The next major step in mobile radio communications 22
Initial eld peomance measements o LTE Researchers have measured the performance of LTE in the eld using different drive routes and radio channel environments. Among other things, they measured different bandwidths, different antenna congurations and layer-1, layer-1, UDP and TCP throughput using two kinds of terminals. Verizon Verizo n Wirel Wireless ess,, Vodafo Vodafone ne and China Mobile, conrmed their commitment Mathias ribacK to LTE as the next-generation mobile network. And wireless industry leaders The 3GPP Long Term Evolution have reached an agreement concern(LTE) standard has made great ing a framework for licensing technostrides during the past pas t year. logy IPR. Measurements from fr om the LTE This article describes some of the SAE Trial Trial Initiative (L STI) have eld-performance eld-performa nce measurements measurements made moved ahead as scheduled and, 1 using different drive routes and radio as expected, shown good results. channel environments while travelIn January 2008, 3GPP coning at velocities of up to 120km/h and rmed that the technology speci at distances of up to 4km. The trials cations for fo r the LTE Terrestrial Terrestrial include measurements measurement s of both 10 MHz Radio Access Network had been and 20 MHz bandwidths and of different differe nt approved and were under Change antenna congurations up to 4×4 MIMO. Control, which means they will be The article also a lso presents the layer-1, UDP included in the forthcoming 3GPP and TCP throughput using two different Release 8. 2 kinds of terminals: a large terminal (the At Mob Mobile ile Wo World rld Congres Congresss 2008, in test bed, built on DSP technology) and Barcelona, major CDMA and WCDMA a prototype handheld terminal. Please operators, such as NTT DoCoMo, note that the performance gures are Jonas Karlsson
BOX A
Tems and abbeviations
3GPP AWGN CDF DSP EMP EPC EPS E-UTRAN EVA FDD GPS IPR ITU LOS LSTI LTE MIMO
Third Generation Partnership Project Additive white Gaussian noise Cumulative distribution function Digital signal processor Ericsson Mobile Platforms Evolved packet core Evolved packet system Evolved UTRAN Extended vehicular A Frequency-division Frequency-divis ion duplex Global positioning system Intellectual property rights International Telecommunication Union Line of sight LTE SAE Trial Initiative Long Term Evolution Multiple input, multiple output
E r i c s s o n r E v i E w • 3 20 20 08 08
Next-generation mobile network Orthogonal frequency-division multiplexing Pedestrian B 3km/h PB3 Packet switched PS System Architecture Evolution SAE SC-FDMA Single-carrier frequency-division multiple access Signal-to-noise ratio SNR S-PARC Selective per-antenna rate control Transmission control protocol TCP Time-division duplex TDD User datagram protocol UDP Universal mobile telecommunications UMTS system UTRAN UMTS terrestrial radio access network Cross-polardiscrimination XPD
NGMN OFDM
taken from the test-bed results; the actual performance of commercial products might differ dif fer somewhat. somewhat. LTE The LTE standard is being developed in two 3GPP work items: The LTE work item targets the evolution of the radio net work; wo rk; and the SAE (Sy (System stemArchi Architectur tecture e Evolution) work item targets the evolution of the packet core network. In contrast to previous standards, the LTE standard solely species a packet-switched (PS) domain for LTE and SAE. Output from the work items is embodied in the Evolved UTR AN (E-UTR AN) and the Evolv Evolved ed Packet Core (EPC) standards. Together, they form the Evolved Packet System (Figure 1). 1). The E-UTRAN standard employs orthogonal frequency-division multi multi-plexing (OFDM) technology for downlink operation and singlecarrier frequency-division multiple access (SC-FDMA) technology for uplink operation. This approach yields excellent exibility in terms ter ms of deploying and allocating spectrum spectru m from 1.4 MHz up to 20 MHz. In addition, addition, the E-UTRAN E-UTRA N standard supports FDD (frequency-division duplex) as well as TDD (time-division duplex) modes of operation. The basic LTE downlink physical resource can be seen as a time2), where each frequency grid (Figure 2), resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. Downlink subcarrier spacing has been set at 15kHz. LTE uplink transmission is based on SC-FDMA with dynamic bandwidth. The single-carrier property of the uplink, which whi ch limits limits transmiss transmission ion to a con contintin-
23
Simplied achitecte o the Evolved Packet System. FIgurE 1
Internet, operator service, etc.
