Analysis of the cdma2000. and UTRA Wireless. 3G Proposals. Final Report. Gabriel Sacripanti EE /7/99

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1 Analysis of the cdma2000 and UTRA Wireless 3G Proposals Final Report Gabriel Sacripanti EE /7/99 Abstract

2 Over 10 proposals were submitted to the International Telecommunications Union (ITU) for the deployment of third generation (3G) wireless systems. These proposals are based on a set of requirements related to the transmission of voice and high speed data known as International Mobile Telecommunications for the year 2000 (IMT 2000). The original goal of having a single global 3G standard will not be achieved since various countries and companies have significant investments in 2G networks and technologies. Two proposals have emerged as the leading standards from the IMT 2000 effort: cdma2000 (USA) and the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access known as UTRA (Europe/Japan). The main aspects of these proposals will be studied and analysis will be conducted to compare the performance of these 2 proposals. 2

3 Introduction Several of the proposals submitted for IMT 2000 are based on wideband CDMA technology. The term wideband refers to bandwidths equal or greater than 5 MHz, as opposed to the 1.25 MHz bandwidths used in 2G CDMA systems (IS 95). UTRA and cdma2000 are the leading wideband CDMA proposals. Short of a single 3G standard, these systems will have a major impact in the final implementation of 3G systems worldwide. The main differences [1] between these proposals are shown in Table 1. Only differences in the frequency division duplexing (FDD) mode will be considered in this paper. Parameter cdma2000 UTRA Base Chip Rate (5 MHz BW) Forward Link Spreading Mbps Mbps or 3.84 Mbps Multi Carrier Spreading or Direct Spreading Direct Spreading Base Station Synchronous (GPS) Asynchronous Synchronization Frame Time 20 msecs 10 msecs Table 1: Main differences between cdma2000 and UTRA proposals. Chip Rate The chip rate parameter in CDMA systems is a measure of the amount of spreading applied to the signal before it is transmitted. The high chip rates used in wideband CDMA systems are more suitable for the higher data rates due to better averaging of the multipath fading. 3

4 It has been argued that the chip rate is directly proportional to the capacity of the system [2], meaning that UTRA should be able to have more capacity since it has higher chip rate than cdma2000. The actual answer to this question is not this simple because of the differences in the frame structure in the 2 proposals. The frames transmitted have a certain amount of overhead consisting of control, pilot, and synchronization signals. The frame time in cdma2000 is twice as long as the frame time in UTRA, therefore, the ratio of the overhead signaling to the total frame time is lower for cdma2000. The longer frame time of 20 msec will result in a longer delay, however this frame time has been used in 2G CDMA and it is not considered to be an issue, even for speech transmissions. The chip rate is closely related to the allocated bandwidth for a given CDMA channel. The signal in the baseband of the mobile unit is sent through a RF filter after spreading. The total bandwidth BW is given by [1]: BW = (1 + α)*chip rate, where α is the raised root cosine filter roll off factor. For cdma2000, the total bandwidth required is 4.5 MHz, which leaves two guard bands on each side of the spread signal equal to 250 khz. The original chip rate proposed for UTRA of Mbps uses up the entire 5 MHz bandwidth with no guard bands. The revised UTRA chip rate of 3.84 Mbps results in a 4.7 MHz bandwidth and 2 guard bands of 150 khz each. The guard bands are used to avoid getting out of band emissions into other frequency bands. A band adjacent to that used by a wideband CDMA operator may be occupied by another type of service, such as TDMA, making the use of guard bands imperative. Different chip rates in UTRA and cdma2000 systems would require the use of dual mode handsets if users of cdma2000 are to have access to UTRA networks and vice versa to fulfill the goal of global access. Dual mode handsets are certainly not a new concept. For example, 4

