure differ in UL compared to DL?


In UL the frequency allocation must be continuous in order to preserve the single carrier properties. This is not the case in DL, where non-contiguous resource blocks  be be allocated to the same user.


4. How much can the data rate be increased with 2x2 MIMO compared to a solution without MIMO?


Up to two times


With MIMO,  multiple antennas and advanced signal processing such as spatial multiplexing, the radio channel can be separated into several
layers, or “data pipes”. Up to four layers can be utilized. This corresponds to up to four times higher data rates for a given bandwidth.


5. Explain the concepts of channel rank, layers, data rate multiplication and codebook.


The radio channel properties decide the maximum channel rank that can be used, i.e. how many layers the channel support at the moment. The number of layers that can be transmitted over the radio channel is equal to the data rate multiplication (e.g. two layers give two times the data rate compared to a solution without MIMO). The complex weights that are applied at each antenna port are selected from a finite
codebook. The codebook index is suggested and indicated by the UE.


6. How HARQ works?


Multiple simple stop-and-wait ARQ processes are processed by the HARQ entity in the MAC protocol. The operation is very fast and has a short round-trip-time thanks to the short TTI and the fact that it is located in the eNodeB, close to the radio interface. Feedback from the receiver is sent in terms of short ACK/NACK messages.


7. How to calculate the maximum theoretical physical peak data rate in LTE radio interface?


Each OFDM symbol contains, if 64-QAM is used, 6 bits per subcarrier (15kHz).


There are, if normal CP is used, 7 OFDM symbols
per slot. This ends up with 6*7 = 42 bits per slot. One slot is 0.5 ms which gives us 42/0.5ms = 84kbps per sub-carrier.


If the full bandwidth, 20MHz, is used, there are 20MHz/15kHz=1333 sub-carriers.


However, only 1200 of these are used for user data. This corresponds to 100 resource blocks.
1200*84kbps = 100,8 Mbps.


With four MIMO layers, we should be able to achieve 403.2 Mbps of raw data rate in the physical layer.


What about the user data rate? The data rates used for L1/L2 signaling, reference signals, PBCH, SCH, layer 3 signaling and
protocol headers has to be subtracted from this figure. Then we end up with approximately 320 Mbps of user data rate on RLC
level??


In UL we have approximately the same calculation, except that the gain from MIMO cannot be included, since no SU-MIMO is used in
UL. Hence, approximately 80-100 Mbps of theoretical bitrate should be possible to reach.

Some questions and answers on LTE EPC architecture

1. Which node handles IP point of presence?


The P-GW


2. Which node handles IP address allocation?


The P-GW


3. Which node handles charging per UE and intra LTE mobility anchor?


The S-GW


4. Which node handles subscription data?


The HSS (Home Subscriber Server)


5. Which node handles Authentication, NAS signaling and paging?


The MME



6. What is the Converged Packet Gateway?


A combined P-GW and S-GW (also called SAE GW) based on the SmartEdge 1200 platform.


7. Which node (function) is handled by SAPC?


PCRF(Policy and Charging Rules Function)


8. Why is MME and S-GW pooling used?


There are two main reasons: redundancy and load balancing.


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What are the functionality of eNodeB?

E-UTRAN consists solely of the evolved Node B (eNB), which is responsible for all radio interface functionality.


eNB is the RAN node in the EPS architecture that is responsible for radio transmission to and reception from UEs in one or more cells.
The eNB is connected to EPC nodes by means of an S1 interface.The eNB is also connected to its neighbor eNBs by means of the
X2 interface. Some significant changes have been made to the eNB functional allocation compared to UTRAN. Most Rel-6 RNC
functionality has been moved to the E-UTRAN eNB. Below follows a description of the functionality provided by eNB.

1. Cell control and MME pool support

eNB owns and controls the radio resources of its own cells.

Cell resources are requested by and granted to MMEs in an ordered fashion. This arrangement supports the MME pooling concept. S-GW pooling is managed by the MMEs and is not really seen in the eNB.

2. Mobility control

The eNB is responsible for controlling the mobility for terminals in active state. This is done by ordering the UE to perform measurement and then performing handover when necessary.

3. Control and User Plane security

The ciphering of user plane data over the radio interface is terminated in the eNB. Also the ciphering and integrity protection of RRC signaling is terminated in the eNB.

4. Shared Channel handling

Since the eNB owns the cell resources, the eNB also handles the shared and random access channels used for signaling and initial access.

