MIMO Introduction in 3G Network

MIMO Introduction

Rev PA1

MIMO Introduction

Rev PA1 2013-10-30 Ericsson AB 2013 2 (13) Commercial in confidence

Contents

1 Introduction ...................................................................................... 3

2 Technical background ..................................................................... 3 2.1 Theoretical introduction ...................................................................... 3

3 MIMO solution .................................................................................. 4 3.1 Hardware ........................................................................................... 4 3.2 Features ............................................................................................ 4 3.3 MIMO with Tx diversity ....................................................................... 5 3.4 MIMO with common pre-coder ........................................................... 6 3.5 Single carrier 64QAM+MIMO ............................................................. 7

4 Main Challenges ............................................................................... 7 4.1 UE Categories and CQI reporting ...................................................... 8 4.2 Mobility ............................................................................................ 10

5 MIMO deployment scenario ........................................................... 10

6 MIMO deployment scenario in IBS ................................................ 11

7 Conclusion ..................................................................................... 13

8 Glossary ......................................................................................... 13

MIMO Introduction


1 Introduction

This document concerns technical background regarding MIMO and can be used to get a better understanding of the involved features.

It also highlights the main challenges and issues that can arise when MIMO is deployed in the network.

2 Technical background

2.1 Theoretical introduction

MIMO (Multiple Input Multiple Output), is a general term used to describe several different transmission techniques where multiple receive and transmit antennas are used. The Shannon-Hartley theorem asserts the following:

C = B log2(1+S/N)

Where:

C is the channel capacity [bps]

B is the bandwidth of the channel [Hz]

S is the total signal power over the bandwidth [W]

N is the total noise power over the bandwidth [W]

S/N is the signal-to-noise ratio (SNR)

With this assumption, the channel capacity in Single stream Rx/Tx processing Multiple Rx and/or Tx antennas, has a logarithmic dependency on SNR, especially for high SNR: C ∼log(1+N x SNR). In a Multi-stream Rx/Tx processing Multiple Rx and Tx antennas (MIMO) the channel capacity increases linearly with the SNR, since SNR is shared between the streams:

CMIMO ∼N x log(1+SNR).

MIMO Introduction


Figure 1: Single stream vs Multistream channel capacity

Clearly MIMO is useful especially at high SNR, where the theoretical peak bit rate is doubled compared to Rel-6 HS-PDSCH. However, since there will be interference between the two streams, very good radio conditions are needed to reach this theoretical limit, even though under less ideal radio conditions, MIMO can still bring gains in user and system throughput.

MIMO, as the name suggests, involves leveraging multiple transmit and receive antennas available at the radio base station and the device to increase data rates, overall capacity and the user experience. Essentially, the MIMO system uses the antennas and “processing” at both transmitter and receiver to create multiple uncorrelated (having different fading characteristics) radio links (called: ‘streams’) between the transmitter and receiver. These streams use the same time and frequency resources, enabling capacity to be increased without an increase in spectrum.

3 MIMO solution

3.1 Hardware

In order to deploy MIMO, RBS6601 with RRUS-11 and DUW has been offered in the solution for required number of sites.

3.2 Features

Following features are been offered in Ericsson solution for MIMO:

FAJ 121 1318, Support for 2x2 MIMO

FAJ 121 1483, 64QAM and MIMO combination

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FAJ 121 1510, Pre-coder for power balancing

3.3 MIMO with Tx diversity

In Ericsson P7FP, the feature MIMO using Transmit Diversity (TX Diversity) implements the 2x2 MIMO transmission scheme for HS-PDSCH as standardized in 3GPP Rel-7. With "2x2" is meant that 2 transmit antennas on the RBS and 2 receive antennas on the UE are employed.

Transmit Diversity is an optional feature that can be activated and deactivated per cell by the operator. Once the feature is activated, TX Diversity will be applied to all channels in the cell. The diversity mode implemented in WCDMA is “open loop” which means that TSTD coding is applied to the synchronization channel and STTD coding is applied to all the other channels (refer to 3GPP TS 25.211 Rel. 7).

With MIMO, two parallel data streams can be transmitted to one UE on the same set of HS-PDSCH codes. This means that two transport blocks are transmitted per TTI. Before being transmitted, the 2 data streams are pre-coded by using the pre-coding matrix. The weighting of the streams and distribution over the two antennas is called pre-coding, and ensures both a well distributed HS-PDSCH power on the two antennas and less impact of the channel fading on the transmitted signal.

Transmission of two parallel data streams (two parallel transport blocks) is denoted dual-stream transmission, while transmission of one stream only is denoted single-stream transmission. Single-stream transmission is quite similar to TX Diversity with closed loop mode 1, with the difference that the antenna weighting is updated once per TTI instead of once per slot as for the TX Diversity case.

In MIMO with TX Diversity implementation, TX Diversity is used as Power Amplifier (PA) balancing method, as illustrated in Figure 2. In this way the system rely on TX Diversity for PA load balancing.