5 2G handsets (TDMA, CDMA) have 1G capabilities (AMPS) in areas where 2G system access was not available. However, the dual mode handset solution may unnecessarily complicate the implementation of a global 3G system. Ultimately, it appears that the selection by the UTRA group of a different chip rate than cdma2000 has more to do with politics than it does with actual technical merit. The choice of chip rate by cdma2000, which is based on the 2G CDMA chip rate, was justified, as it was the first CDMA system in place. A solution to this controversy [3] has been proposed by the 3G Operators Harmonization Group which, was set up to try to resolve some of these differences. It has recommended the use of the UTRA chip rate for the direct spread (DS) and the cdma2000 chip rate for the multi carrier (MC) spread mode. Forward Link Structure The two systems use other chip rates derived from their base chip rates [3]. In UTRA, the basic chip rates is defined as 3.84 Mbps, with additional rates at 2 and 4 times that number, to be used in 5, 10 and 20 MHz bands, respectively. Cdma2000 has chip rates of 3X, 6X, 9X and 12X, for bandwidths equal to 5, 10, 15 and 20 MHz respectively (X is equal to Mbps). In cdma2000, there are two spreading modes [3] in the forward link: direct spread (DS) and multi carrier (MC). For example, in a given channel with a 5 MHz bandwidth, the data for the users in this channel could be spread in the DS mode with a Mbps chip rate. It could also be de multiplexed into 3 parts and have each part spread in the MC mode with a Mbps chip rate. In a UTRA system, given that same 5 MHz bandwidth, a DS mode with a chip rate of 3.84 Mbps is the only choice. 5

6 The use of the MC mode results in more flexibility in the use of the allocated bandwidth. Table 2 shows the calculations for the number of channels that can used for various channel bandwidths. It can be seen that for bandwidths equal or greater than 10 MHz at least 1 more 1X channel can fit over the number of channels that can be used with DS mode only. Bandwidth Mhz Number of Channels W CDMA cdma2000 DS mode DS Mode MC Mode MHz 3.84 Mbps 3X 1X 1X only Table 2: Channel allocation in cdma2000 and UTRA. MC mode allows backward compatibility with IS 95 2G systems. Cdma2000 and 2G CDMA users can actually transmit data over the same 1X channel (overlay mode), provided they use different spreading codes. This is certainly an advantage for cdma2000 over UTRA, as it provides a more gradual upgrade path from 2G CDMA systems. Analysis of the simulation results in the cdma2000 proposal [4] indicate that the MC spread method is less spectrally efficient than the DS method, as shown in Table 3. The DS is certainly much better in the vehicular environment, whereas the DS mode is only slightly better or comparable in the pedestrian and indoor environments. Base Station and Mobile Synchronization 6

7 A mobile unit in a cell needs to synchronize to the base station in that cell in order to communicate. The mobile needs to figure out the scrambling code applied to the data sent from the base station. In cdma2000, the synchronization is achieved by detecting the pilot signal [3]. All the base stations use the same code for the pilot signal, but a time offset is applied to the pilot signal that is different for each base station. The Global Positioning System (GPS) is used to establish a common time reference so that the Service Environment Spectrum Efficiency, users/mhz/cell (for voice) kbps/mhz/cell (for data) Multi Carrier Direct Spread Voice Vehicular (120 km/hr) Pedestrian (3 km/hr) Indoor (3 km/hr) Packet Data 76.8 kbps Vehicular (120 km/hr) Pedestrian (3 km/hr) Indoor (3 km/hr) Table 3: Spectrum efficiency comparison of MC and DS modes in cdma2000. mobile unit can then figure out the time offset. In the UTRA system, the base stations are not synchronized but this means that the mobile unit must synchronize to the base station in the cell area. The UTRA base stations first send out a sync signal SCH1 followed by a second signal SCH2 which gives the mobile a hint to decode the scrambling code Synchronization using GPS is achieved quickly since the signals from GPS satellites can be received anywhere in the world. The disadvantage is that reception of the GPS signal may be difficult in underground (subways) or in crowded city areas with 7