EPS
LTE test bed is a one-sector, one-cell system designed for early testing and proofof-concept. Some updates have been made to the hardware since 2006, but it is essentially the same exible, highperformance platform described in LTE 3 test bed. It supports a variety of congurations, antenna setups, and system bandwidths; and measurements of layer-1, layer-2 and application-level performance.
BOX B
3gPP LTE 3GPP LTE (Long Term Evolution) is the name given to a project to improve the UMTS mobile phone standard to cope with future technology evolutions. Goals include improving spectral efciency, lowering costs, improving services, making use of new spectrum and refarmed spectrum opportunities, and better integration with other open standards.
to perform modem use case s with a laptop; to demonstrate multimedia telephony (for example, VoIP); to demonstrate video telephony; to perform le transfers; to stream video applications; and to test multiple terminals simultaneously.
Experimental The results were obtained using the test bed in the laboratory and in extensive outdoor eld tests. Fading emulators and additive white Gaussian noise (AWGN) sources were used in the lab to measure the performance of different channel proles at different signal-to-noise ratios (SNR). For the eld trials, an LTE test site was installed on the roof at Ericsson’s headquarters in Stockholm (Figure 4). Commercially available antennas for the 2.6 GHz band were used at the base station and in the terminal. To evaluate the use of spatially separated and dualpolarized antennas, the testers used different antenna setups and benchmarked them against each other for different MIMO congurations. The test-bed terminal was installed in a measurement van with antennas mounted on the roof (Figure 5). A GPS receiver was also used to make it possible to associate logged performance with the position of the terminal. The eld tests were performed in two sectors that represent different instal-
The baseband software is based on a common platform, but different versions of the code are used for differEPC ent hardware congurations in order to match the current antenna conguration. Test bed development has taken place in parallel with the 3GPP stanE-UTRAN dardization process. Therefore, the LTE test bed does not exactly mirror the most recent 3GPP LTE specication. The MIMO scheme in the test bed, for example, employs selective per-antenna rate control (S-PARC), which means streams can be individually encoded and modulated and a single data stream is always E-UTRA Terminal mapped to a single transmit antenna.4 In addition to measurements made using the LTE test-bed hardware, this article includes measurements peruous frequency block for each termi- formed with a prototype handheld LTE nal, greatly improves power efciency device developed by Ericsson Mobile in terminals. Platforms (EMP) and shown at Mobile One important aspect of LTE is its sup- World Congress 2008 in Barcelona port of various multi-antenna schemes. (Figure 3). The terminal, which is comIn addition to receive diversity, both at patible with the base station, has been the base station and at terminals, LTE used downlink transmission supports openloop transmit diversity, two types of spaFIgurE 2 The LTE downlink physical esoce id. tial multiplexing, and the scheduling of multiple terminals on the same time and frequency resource – a technique called multi-user MIMO (multiple-input multiple-output). Spatial multiplexing (MIMO) makes it possible to transmit Subcarrier spacing = 15 kHz parallel streams of data, which substantially increases the peak data rate. In theory, the peak rate of a system with a 2×2 antenna conguration (two transmit and two receive antennas) using two-stream MIMO should be double that of a 1×2 system (one transmit Tim e and two receive antennas). Likewise, the theoretical peak rate of a 4×4 system with four-stream MIMO should be four times that of a 1×2 system. Ericsson LTE test bed The hardware architecture of Ericsson’s
One OFDM symbol
One resource element
Frequency
E r i c s s o n r E v i E w • 3 2 00 8
The next major step in mobile radio communications 24
FIgurE 3
“Beta” – Eicsson’s pototype handheld LTE device.
lation scenarios. The rst sector is quite sparsely settled and dominated by low-rise buildings (compared to the site installation height) – line-of-sight (LOS) conditions were thus fairly common. The second sector is more dense and building heights are comparable to the site installation – LOS conditions were thus quite rare. The majority of the eld tests were performed within a radius of 1km from the test site. The tests also included distances of more than 4km. During most parts of the eld tests, the speed of the terminal was between 5 and 40km/h. Some of the tests were conducted while driving on a freeway at speeds of more than 100km/h. Measurements Lab results
FIgurE 4
Installation o the base station antenna.
