3gpp Umts Specifications And Management

Published Date: 02 Nov 2017

The 2G data network started in the commercial level in the early 90’s. Then the cellular data network started growing in a rapid phase. As of September 2002, there were 460 GSM networks throughout the world and there were seven hundred and forty seven and a half million subscribers in these networks.

ETSI had started the standardization works for the next generation mobile telecommunication, when the GSM was commercially announced. This new system was came to know as the UMTS – Universal Mobile Telecommunications System. This was the outcome of ETSI’s technical committee Special Mobile Group (SMG). There were many divisions in SMG, each specializing in a specific area. The ETSI was not the only organization running research programs in this area. There were many programs like Research on Advanced Communication Technologies in Europe (RACE) and Advanced Communication Technologies and Services (ACTS). In 1996 the UMTS Forum was created to define and manage standards needed for 3G

2.1 Overview of 3G networks

http://www.gsma.com/aboutus/wp-content/uploads/2012/03/hspagraphic.jpg

Figure 2.1: 3G technologies and its evolution. Source: www.gsma.com

3G is a family of technologies (Seen in Figure 2.1) which includes simple 3G, HSPDA and HSUPA. The radio leg of 3G is based on WCDMA (Wideband Code Division Multiple Access). The third generation has to be able to offer an ample range of services. It is the 1990’s, when the mobile telecommunications underwent a great revolution.

Slowly and constantly the 3G technologies started moving forward with the development of HSDPA and HSUPA, which are capable of supporting more bandwidth. The Properties of the 3G family can be seen in the Table 2.1.

WCDMA

(UMTS)

HSPA

HSDPA / HSUPA

HSPA+

LTE

LTE ADVANCED

(IMT ADVANCED)

Max downlink speed

bps

384 k

14 M

28 M

100M

1G

Max uplink speed bps

128 k

5.7 M

11 M

50 M

500 M

Latency round trip time approx

150 ms

100 ms

50ms (max)

~10 ms

less than 5 ms

3GPP releases

Rel 99/4

Rel 5 / 6

Rel 7

Rel 8

Rel 10

Approx years of initial roll out

2003 / 4

2005 / 6 HSDPA

2007 / 8 HSUPA

2008 / 9

2009 / 10

Access methodology

CDMA

CDMA

CDMA

OFDMA / SC-FDMA

OFDMA / SC-FDMA

Table 2.1: Characteristics of different technologies. Source: www.radio-electronics.com

2.1.1 3GPP UMTS Specifications and Management

There was a necessity to develop a very large number of documents and specifications because the UMTS or Wideband CDMA is very complex. 3GPP- Third Generation Partnership Programme is the group responsible for managing this WCDMA or UMTS. The 3GPP is a global organization, which is a joined venture by six groups ARIB, CCSA, ETSI, ATIS, TTA and TTC. The ultimate ail of 3GPP is to develop and deliver a globally applicable Technical specification and technical report for a third generation mobile telecommunications system. The GSM core network and the radio access technologies are the key technologies this third generation telecommunication specification relies on. The radio access technologies that the GSM core network supports are Time Division Duplex (TDD) modes and Universal Terrestrial Radio Access (UTRA) both Frequency Division Duplex (FDD). Even though it is formed to take care and of 3G and the data communication, It is taking care of the standards for the GSM as well. It is also responsible for future developments, of which LTE is the key technology.

2.1.2 UMTS Capabilities

Wideband CDMA - WCDMA is used as the radio transmission standard for UMT. It uses a 5 MHz channel bandwidth. The UMTS can handle more than 100 voice calls with this 5MHz bandwidth, or while transferring data it can reach up to 2Mbps, which is a good data rate. Then there occurred some advancement like HSDPA and HSUPA, using which a bandwidth of up to 14.4 Mbps is made possible. These advancements are later included in the releases.

Many capabilities of GSM have been enhanced for UMTS. Components like SIM have been enhanced into a more powerful component, USIM (Universal SIM). Moreover, there were enhancements made to the network design for GPRS and EDGE technologies. These enhancements are made useful by this technology. The initial cost was kept low and the technology migration was made seamless.