MIMO Introduction


Figure 2: Load balancing with and without TX Diversity

3.4 MIMO with common pre-coder

An alternative way to use MIMO is with common pre-coder (FAJ 121 1510). When TX Diversity is not applied to the control channels, and MIMO is using common pre-coder, the cell will be configured with a Primary and a Secondary CPICH (S-CPICH) as a phase reference.

Since TX Diversity is affecting throughput performance for legacy HS UEs, MIMO technique has been developed without TX Diversity, by using a common pre-coder. , by which it is possible to balance the power levels in the antenna branches, as done with TX Diversity mode, but without affecting performance for legacy HS UEs.

Common pre-coder in the RBS baseband aims to balance the power on the 2 streams, but without using TX Diversity which affects legacy UEs.

It is also recommended to implement common pre-coder with cross-polarized antennas, in order to avoid nulls (which could happen if common pre-coder is implemented in cells with spatially separated antennas).

MIMO Introduction


3.5 Single carrier 64QAM+MIMO

The combination of 64QAM and MIMO is standardized in 3GPP Rel-8. It is an optional feature for both the network and the UE. In Ericsson product, the support for simultaneous 64QAM and MIMO on single carrier is an optional feature (FAJ 121 1483). Hardware that supports MIMO also supports 64QAM+MIMO. The enhanced layer 2 (EL2) protocol MAC-ehs is needed to enable 64QAM and MIMO. For power balancing between the two power amplifiers, MIMO requires TX diversity or Pre-coder for Power balancing.

The main benefits of the feature are:

Increased theoretical throughput on HS-PDSCH. The theoretical maximum L1 bit rate is 42 Mbps if 15 HS-PDSCH codes are available. However, when MIMO is configured without TX Diversity in the cell, i.e. using the common pre-coder, only 14 codes will be available. This configuration limits the maximum throughput to ~38.5 Mbps.

Enable single stream transmission with 64QAM modulation when dual stream transmission cannot be done.

Note: the benefit of 64QAM+MIMO over legacy MIMO (using 16QAM modulation) in realistic scenarios is expected to be very limited, since extremely friendly channel conditions are needed to achieve the higher rates.

4 Main Challenges

The main challenges to introduce MIMO in the network can be divided into the following macro area:

1) To get maximum throughput

2) UE dependency

In order to reach the maximum HS throughput, the maximum number of HSPDSCH codes needs to be available. In case of MIMO with TX Diversity 15 HS-PDSCH codes can be used, provided that the all the common channels occupy the have to be available on cell level.

It has to be noted that this configuration is possible only for MIMO with TX Diversity. When MIMO is deployed with common precoder, 1 more SF256 code is used by the common channels for S-CPICH, hence the maximum number of available HS-PDSCH codes in a cell will be 14, since no space for A-DCH is left in the 15th SF16 branch.

MIMO Introduction


It is also very important to have good UL performance in order to guarantee good acknowledgment process, especially for TCP related applications. Enhanced uplink is generally able to provide the performance needed to achieve the very high DL throughput which is peculiar of MIMO. Nevertheless heavy loaded cell can also affect UL performance in term of UL interference.

4.1 UE Categories and CQI reporting

All the traditional UEs are not capable of supporting MIMO. UEs belonging to HS-DSCH categories 15, 16, 17, 18, 19, 20 support MIMO and are said MIMO-capable. Category 17/18 UEs work as category 15/16 UEs when using MIMO-activated HS-DSCH. Category 17/18 UEs support MIMO and 64QAM, but not simultaneously. For these UEs, if the cell supports both MIMO and 64QAM, then the HS-DSCH will be configured to be MIMO-activated and 64QAM deactivated. Category 19/20 UEs can support simultaneous MIMO and 64QAM.

The CQI reported by the UE in HS-DPCCH when MIMO is configured is called CQI Type A (RBS only supports Type A CQI reports), and can be single stream or dual stream:

Single-stream CQI: Values 0...30, same as Rel-6, sent when single transport block stream is preferred by the UE.

Dual-stream CQI: Values 31...255 (=15*CQI1 + CQI2 + 31 where CQI1, CQI2 = 0...14) sent when 2 transport block streams are preferred by UE.

In 3GPP 25.214, different CQI mapping tables are provided for the different UE categories indicating standard values of TBS, number of HS-PDSCH codes and modulation for the different cases. A summary is reported in Table-1.

MIMO Introduction


Table 1: CQI table for different UE categories

Few reference examples are reported below:

• UE category either 15, 17 or 19, MIMO used without 64QAM: maximum HS throughput at Layer 1 is 23.37 Mbps.

• UE category either 16, 18 or 20, MIMO used without 64QAM: maximum HS throughput at Layer 1 is 27.952 Mbps.

• UE category 19, MIMO used in combination with 64QAM: maximum HS throughput at Layer 1 is 32.264 Mbps.