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Some questions and answers on LTE Radio Interface - Part1

1. How is the UE getting information that it is scheduled?


By reading the PDCCH (this is valid for both UL scheduling grants and DL scheduling assignments).


PDCCH contains DCI(DL control information), which indicate 3 different messages:-


1. Uplink scheduling grants for PUSCH
2. Downlink scheduling assignment for PDSCH
3. TPC command for PUSCH and PUCCH


2. In which node is PDCP located and what are the tasks of that protocol?


PDCP is located in the eNodeB and handles encryption of user data streams and reordering at handover.


Each radio bearer also uses one PDCP instance. PDCP is responsible for header compression(ROHC Robust Header Compression) and ciphering/deciphering. Obviously header compression makes sense for IP diagram's, but not for signalling. Thus the PDCP entities for signalling radio bearers will usually do ciphering/deciphering only.


3. What is a resource block?


A Resource Block (RB) is a time- and frequency resource that occupies 12 subcarriers (12x15 kHz = 180 kHz) and one slot
(= 0.5 ms). RBs are allocated in pairs by the scheduler (then referred to as Scheduling Blocks).



4. What are two radio interface solutions that increase the spectrum efficiency ?


Higher order modulation:-LTE support all types of modulation schemes like QPSK,16 QAM, 64 QAM that results in high data rate
MIMO:- MIMO increase data rate by doubles in 2*2 and 4 folds in 4*4 case.


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Radio Interface Structure of LTE

The radio interface is structured in a layered model, similar to WCDMA, with a layer 2 bearer (here called EPS Bearer Service), which corresponds to a PDP-context in Rel. 6, carrying layer 3 data and the end-to-end service. The EPS bearer is carried by the EUTRA Radio Bearer Service in the radio interface. The E-UTRA radio bearer is carried by the radio channels. The radio channel structure is divided into logical, transport and physical channels. The logical channels are carried by transport channels, which in
turn are carried by the physical channels as illustrated in Figure.

What are the functionality of eNodeB?

E-UTRAN consists solely of the evolved Node B (eNB), which is responsible for all radio interface functionality.


eNB is the RAN node in the EPS architecture that is responsible for radio transmission to and reception from UEs in one or more cells. The eNB is connected to EPC nodes by means of an S1 interface.The eNB is also connected to its neighbor eNBs by means of the X2 interface. Some significant changes have been made to the eNB functional allocation compared to UTRAN. Most Rel-6 RNC functionality has been moved to the E-UTRAN eNB. Below follows a description of the functionality provided by eNB.


1. Cell control and MME pool support


eNB owns and controls the radio resources of its own cells. Cell resources are requested by and granted to MMEs in an ordered fashion. This arrangement supports the MME pooling concept. S-GW pooling is managed by the MMEs and is not really seen in the eNB.


2. Mobility control


The eNB is responsible for controlling the mobility for terminals in active state. This is done by ordering the UE to perform measurement and then performing handover when necessary.


3. Control and User Plane security


The ciphering of user plane data over the radio interface is terminated in the eNB. Also the ciphering and integrity protection of RRC signaling is terminated in the eNB.

LTE EPC ARCHITECTURE

As illustrated in Figure, the EPC is made up of a control plane node called MME (Mobility Management Entity) and two user plane nodes called S-GW (Serving GW) and P-GW (Packet Data Network GW). In the non-roaming case the S-GW and P-GW functionalities are both located within one operators network and can be implemented in a combined S- and P-GW node.


The E-UTRAN is made up of eNB nodes, which are connected to each other via the X2 interface. Both the S1 and the X2 interface can be divided into control plane (dashed lines) and user plane (solid lines) parts. The S1 and X2 are multi-to-multi capable interfaces meaning that one eNB can be connected to multiple MMEs, S-GWs and other eNBs. A given terminal however can only be connected to one eNB, MME and S-GW at a given moment. Two terminals
connected to the same eNBs can be connected to different S-GWs and vice versa.

LTE in a Nutshell: White paper on physical layer

The design of the LTE physical layer (PHY) is heavily influenced by the requirements for high peak transmission rate (100 Mbps DL/50 Mbps UL), spectral efficiency, and multiple channel bandwidths (1.25-20 MHz). To fulfill these requirements, orthogonal frequency division multiplex (OFDM) was selected as the basis for the PHY layer. OFDM is a technology that dates back to the 1960’s. It was considered for 3G systems in the mid-1990s before being determined too immature. Developments in electronics and signal processing since that time has made OFDM a mature technology widely used in other access systems like 802.11 (WiFi) and 802.16 (WiMAX) and broadcast systems (Digital Audio/Video Broadcast – AB/DVB).