E r i c s s o n r E v i E w • 3 20 08
Figure 6 contrasts the layer-1 throughput with SNR of different antenna congurations. The measurements, made with 10 MHz bandwidth and the Extended Vehicular A (EVA) 3km/h channel model5, were complemented with an emulated cross-polar discrimination (XPD) of 10dB, which simulates the use of dual-polarized antennas. As anticipated, the 1×4 setup outperforms the 1×2 setup, especially when SNR is low. Compared to the 1×2 setup, the 2×2 setup yields gains of approximately 10 to 15dB and up. By contrast, the 2×4 setup, yields multi-stream gains of around 5dB. The gain with four streams compared to two is seen from 20dB and up. With this specic channel model, the peak throughput of the 4×4 setup yields a gain of approximately 150 percent compared to the 1×2 setup. Other channel models yield different gains – Figure 7, for example, shows the gain that can be obtained from using an ideal 4×4 AWGN MIMO channel. To evaluate application level performance, measurements were taken using UDP and TCP trafc over a 2×2 20 MHz MIMO Pedestrian B 3km/h (PB3) channel.6 As can be seen in Figure 8, the difference in performance between UDP and TCP is very small. UDP is a best-effort transmission technique without acknowledgements from the terminal to the base station. It is thus insensitive to uplink performance and IP-related issues, such as system buffer sizes. TCP, on the other hand, transmits
25
TABLE 1
Example o elative ain compaed to 1×2 antenna setp.
Throughput gain relative 1 x 2 setup 10th percentile
Mean
+ 15 %
+ 12 %
+1%
2x2
+ 27 %
+ 42 %
+ 55 %
2x4
+ 86 %
+ 87 %
+ 91 %
4x4
+ 104 %
+ 108 %
+ 100 %
1x4
acknowledgements in the uplink. The results indicate that the test system has a well-tuned and reliable uplink.
90th percentile
the mean throughput by 10 percent to 20 percent compared to the 1×2 setup. Combining a second stream at the base station with four receive antennas Field results (2×4) signicantly increases the mean In good radio conditions, the system throughput (50 percent to 70 percent) achieved the theoretical maximum compared to a 1×2 setup. Large multithroughput over the downlink of a stream gains were seen in both sectors 2×2 system using link adaptation and using dual-polarized as well as spatial20 MHz bandwidth: 170Mbps. Using ly separated antennas. The addition four transmit streams (the maximum of more transmit streams (up to four) number supported in the LTE standard), increases peak throughput by approxfour receive antennas (4×4 MIMO) and imately 60 percent, whereas the mean 10MHz bandwidth, the measured peak throughput increased by 8 percent to rates exceeded 130Mbps. This trans- 25 percent compared to the 2×4 setup. lates into approximately 260Mbps, giv- In conclusion, besides increasing peak en the maximum bandwidth of 20MHz, data rates, we see that multi-stream which is more than three times the peak transmission improves the mean and throughput that can be achieved from a 10th percentile data rates. basic 1×2 setup. Figure 9 shows the cumulative disFIgurE 5 Test measement van. tribution functions (CDF) for different antenna congurations along a drive route where vehicle velocity was between 5 and 40km/h. The measurements show that adding more transmit and receive antennas improves performance. Table 1 compares the relative gains to a 1×2 setup along a given drive route. The MIMO-related gains are strongly dependent on radio channel conditions and should therefore solely be seen as an example of what can be achieved. From the measurements, including those derived from different drive routes, it is seen that adding a second stream at the base station (2×2) can increase the mean throughput by approximately 15 percent to 40 percent and more or less double the peak throughput. Adding two more receive antennas but keeping a single transmit antenna at the base station improves