• UE category 20, MIMO used in combination with 64QAM: maximum HS throughput at Layer 1 is 42.192 Mbps.

Table-2 reports the highest Transport Block Size (TBS) achievable for 64QAM+MIMO. If common precoder is used, max 14 HS-PDSCH codes are possible. In case of 64QAM+MIMO with TX Diversity, the left most table is still valid; in this case though, use of 15th HS-PDSCH code is allowed and highest TBS are indicated in the right most table.

MIMO Introduction


Table 2: Highest TBS for 64QAM+MIMO UEs (cat 19/20)

4.2 Mobility

Regarding mobility, it is important to notice that if Enhanced Layer 2 is used in a cell configured as 64QAM or MIMO capable or 64QAM+MIMO capable, and if the UE (64QAM/MIMO capable) is performing a HS cell change to a new serving cell that does not support the same capability, it will trigger a reconfiguration away from 64QAM or MIMO or 64QAM+MIMO. This reconfiguration might also be triggered by IFHO, if the target cell does not support 64QAM or MIMO or 64QAM+MIMO. Note that the reverse functionality is not implemented; the connection is not upgraded to gain EL2, 64QAM or MIMO or 64QAM+MIMO capability triggered by HS cell change or by IFHO. In order to gain back 64QAM/MIMO capability it has to go to idle or FACH.

5 MIMO deployment scenario

MIMO successful feature introduction depends on parameter setting as well as the particular layer strategy adopted by the operator.

Nevertheless, different scenarios require different optimization of MIMO related parameters. MIMO can be deployed in four types of scenarios:

MIMO Introduction


1) 1 carrier with R99 and MIMO

2) 2 carriers with MIMO on 2nd carrier, R99 on 1st carrier

3) HS on both carriers

4) MIMO on a dedicated carrier

For Robi, the first scenario is applicable as they will operate the 3G network on single carrier.

In first scenario MIMO with STTD TX Diversity, legacy UEs will experience bad performance for HSDPA throughput. In this case MIMO with common pre-coder can work instead.

Figure 3: Scenario 1: 1 carrier with R99 and MIMO

In this scenario, network capacity is the major issue. The minimum number of codes statically allocated to HSDPA cannot be high. Values lower or equal to 5 are suggested. As a consequence, in order to optimize HSDPA throughput whenever R99 traffic allows, it is recommended to enable Dynamic Code Allocation. The maximum number of codes that can be allocated to HSDPA has to be set to 14, since the S-CPICH is requiring one more code, and the 15th code will be blocked. Considering there is only one carrier, all HSDPA users will camp on that carrier and high HS number of user is expected, so code multiplexing is useful to multiplex user in the same TTI.

6 MIMO deployment scenario in IBS

MIMO can be deployed in IBS solution as an option. To support 2x2 MIMO, special antennas can be employed, using dual polarizations to be capable of handling two separate streams simultaneously. Alternatively, two separate antennas, one for each MIMO stream, can be used, with spatial separation to achieve diversity. If 2x2 MIMO is used, a double passive RF distribution network is required in order to support the two MIMO streams.

MIMO Introduction


As a general Ericsson recommendation, MIMO deployment should be considered on a case by case basis, depending on the specific capacity requirements of the building at hand. The general Ericsson approach to MIMO design philosophy is summarized as follows:

MIMO deployment

Areas where we do observe maximum data users.

MIMO is an important key to higher capacity.

An in-building solution based on a main-remote RBS6601 involves the main unit (MU) connected to one or more remote radio units (RRU) via optical fiber. Each RRU is then connected to a passive DAS covering a certain portion of the area to be covered.The main unit can be stored in a central equipment room while the RRUs can be installed far away from the main unit. The RRUs can be connected to antenna systems in e.g. both a main building as well as some satellite buildings, high up in skyscrapers etc, which are impossible to reach via coaxial cables from a centralized traditional RBS because of the feeder losses.

Figure 4:An in-building solution with one main-unit RBS covering different buildings.

MIMO Introduction


MIMO cannot be deployed with traditional DAS system omni antenna. MIMO supportive omni antenna or panel antenna can be used as well to deploy the solution in the DAS system.

7 Conclusion

Demand for mobile data services is growing in epic proportions. Faced with this growth, mobile operators are looking for opportunities to cost effectively increase capacity. MIMO is one of the powerful tools in their arsenal, as it can significantly improves data rates, user experience and capacity within 3G spectrum and through an upgrade to existing infrastructure.

Undoubtedly the main deployment challenge, as witnessed in the initial roll outs of MIMO networks, is to ensure that MIMO can co-exist with legacy HSPA terminals and ensure high peak rates.

8 Glossary

HSDPA – High Speed Downlink Packet Access

MIMO – Multiple input Multiple output

QAM – Quadrature Amplitude Modulation

SNR- Signal to Noise Ratio

STTD- Space Time Transmit Diversity

Documents

MIMO Introduction in 3G Network