In addition to OFDM, LTE implements multiple-antenna techniques such as MIMO (multiple input multiple output) which can either increase channel capacity (spatial multiplexing) or enhance signal robustness (space
frequency/time coding).

LTE Frequently Asked Questions

Some Frequently asked LTE questions with their ansrwers.



What speed LTE offers?


Ans:- LTE provides downlink peak rates of at least 100Mbit/s, 50 Mbit/s in the uplink and RAN (Radio Access Network) round-trip times of less than 10 ms.


What is LTE Advanced?


Ans:-LTE standards are in matured state now with release 8 frozen. While LTE Advanced is still under works. Often the LTE standard is seen as 4G standard which is not true. 3.9G is more acceptable for LTE. So why it is not 4G? Answer is quite simple - LTE does not fulfill all requirements of ITU 4G definition.


Brief History of LTE Advanced: The ITU has introduced the term IMT Advanced to identify mobile systems whose capabilities go beyond those of IMT 2000. The IMT Advanced systems shall provide best-in-class performance attributes such as peak and sustained data rates and corresponding spectral efficiencies, capacity, latency, overall network complexity and quality-of-service management. The new capabilities of these IMT-Advanced systems are envisaged to handle a wide range of supported data rates with target peak data rates of up to approximately 100 Mbit/s for high mobility and up to approximately 1 Gbit/s for low mobility.



What is EUTRAN?


The E-UTRAN (Evolved UTRAN) consists of eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U.

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Automatic Neighbour Relation in LTE

Manually provisioning and managing neighbor cells in traditional mobile network is challenging task and it becomes more difficult as new mobile technologies are being rolled out while 2G/3G cells already exist. For LTE, task becomes challenging for operators, as in addition of defining intra LTE neighbour relations for eNBs operator has to provision neighboring 2G, 3G, CDMA2000 cells as well.


According to 3GPP specifications, the purpose of the Automatic Neighbour Relation (ANR) functionality is to relieve the operator from the burden of manually managing Neighbor Relations (NRs). This feature would operators effort to provision 


Figure below shows ANR and its environment as per 3GPP. It shows interaction between eNB and O&M due to ANR.




The ANR function resides in the eNB and manages the conceptual Neighbour Relation Table (NRT). Located within ANR, the Neighbour Detection Function finds new neighbours and adds them to the NRT. ANR also contains the Neighbour Removal Function which removes outdated NRs. The Neighbour Detection Function and the Neighbour Removal Function are implementation specific.


An existing Neighbour cell Relation (NR) from a source cell to a target cell means that eNB controlling the source cell knows the ECGI/CGI and Physical Cell Identifier (PCI) of the target cell and has an entry in the NRT for the source cell identifying the target cell.


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How to calculete LTE peak capacity?

In this blog, I will look at the peak capacity of LTE. This is the maximum possible capacity which in reality can only be achieved in lab conditions. To understand the calculations below, one needs to be familiar with the technology (I will provide references at the end). But for now, let’s assume a 2×5 MHz LTE system. We first calculate the number of resource elements (RE) in a subframe (a subframe is 1 msec):


12 Subcarriers x 7 OFDMA Symbols x 25 Resource Blocks x 2 slots = 4,200 REs


Then we calculate the data rate assuming 64 QAM with no coding (64QAM is the highest modulation for downlink LTE):


6 bits per 64QAM symbol x 4,200 Res / 1 msec = 25.2 Mbps


The MIMO data rate is then 2 x 25.2 = 50.4 Mbps. We now have to subtract the overhead related to control signaling such as PDCCH and PBCH channels, reference & synchronization signals, and coding. These are estimated as follows:


1. PDCCH channel can take 1 to 3 symbols out of 14 in a subframe. Assuming that on average it is 2.5 symbols, the amount of overhead due to PDCCH becomes 2.5/14 = 17.86 %.


2. Downlink RS signal uses 4 symbols in every third subcarrier resulting in 16/336 = 4.76% overhead for 2×2 MIMO configuration.


3. The other channels (PSS, SSS, PBCH, PCFICH, PHICH) added together amount to ~2.6% of overhead.


The total approximate overhead for the 5 MHz channel is 17.86% + 4.76% + 2.6% = 25.22%.


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