Figures 10 and11show performance along a drive route using 10 MHz band width for 4×4 and 2×2 MIMO setups. Good throughput was achieved along the entire route. In some areas the peak rates were very high. The 4×4 setup performed better than the 2×2 setup except in line-of-sight conditions where performance was similar. The lab measurements (Figure 11) assessed the TCP and layer-1 bit rates available for applications using 20MHz bandwidth. The difference between layer-1 bit rates and corresponding TCP bit rates can be attributed to protocol overhead. The layer-1 bit rate (no protocol overhead) is available to higher-level applications. The TCP bit rate was more than 40Mbps at least 50 percent of the time and more than 100Mbps at least 10 percent of the time. The impact of velocity on performance was evaluated on a freeway. Given good radio conditions, a bit rate of more than 100Mbps was obtained while traveling at more than 100km/h and using 20 MHz bandwidth. Under the same conditions, but at a distance of more than 4km from the base station, throughput was 40Mbps or greater. Field measurements using the prototype handheld terminal were conducted in Nuremberg, Germany
E r i c s s o n r E v i E w • 3 2 00 8
The next major step in mobile radio communications 26
FIgurE 6 Laye-1 thohpt meased at 10 MHz bandwidth sin Extended Vehicla A (EVA) 3km/h channel model.
Throughput (Mbps)
FIgurE 7 Laye-1 thohpt meased at 10 MHz bandwidth sin ll ank AWgN channel model.
Throughput (Mbps)
80 120
4 x 4 MIMO 2 x 4 MIMO 1 x 4 receive diversity 2 x 2 MIMO 1 x 2 receive diversity
70 60
4 x 4 MIMO 2 x 4 MIMO 1 x 4 receive diversity 2 x 2 MIMO 1 x 2 receive diversity
100 10 MHz bandwidth
50
10 MHz bandwidth
80
40
60
30 40 20 20
10 0
0 0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
SNR (dB)
Peomance o uDP and F TP at 20 MHz bandwidth sin 2×2 setp with Pedestian B 3km/h (P B3) channel model. FIgurE 8
35
SNR (dB)
Cmlative distibtion nctions (CDF) o difeent antenna conations meased with 10 MHz bandwidth and dal-polaized antennas. FIgurE 9
CDF
Throughput (Mbps)
1.0
90 UDP, 64QAM, R= 0.5 80
0.9
FTP, 64QAM, R=0.5 UDP, 16QAM, R=0.5
70
1x2
0.8
1x4
FTP, 16QAM, R=0.5
2x2 20 MHz bandwidth
UDP, QPSK, R =0.5
60
FTP, QPSK, R =0.5
0.7
2x4 4x4
0.6
50
10 MHz bandwidth 0.5
40 0.4
30
0.3
20
0.2
10
0.1
0 0
5
10
15
20
25
30
35
SNR (dB)
E r i c s s o n r E v i E w • 3 20 08
0 0
20
40
60
80
100
120
Throughput (Mbps)
27
(Figure 12). Commercially available antennas were installed on the fourth oor at Ericsson’s premises. The frequency band was 2.6 GHz; band width was 20 MHz. In a static measurement made close to the base station (Point A), the maximum layer-1 bit rate over the downlink was 25Mbps. This is also the maximum throughput of the prototype terminal in this conguration when connected via cable. UDP throughput over the downlink was close to 23Mbps. As the terminal moved away from the base station (Point B), the maximum layer-1 data rate remained steady at 25Mbps, whereas UDP throughput fell slightly to just above 20Mbps. Driving with the terminal at close to 30km/h, the layer-1 data throughput over the downlink was around 22Mbps; the UDP data rate was around 18Mbps. To further demonstrate the strength of the prototype, three additional terminals were placed in radio conditions that varied from high to considerably low SNR. Despite a variance in SNR of more than 20dB, all three terminals reached a throughput of about 14Mbps. Keeping in mind that the terminal is still an early prototype, these performance gures are most promising. Summary and future perspectives Measurements made in the lab and extensive field trials show that LTE performs well in both the physical and application layers. Multistream MIMO yields good gains in realistic environments, improving peak rates as well as the mean and 10th percentile throughput. A small, prototype handheld LTE terminal has been used, among other things, to stream video from an LTE test-bed base station and to illustrate that LTE is quickly becoming a mature technology. Workis also under wayto standardize the next releases of 3GPP to ensure that future end-user and operator requirements are met. The International Telecommunication Union (ITU) is dening the requirements for IMT Advanced, sometimes also referred to as “4G.” Some of the requirements being discussed are bit rates of up to 1Gbps in the downlink, more than 500Mbps in the uplink, and a scalable bandwidth of up to 100 MHz. Other requirements
FIgurE 10 Let: Thohpt elative to location o a 4x4 MI MO setp sin 10 MHz bandwidth and dal-polaized antennas . riht: Thohpt elative to location o a 2x2 MIMO setp sin 10 MHz bandwidth and dal-polaized antennas. Base station located at X in both plots.