It was necessary for the UMTS to have some specification for Duplexing. Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes are defined in the specification. The FDD modes are introduced first, which had different uplink and downlink frequencies. In the networks that are being used and rolled out, the frequencies are spaced with 190 MHz Band.

However the TDD mode in which the uplink and downlink are split in time with the base stations and then the mobiles transmitting alternately on the same frequency is specifically suited to a wide range of applications. Obviously there are some places where spectrum is limited and paired bands suitably spaced, which are not available. This also performs well relatively, where small cells are to be used. When short distance is being covered, this baud will be smaller, because a guard time is always required. It is an universal fact that the down traffic is more than the up traffic in the internet. This is advantageous for this kind of system. It is also necessary to allocate more bandwidth to the downlink, which is not possible in paired spectrum. When a TDD system is used, it is possible to allocate more bandwidth for downlink than uplink and there by improve the efficiency. By employing this method an efficient management of picocells can be done. It is often called as TD-CDMA (Time Division CDMA). This UMTS WCDMA provided a far more efficient system when compared to the previous 2G technologies. The table below, Table 2.2 shows the UMTS parameters and their specification.

PARAMETER

SPECIFICATION

Data rate

2048 kbps low range

384 kbps urban and outdoor

RF channel bandwidth

5 MHz

Multiple access scheme

CDMA

Duplex schemes

FDD and also TDD

Table 2.2: UMTS Parameters and Specification

2.1.3 UMTS frequencies

Six bands are specified for use for UMTS currently / WCDMA, even though operation on other frequencies is not prohibited. Frequency allocations around 2 GHz are focused for the transmission of UMTS. The bands 1885 - 2025 and 2110 - 2200 MHz were reserved at the World Administrative radio Conference (WARC) in 1992, for implementing International Mobile Telecommunications-2000 (IMT-2000). Easy roaming for UMTS / WCDMA users can be promoted by allocating spectrum on a worldwide basis.

There are reservations within these bands for different purposes, which of those are as follows: 

Frequency Ranges

Purpose/Modulation

Channel Spacing

Raster

1920-1980 and 2110-2170 MHz

Frequency Division Duplex (FDD, W-CDMA) Paired uplink and downlink

5 MHz

200 kHz

1900-1920 and 2010-2025 MHz

Time Division Duplex (TDD, TD/CDMA) Unpaired

5 MHz

200 kHz

1980-2010 and 2170-2200 MHz

Satellite uplink and downlink

Table 2.3: Frequency ranges and characteristics specification

UMTS carrier frequencies are identified and allocated by a UTRA Absolute Radio Frequency Channel Number (UARFCN). This is calculated from:

UARFCN = 5 x (frequency in MHz)

Wideband CDMA is the radio transport mechanism used by UMTS. The channel spacing is 5MHz for consumer implementations.

2.1.4 UMTS power control

Like any CDMA system, it is essential that the power levels of all the user equipment are same at the base station. They will not be heard, If not. This may be due to the fact that the UEs that are further away will be lower in strength than those closer to the base station. This effect is known as the near-far effect. The Base station instructs those stations closer in a cell to reduce the transmitting power in order to overcome this. This facilitates the transmission through same power levels.

It is also important for the base stations to control their own power levels as well. Signals from different ones will interfere, if the signals transmitted by the different base stations are not orthogonal to one another. Accordingly their power is also kept to the minimum required by the UEs being served.

Two techniques that are employed to achieve the power control: open loop; and closed loop.

During the initial access, .i.e. before communication between the UE and Base station has been fully established, Open loop techniques are used. It operates by calculating the received signal strength and with the help of that estimate the transmitter power required. The path losses in either direction are different as the transmit and receive frequencies are different, different and hence it can be said that this method is a close estimate, which has no room to improve.

Once the UE has established a connection with the node B, closed loop techniques are intiated. The signal strength is measured in each time slot. A power control bit is sent to signal the UE to step up or down its power. This process is done on both the up and downlinks. Only one bit is assigned to power control to enable the power to change continually. This will not create a overhead at any level

2.2 LTE Systems Overview

After the 3G data networks (e.g. GSM to UMTS to HSPA to LTE or CDMA to LTE), the next step to step on is Long Term Evolution. LTE is based on standards developed by the 3rd Generation Partnership Project (3GPP).  It can be said that UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) transformed to the next stage, which is LTE. The standards for GSM/UMTS family were created by 3GPP. The standards for LTE are totally new, with some exceptions where it made sense. The key objectives for LTE are listed as follows.