8 tall buildings because GPS are line of sight signals [5]. Reception of the UTRA synch signals is not hampered by obstructions although it takes longer for the mobile to synchronize to the base station. This could potentially be a problem for handoffs, even if soft handoffs are used. A study showed that the cell search time could take as much as 150 to 500 msecs, which is equivalent to 15 to 50 UTRA frames [6]. Certainly, more research is required in this area to develop faster cell search algorithms in order for this synchronization method to be viable. The Europeans have been reluctant to adopt the GPS synchronization method. They argue that the US military, which developed and supports the GPS technology, could stop supporting it, rendering the synchronization method useless. This is a not a very good argument, since GPS is being adopted in many civilian applications and it is very unlikely that it will stop being supported in the future. Spectrum Efficiency and Capacity Comparison of the two systems is better gauged by analyzing their spectrum efficiencies. Table 4 compares the spectrum efficiency in the 2 systems assuming the DS mode is used in both systems. System Chip Rate, Mbps Spectrum Efficiency Forward / Reverse Link (Erlangs/MHz/Cell) Self Evaluations Chinese Evaluation cdma / / 27.2 UTRA / / 22 Table 4: Spectrum efficiency for speech in a vehicular environment. 8

9 This table shows that cdma2000 is more spectrally efficient for voice transmissions than UTRA, particularly more in the forward link [2]. Speech transmissions have been carried on 2G IS 95 systems for years, and, since cdma2000 borrows many things from it, it has given them the opportunity to optimize the system. Table 4 shows comparisons for data rates of 144 kbps for UTRA and cdma2000 taken from results in the respective proposals [4, 7]. The results in Table 5 show that both systems perform comparably for high speed data transmissions. In the long delay case in UTRA, 2 code sets were used, so the actual forward link spectrum efficiency is half the value shown in the table, making it almost the same as the cdma2000 value. Comparisons could not be made at other data rates due to differences in the data rates and environments simulated, which would make these comparisons invalid. System Environment Spectrum Efficiency Forward/Reverse Link (Erlangs/MHz/Cell) cdma2000 Long Delay 108/221 UTRA Long Delay 210/204 (2 code sets in FL) Table 5: Spectrum efficiency for 144 kbps rate in vehicular environment. The capacity of a CDMA channel is related to the number of Walsh codes available. In the 5 MHz channels, the number of Walsh codes is typically equal to 256. This channel could support at the most approximately 250 or so speech channels (control channels use some of the available codes). Users with high data rates use shorter spreading codes [3]. For example, to transmit data at 1 Mbps, the Walsh code length is usually 4 bits. If the short code starts with 1, 1, 1, 1, one of every four of the Walsh codes start with the same sequence. Although the rest of the bit sequence may be 9

10 different, the system can not differentiate the codes of different lengths because of the common initial bits. One user with high data rate effectively uses up of 1/4 of the total capacity of this channel. This limitation is equally applicable to UTRA and cdma2000 systems. Conclusions The comparison between UTRA and cdma2000 shows that for the most part, both systems will perform comparably in terms of performance and spectrum efficiency. The cdma2000 system has advantages in terms of flexibility and ease of implementation. The base station to mobile synchronization is an area where both proposals have proposed schemes that do not appear to work equally well in all of the environments where they will be used. More research should be conducted in this area. More work should also be devoted to reduce the remaining differences between the systems, which would benefit wireless users worldwide. References 1. Tero Ojanperä et al., An Overview of the Air Interface Multiple Access for IMT 2000/UMTS, IEEE Communications Magazine, September 1998, pp CDMA Development Group White Paper, Third Generation Systems, November Hewlett Packard, 3G Market Overview and Technology Basics for cdma2000 and UTRA (ETSI W CDMA), /literature/english/wireless_mkttechb.html, October TIA/TR45.5, The cdma200 ITU R RTT Candidate Submission, radio dev/proposals/cdma2000(0.18).pdf,

11 5. Erik Dahlman et al., UMTS/IMT 2000 Based on Wideband CDMA, IEEE Communications Magazine, September 1998, pp , pp ETSI/SMG2, The ETSI UMTS Terrestrial Radio Access (UTRA) ITU R RTT Candidate Submission, radio dev/proposals/etsi/utra.pdf, July Fumiyuki Adachi et al., Wideband DS CDMA for Next Generation Mobile Communications Systems, IEEE Communications Magazine, September 1998, pp

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