Fundamental of the LTE (long-term evolution) system, which is my note for a related video on Youtube. This is the second part. MIMO technology and carrier aggregation technique.

Video 8 - MIMO Techniques

Prologue

This part is the note for the 8th video in the playlist.

Portal: the post of Part 1.

Topic

• Spatial diversity - enhance link performance
• Spatial multiplexing - enhance link efficiency
• Antenna beamforming - improve the coverage
• Massive MIMO - increase link capacity by several folds

Spatial Diversity

• Time diversity

The same message is transmitted in different time slots.

• Frequency diversity

Same message is transmitted at different frequencies.

• Space diversity

It uses separate antennas, which are located in different positions to take advantage of the different radio paths, that exist in a typical terrestrial environment. This is the foundation of MIMO.

MIMO Technology

Multiple inputs and multiple outputs. Capacity and coverge enhancement.

Degradation due to multipath propagation decrease link capacity and link reliability. This is addressed by adding more independent fading channels.

Term: diversity gain = # of Tx Antenna x # of Rx Antenna

For example, if we have multiple Rx antenna, we receive multiple copies of the data, thus we can recover original information and increase the robustness.

Shannon’s channel capacity theorem

Capacity (bits/s) = BW x log₂(1+SNR)

When using a higher order of modulation scheme, the capacity is increased, meanwhile, the SNR shall be improved to guarantee the fidelity. Therefore it is a trade-off. Sometimes it is difficult, expensive or not compromisable.

Spatial Multiplexing

MIMO in spatial multiplexing

For example, we have two antennae in both Tx and Rx. Then we have information “10101010” to transmit. Then we use T1 antenna to transmit the odd digits and T2 for even digits. The receiving antennas are likewise. Such that we have increased data rate.

• Channel transfer matrix

\[ R = H\times T \]

or

\[ T = H^{-1} \times R \]

where \(R\) is the received data, \(T\) is the transmitted data, \(H\) is channel transfer matrix. \(H_{ij}\) is the channel property.

A considerable amount of signal processing is required at the Rx side.

First of all, Rx estimates the individual channel to find the transfer characteristic, which is the element in \(H\).

There are many combinations: SISO, SIMO, MISO, MIMO. Each of them has its advantages and disadvantages.

Beamforming Technology

• Phase array system

Use predefined antenna pattern. Transmit the data using the antenna facing the device.

• Adaptive array system

It leverages real-time beamforming, focusing on the direction of the device. We have infinite number of choices in this case.

In Massive MIMO system, eNB has more than 100 antennae. It is popular because it increases data rate (throughput)), SNR and hardens the channel.

Antenna design rule of thumb: the adjacent antenna is separated by \(\frac{\lambda}{2}\) (physically), where \(\lambda\) is the wavelength of the signal. This can be achieved by

• Use of higher frequency (lower wavelength)
• Use of volumetric spacing
• Use of spatial modulation

Video 9 - Carrier Aggregation Technique

Video Link: LTE - 9

MIMO and OFDM are two cornerstones of the modern wireless communication system. Besides these two, there are

• Carrier aggregation
• Coordinate multipoint
• Small cell
• LTE relaying

Video 9 focus on carrier aggregation (CA).

Analog to building more traffic lanes to increase throughput, CA is employed to increase bit rate demand.

Component carrier (CC) is achieved by aggregating separate spectrum. The maximum bandwidth supported by LTE is 20 MHz. When we aggregate 5 CC, the maximum aggregated bandwidth is 100 MHz.

CA can be applied to both FDD and TDD. In FDD, DL >= UL, while in TDD, DL = UL.

The cells providing CC is called “serving cell”. Although we have multiple serving cells, there is only one primary cell, who handles the RRC connection. SIB broadcast message is sent to the mobile device, and then mobile device sends the uplink control information back to the primary cell (PCC). The other serving cells are called “secondary cell” (SCC). SCC is added or removed as required, while the PCC is changed at handover.

Intra Band Contiguous

Take Band 40 as an example. The 2.4GHz spectrum is from 2300 MHz to 2360 MHz, it is divided into 3 sections: [2300, 2320], [2320, 2340], [2340, 2360]. Intra band contiguous is the two sections that cover half of each section above: [2310, 2330], [2330, 2350].

Intra band contiguous is the simplest CA deployment. It aggregates multiple adjacent component carriers in a single operation band.

Intro band non contiguous

In practical (in many countries), the spectrum is more fragmented, the contiguous aggregation within the same frequency band may not be possible. There is no intra band contiguous in some cases.

Inter band CA

Different frequency Bands are aggregated. For example, Band 40, Band 3 and Band 5.

• It serves a huge customer base.
• It has more complex techniques than others.
• It requires a more advanced transceiver in the user equipment.

CA Configuration

Some terminologies:

• Aggregated Transmission Bandwidth Configuration (ATBC)
  • The total number of aggregated physical resource blocks (PRB)
• CA bandwidth class
  • Combination of maximum ATBC and the maximum number of component carriers.
  • In Release 10 and Release 11, three classes are characterized.
    • Class A: ATBC <= 100, maximum number of CC = 1;
    • Class B: ATBC <= 100, maximum number of CC = 2;
    • Class C: 100 < ATBC <= 200, maximum number of CC = 2.
  • eUTRA operation band
    • Chunks of frequency allocated by ITU to deploy LTE in FDD or TDD mode in different parts of the world.

CA configuration is eUTRA band + bandwidth classes. For example,

• “CA\_40C” means intra band Carrier aggregation on Band 40, in Class C.
• “CA\_3A\_3A” means, intra band non contiguous CA on Band 3, in Class A.
• “CA\_3A\­5B” means, inter band CA on Band 3 Class A and Band 5 Class B.

Deploy CA products

• There is a “UE category” that indicates the capabilities of the mobile device. The more powerful phone, the more # of AC DL CCs as well as UL CCs it can support.
• When the cell knows the capability of a UE, it can scale the throughput of the channel connected to the UE.

CA advantages and disadvantages

From UE’s perspective:

• Pro
  • Provide a bandwidth of up to 100 MHz.
  • Users can get more data rate.
• Con
  • Requires more receiver activity and processing.
  • Power consumption is higher.

From cell’s perspective:

• Pro
  • It can club fragmented bandwidths.
  • Greater bandwidth utilization.
  • Cross component carrier scheduling.
  • Powerful technology for effective utilization of radio resources.
  • Handle CA with CCs from different sites.
  • Improve the utilization of small cells and picocells.