Better downlink and uplink peak data rates.

Scalable bandwidth

Improved spectral efficiency

All IP network

A standard’s based interface, which is capable of supporting a wide multitude of user types.

The main goal or intension of LTE networks is to bridge the data transfer speed   gap   between   very high data rate   fixed wireless   Local   Area   Networks (LAN) and highly mobile cellular networks.

2.2.1 Overview of the LTE Standard

The original study on Long Term Evolution (LTE) of the 3GPP Radio Access Technology family was started with the aim of making sure that 3GPP RAT is more advanced in the future than that of the predecessors. The motive of the investigation was to enhance and improve the radio-access technology (UTRA) and optimization of radio access network (UTRAN). The main characteristics of LTE are:

Efficient spectrum utilization

Flexible spectrum allocation

Reduced cost for the operator

Improved system capacity and coverage

Higher data rate with reduced latency

2.2.1 Targets for LTE

Some of the key targets set for LTE are listed below (as per [3GPP TR 25.913])[17]

Increased peak data rate: 100Mbps for DL with 20MHz (2 Rx Antenna at UE), up to 50Mbps for UL with 20MHz

Improved spectrum efficiency: 5bps/Hz for downlink and 2.5bps/Hz for Uplink

Improved cell edge performance (in terms of data rate)

Relatively low latency.

2.2.3 Overall Network Architecture

The E-UTRAN makes use of simplified version single node architecture. This has the eNBs (E-UTRAN Node B). eNB and the Evolved Packet Core (EPC) communicates with each other through the S1 interface; specifically with the MME (Mobility Management Entity) and the UPE (User Plane Entity) identified as S-GW (Serving Gateway). This S-GW uses S1-C and S1-U for control plane and user plane respectively. MME and UPE are mostly implemented as separate network nodes so that independent scaling of the control plane and the user plane can be implemented. This is clearly illustrated in the figure 2.5.

https://sites.google.com/site/lteencyclopedia/_/rsrc/1293465421244/home/E-UTRAN.bmp

Figure 2.2: Overall Architecture [8] 

Multicast/Broadcast over a Single Frequency Network (MBSFN) is efficiently supported by LTE, where multiple cells transmit a common signal with appropriate time synchronization. Even though eNB is the only entity of the E-UTRAN, all the functions in a typical radio network, for example, Radio Mobility management, Bearer control, scheduling and Admission control are efficiently supported in it. The Access Stratum resides at the eNB.

https://sites.google.com/site/lteencyclopedia/_/rsrc/1293467072241/home/E-UTRAN2.bmp

Figure 2.3 Functional Split between E-UTRAN and EPC [8]

2.2.4 LTE Physical layer

To achieve the aim of high data rate and improved spectral efficiency, the LTE physical layer is built with Orthogonal Frequency Division Multiplexing scheme OFDM. Both time also called as slot and frequency units, which is also called as subcarrier makes the spectral resources that are allocated/used as a combination. 2 or 4 Antennas are supported. UL and DL supports Multi-user MIMO. QPSK, 16QAM and 64QAM are the modulation schemes supported in both uplink and downlinklink spectrum.

2.2.4.1 Downlink (DL) Physical Channel

OFDM with cyclic prefix is used for the downlink transmission. OFDM is used due to the reasons that are described below:

For the narrow band subcarrier, the channel appears to have nearly flat frequency response and to mitigate this selective fading is countered by multiple carrier modulation (MCM).

By changing or adapting to the channel condition like the number of resource blocks and the frequency range of each of the resource block, flexible spectrum allocation is achieved.

Higher peak data rates are achievable with the help of combining several resource blocks and not by reducing the duration of each of the symbols in it or by using still higher order modulation.

Higher spectral efficiency is an advantage obtained by the multiple orthogonal subcarriers.

