LTE Basics

LTE stands for Long Term Evolution. It is the technology that superseded GSM (2G), and UMTS (3G) as a packet switched network. The main driver by 3GPP was to consolidate its pre-decessor technologies and offer faster data rates to enhance user experience. For these reasons, initial requirements for peak data rates were 100Mbps in DL and 50Mbps in UL with low latency of 10-20ms. In summary LTE was developed to:

  • Simplify the RAN (removing the UMTS RNC and replacing with e-nodeB as well as MME)

  • Increase throughput (with features like MIMO and HARQ)

  • Reduce Latency

  • Improve spectrum efficiency

  • Provide frequency flexibility (thus scalable form 1.4 to 20 MHz)

  • Improve terminal power efficiency

The multiple access technique utilized is OFDM (Orthogonal Frequency Division Multiplex) in the downlink and SC-FDMA (Single Carrier - Frequency Division Multiple Access) in the uplink. This modulation technique allows for many subcarriers to be close together, hence making the system robust to interference. This technique also allows for improving spectral efficiency. As a result, more data can be sent in the same frequency bandwidth - which also translates to faster data rates. OFDMA supports various modulation schemes from BPSK, QPSK, 16QAM to 64QAM. Peak to Average Power Ratio is a main challenge of OFDM, thus in the uplink SC-FDMA is utilized.

 

There are two ways in which LTE can operate. It can operate in either the frequency or time domain as FDD and TDD.

•LTE FDD (Frequency Division Duplex) requires two separate wireless communication channels on separate frequencies, one for transmit and the other for received data

•LTE TDD (Time Division Duplex) TDD uses a single frequency band for both transmit and receive. Then it shares that band by assigning alternating time slots to transmit and receive operations

The diagram below helps illustrate the differences.

 

LTE AIR INTERFACE

The basic EUTRAN radio frame is 10ms long - and is divided into 20 slots with each one being 0.5ms long. The two slots form a subframe 1ms long - and is the smallest time unit the scheduler assigns to physical channels. In FDD, there is a time offset between downlink and downlink transmission

 

Furthermore, each slot is divided into 7 OFDM symbols (depending on choice of Cyclic Prefix). If it is a short cyclic prefix then 7 symbols are used. If it is a long cyclic prefix, 6 symbols are used.

Thus, as a unit, each RB is made up of 12 OFDM subcarriers and 7 OFDM symbols, with each subcarrier having bandwidth of 15 KHz, giving us total bandwidth of 180 KHz per RB.

 

 

 

LTE UE Category & Class Definitions

3GPP Release 8 defines five LTE user equipment categories depending on maximum peak data rate and MIMO capabilities support. With 3GPP Release 10, which is referred to as LTE Advanced, three new categories have been introduced, and four more with 3GPP Release 11

User
equipment
Category

Max. L1
datarate
Downlink
(Mbit/s)

Max. number
of DL MIMO
layers

Max. L1
datarate
Uplink
(Mbit/s)

3GPP Release

User
equipment
Category

Max. L1
datarate
Downlink
(Mbit/s)

Max. number
of DL MIMO
layers

Max. L1
datarate
Uplink
(Mbit/s)

3GPP Release

NB1

0.68

1

1.0

Rel 13

M1

1.0

1

1.0

0

1.0

1

1.0

Rel 12

1

10.3

1

5.2

Rel 8

 

 

 

 

2

51.0

2

25.5

3

102.0

2

51.0

4

150.8

2

51.0

5

299.6

4

75.4

6

301.5

2 or 4

51.0

Rel 10

7

301.5

2 or 4

102.0

8

2,998.6

8

1,497.8

9

452.2

2 or 4

51.0

Rel 11

10

452.2

2 or 4

102.0

11

603.0

2 or 4

51.0

12

603.0

2 or 4

102.0

13

391.7

2 or 4

150.8

Rel 12

 

14

3,917

8

9,585

15

750

2 or 4

226

16

979

2 or 4

n/a

17

25,065

8

n/a

18

1174

2 or 4 or 8

n/a

Rel 13

 

19

1566

2 or 4 or 8

n/a

Note: Maximum datarates shown are for 20 MHz of channel bandwidth. Categories 6 and above include datarates from combining multiple 20 MHz channels using Carrier Aggregation. Maximum datarates will be lower if less bandwidth is utilized.

Note: These are L1 transport data rates not including the different protocol layers overhead. Depending on cell BW, cell load, network configuration, the performance of the UE used, propagation conditions, etc. practical data rates will vary.

Note: The 3.0 Gbit/s / 1.5 Gbit/s data rate specified as Category 8 is near the peak aggregate data rate for a base station sector. A more realistic maximum data rate for a single user is 1.2 Gbit/s (downlink) and 600 Mbit/s (uplink). One vendor has demonstrated downlink speeds of 1.4 Gbit/s using 100 MHz of aggregated spectrum.

Other Key Configurations

Description

Other Key Configurations

Description

Duplexing

FDD, TDD, Half duplex FDD

Channel Coding

Turbo Code

Channel Bandwidth (# of subcarriers)

1.4(72), 3(198), 5(330), 10(660), 15(990),20(1320)

Transmission Bandwidth Configuration NRB : (1 resource block = 180kHz in 1ms TTI=Transmission Time Interval )

6, 15, 25, 50, 75, 100

Modulation Schemes

UL: QPSK, 16QAM, 64QAM(optional)

DL: QPSK, 16QAM, 64QAM

Multiple Access Schemes

UL: SC-FDMA (Single Carrier Frequency Division Multiple Access) supports 50Mbps+ (20MHz spectrum)

DL: OFDM (Orthogonal Frequency Division Multiple Access) supports 100Mbps+ (20MHz spectrum)

Peak data rate in LTE

UL: 75Mbps(20MHz bandwidth)

DL: 150Mbps(UE Category 4, 2x2 MIMO, 20MHz bandwidth)

DL: 300Mbps(UE category 5, 4x4 MIMO, 20MHz bandwidth)

Some Parameters

Physical Cell identifier (PCI), RSRP (Reference Signal Received Power), RSRQ (Reference Signal Received Quality)

Uplink Channels

PUSCH: Physical uplink shared channel

PUCCH: Physical Uplink Control Channel

PRACH: Physical Random Access Channel

Downlink Channels

PBCH: Physical Broadcast Channel

PCFICH: Physical Control Format Indicator Channel

PHICH: Physical Indicator Channel

PDCCH: Physical Downlink Control Channel

PDSCH: Physical Downlink Shared Channel

Operating Bands

LTE In Signal Pro

Signal Pro is a market leading software that caters for a suite of technologies, including LTE. There are two options available. LTE and LTE PRO. LTE is the legacy LTE module. LTE-PRO module is the updated LTE module with the latest 3GPP standards for LTE-A pro.

To navigate to the LTE module, click on: Network Design/Analysis-> LTE Networks. You will see the options available as illustrated in the screenshot below

In System Details/Service Area, there are configuration options available to select duplex type and bandwidth to cater for all possible scenarios as detailed in the configuration section.

The System Profile option chosen from above will automatically self populate information below.

At the radio transmitter and sector level, there are also options to identify the system type as LTE or LTE PRO