FIgurE 11 TCP and laye-1 thohpt o a 2x2 setp with 20 MHz bandwidth.
CDF 1.0 0.9 0.8 20 MHz bandwidth
0.7 TCP
Layer 1
0.6 0.5 0.4 0.3 0.2 0.1 0 0
20
40
60
80
100
120
140
160
Throughput (Mbps)
E r i c s s o n r E v i E w • 3 2 00 8
The next major step in mobile radio communications 28
FIgurE 12 Field testin the pototype hand held LTE teminal in Nembe, ge many.
Jonas Kalsson received an M.Sc. from Linköping University (Sweden), a Licentiate degree from the Royal Institute of Technology (Sweden) and a Ph.D. from the University of Tokyo (Japan), all in electrical engineering. Since 1993 he has worked for Ericsson Research in Sweden and Japan. The focus of his research has been on advanced antennas and interference cancellation. In 2007 he moved to the LTE System Management department, where he is a Senior Specialist in multiple antenna systems.
Mathias riback
being discussed have already been met by LTE. The 3GPP has begun working on LTE Advanced, which is scheduled for inclusion in 3GPP Release 10. LTE Advanced should fulll, but not be limited to, the IMT Advanced requirements. The 3GPP view is that LTE Advanced should
joined Ericsson in 2004 to work as a research engineer at Access Technologies and Signal Processing, Ericsson Research, Stockholm. The focus of his work has been on radio propagation research and test-bed development (HSPA and LTE) with emphasis on multi-antenna aspects. Mathias is currently responsible for the LTE test bed activities at Ericsson Research. He holds an M.Sc. in electrical engineering from the Royal Institute of Technology (Sweden).
aim for NGMN (next-generation mobile network) system performance. The NGMN intends to complement and support the work within standardization bodies by providing a coherent view of what the operator community will require in the decade beyond 2010; fulll 3GPP compatibility requirements – that is, it should allow LTE Release 8 terminals in LTE Release 10 networks and vice versa; and support smooth migration from LTE Release 8.
Acknowledements Werner Anzill, Robert Jansen, Hanna Maurer-Sibley and David Wisell
In addition, the 3GPP is studying extended multi-antenna deployments as well as ways of lowering terminal and net work power consumption.
reeences 1. www.lstiforum.org 2. The Third Generation Partnership Project, www.3gpp.org. 3. Johansson, B. and Sundin, T.: LTE test bed. Ericsson Review, vol. 84(2007):1 pp. 9–13
E r i c s s o n r E v i E w • 3 20 08
4. Grant, S., Molnar, K. and Krasny, L,: System-level performance gains of per-antenna-rate-control (S-PARC), VTC Spring 2005, vol. 3, pp. 1696–1700 5. 3GPP spec. 36.101, ver. 8.1.0, Annex B 6. 3GPP spec. 25.101, ver. 8.2.0, Annex B