2.2.4.2 Uplink (UL) Physical Channel

The uplink transmission is built with the SC-FDMA (Single Carrier FDMA) scheme. A two stage process makes up the SC-FDMA scheme. The first stage is where the input signal is converted to frequency domain, which are represented by DFT coefficients and the second stage is where OFDM scheme is used to change these DFT coefficients to an OFDM signal. The SC-FDMA scheme is a scheme which is known as DFT-Spread OFDM because of this association with OFDM. The reasons for this choice are described below:

The two stage process facilitates the selection of appropriate frequency range for the subcarriers while mapping the set of DFT coefficients to the Resource Blocks. At any given time, Users get unique frequency. This avoids co-channel interference among the users of a cell.

The transformation is same as the shift in the centre frequency of the single carrier input signal. The subcarriers do not combine in random phases, which cause large variation in the modulated signal in terms of instantaneous power. This implies Peak to Average Power Ratio is low.

The Peak to Average Power Ratio (PAPR) of SC-FDMA is lesser than the PAPR of the conventional OFDMA.

2.3 Introduction to LTE Advanced

Observing the growth and success rate of the 3G technologies, it is obvious that the growth rate of cellular network should not slow down. The ideas to start up 4G technology started to flow in and the investigation started. In an initial investigation happened on 25th of December, 2006 which was released on 9th of February 2007, NTT DoCoMo briefed out the information about the their trial which succeeded in sending data to a mobile station moving at a rate of 10Km/h, with a speed up to 5 Gb/s. This was done with a 10Mhz bandwidth. The Technique which made this possible includes several technologies to achieve this, of which the significant technologies are variable spreading factor spread orthogonal frequency division multiplex, MIMO, multiple input multiple output, and maximum likelihood detection. Methods and procedures for these new 4G experiments were passed to 3GPP for their consideration

3GPP organized two workshops in 2008 on IMT Advanced. This is where the "Requirements for Further Advancements for E-UTRA" were outlined. Technical Report 36.913 was made out of this and then published in June 2008. The LTE-Advanced system was submitted to the ITU-R as their proposal for IMT-Advanced.

The evolution from the 3G services was developed by making use of UMTS / W-CDMA Technologies. This is in turn followed by the development of LTE Advanced / IMT Advanced technologies.

There exists another technology that competes with LTE. WiMAX offers very high data rates with high levels of mobility. However it is less likely that the WiMAX may be adopted as the 4G technology. LTE Advanced seems to have a better probability.

2.3.1 LTE Advanced key features

With a large number of improvements are happening in LTE Advanced, a number of key features and requirements are found. In spite of the specification not being fixed now, there are many high level targets for the new LTE Advanced specification. These specifications need to be verified and validated. Much work has to be done in the specifications of this technology, before these are all fixed. Some of the main targets for LTE Advanced as of now are listed as follows:

Peak data rates: downlink - 1 Gbps; uplink - 500 Mbps.

Spectrum efficiency: 3 times greater than LTE.

Peak spectrum efficiency: downlink - 30 bps/Hz; uplink - 15 bps/Hz.

Spectrum use: the ability to support scalable bandwidth use and spectrum aggregation where non-contiguous spectrum needs to be used.

Latency: from Idle to Connected in less than 50 ms and then shorter than 5 ms one way for individual packet transmission.

Cell edge user throughput to be twice that of LTE.

Average user throughput to be 3 times that of LTE.

Mobility: Same as that in LTE

Compatibility: This is backwards compatible. LTE Advanced shall be capable of interworking with 3GPP legacy systems and LTE.

These are some of the key development targets for LTE Advanced. Their actual specifications and the actual implementation have to be done during the implementation stage of the system.

2.3.2 LTE Advanced technologies

There are various key technologies to achieve the high data throughput rates that are required as per the target of LTE advanced. MIMO and OFDM are the two of the base technologies that enable this amount of precision and efficiency. There exists number of other techniques and technologies apart from these.

OFDM forms the foundation of the radio bearer. Apart from that, there is OFDMA (Orthogonal Frequency Division Multiple Access) along with SC-FDMA (Single Channel Orthogonal Frequency Division Multiple Access). A hybrid format of these will be implemented. Anyhow all these schemes work based on OFDM.