Frames FMA2 wideband-CDMA for UMTS

FRAMES FMA2 Wideband-CDMA for UMTS

ANTTI TOSKALA Nokia Telecommunications, New Radio Systems, P.O.Box 300, FIN-00045, Nokia Group - Finland

[email protected] JONATHAN P. CASTRO

2CSEM, Jaquet-Droz 1, CH-2007 Neuchatel- Switzerland ERIK DAHLMAN

Ericsson Radio Systems AB, S-164 80 Stockholm - Sweden- MATTI LATVA-AHO

University of Oulu, Centre for Wireless Communications, P.O.Box 44.4, FIN-90571, Oulu - Finland

TERO O J A N P E ~ Nokia Research Center, 2300 Valley View Ln, Irving, TX 75038 - USA

Abstract. The basic principles of the physical layer of the FRAMES (Future Radio Wideband Multiple Access System) FMAZ (FRAMES Multiple Access) wideband CDMA system are described in this paper. The FMAZ is part of the FRAMES dual mode platform designed as UMTS air interface, where the harmonisation with GSM is also taken into account. The main characteristics features in the FMAZ are the asynchronous network operation and flexible layer 1 support for the variable rate transmission needs. This paper presents also performance results. which allow to conclude that the FMAZ meets UMTS air interface requirements. The FMAZ concept was a major part of the ETSI (European Telecommunications Standards Institute) wideband CDMA concept group, selected by ETSI as a basis for UMTS air interface for the paired frequency band.

1. INTRODUCTION

The European Research Program ACTS (Advanced Communication Technologies and Services) started in I995 to support research and development in the area of mobile communication. Within the ACTS program the project FRAMES was set up with an objective of defin- ing a proposal for a UMTS (Universal Mobile Telephone System) terrestrial radio-access scheme. After an initial evaluation phase, the FMA platform was set up [I]. The FMA platform consists of two modes, FMA1, a wideband TDMA scheme with and without spreading [2], and FMA2, a wideband CDMA scheme. This paper describes the FMA2 Wideband CDMA (W-CDMA) radio-access scheme physical layer solution. The UMTS satellite aspects based on the FMA concept are being studied by the ACTS SINUS [3] project, where the FMA2 has been chosen as the basis for UMTS satellite component development.

The FMA2 is a W-CDMA radio-access scheme that has been designed based on the UMTS requirements and on the flexibility needs of the third generation cellular services. The main features of the FMA2 include:

- Support for high data rate transmission; 383 kbits/s with wide-area coverage and up to 2 Mbits/s for indoor and local outdoor coverage.

- High service flexibility with support of multiple parallel variable rate services for each active user.

- Efficient packet access. - High initial capacity and coverage with build-in

support for future capacity and coverage enhancing technologies, such as smart antennas, advanced receiver structures and macro diversity.

- Support for soft handover as well as inter-frequency handover for e.g. operation with hierarchical cell structures.

-Backwards compatibility to GSM; handover between GSM and FMA2 supported.

Clearly, the FMA2 concept satisfies the UMTS requirements. The key technical characteristics of the FMA2 air interface are listed below.

- Basic chip rate: 4.096 Mchipds; expandable to

- Frequency Division Duplex (FDD) operation. - Dual-channel QPSK spreading modulation, BPSK

data modulation for each data channel ( I ) .

- Spreading factor may vary from 4 to 256.

8.192 and 16.384 Mchipsk

( I ) By the detinition a physical data channel is one BPSK modulated and spread I or Q branch signal. Hence. one I-Q pair carries two physical channels.

Vol. 9. Nu. 4 July -August 19% 325

- Orthogonal Variable Spreading Factor (OVSF) codes are used for channel separation: the scrambling codes in the uplink are the extended very- large Kasami codes of length 256 unique to each terminal, extended Gold codes of length 256 are used for the downlink. The optional long scrambling code (40960 chips) for the uplink is derived from a Gold code of length Z4' - 1.

- The coded bit rate of single data channel may vary from 16 kbit/s to 1.024 Mbids. Higher bit rates with parallel channels.

- Carrier raster 200 kHz with the basic carrier spacing from 4.4 MHz to 5 MHz for basic chip rate of 4.096 Mchipds; square-root raised-cosine filter- ing, with roll-off 0.22, is used for band limitation.

- Frame length 10 ms; a super-fraine consists of 60 frames.

- Asynchronous operation, i.e., no need for accurate inter-BS synchronisation.

- Both variable spreading factor and multi-code operation to support multi-rate transmission.

- Rate matching with either unequal repetition or puncturing.

- Convolutional codes of constraint length 9 with rates 112 and 113 used for BER = IOE-3 services. An additional Reed-Solomon code is used as an outer code for BER = 10E-6 services.

- Flexible interleaving with variable interleaving depth to support ARQ with packet data and to support sewice multiplexing.

- Fast power control for both the uplink and for the downlink with the dynamic ranges 80 dB and 20 dB, respectively. The power control command rate and the step-size are cell specific parameters in the range of 400 - 1600 Hz and 0.5 - 2 dB, respectively.

- Flexible support of variable-rate services; data rate is allowed to vary frame-by-frame.

- Pilot symbols to assist coherent reception with the number of pilot symbols as a cell specific parameter; in case no beamforming is used in the downlink, common pilot used as phase reference.

The rest of this paper is organised as follows: sections 2 and 3 introduce the logical and physical channel structures respectively. Section 4 covers channel coding and service multiplexing while random access is covered in

section 5. The physical layer elements of the packet access are presented in section 6 and additional features are described in section 7. The link level performance is presented in section 8 and physical layer aspects relevant to the network deployment are outlined in section 9. The conclusions are presented in section 10.

2. LOGICAL-CHANNEL STRUCTURE

The FMA2 basically follows the ITU recommendation ITU-R M-1035 [4] in the definition of the logical channels. The following logical channels are defined for the FMA2.

Three different common control channels are available:

- BCCH (Broadcast Control Channel) carrying

- PCH (Paging Channe1):for messages to the mobiles

- FACH (Forward Access Channel) for messages

system- and cell specific information

in the paging area

from the BTS to the MS in one cell.

There exist two dedicated channel types:

- DCCH (Dedicated Control CHannel) covers the two dedicated control channels Stand-Alone Dedicated Channel (SDCCH) and Associated Control Channel (ACCH)

- DTCH (Dedicated Traffic CHannel) for point-to- point data transmission in the uplink and downlink.

3. PHYSICAL-CHANNEL STRUCTURE

3.1. Dedicated physical channels

The FMA2 defines two types of dedicated physical channels for both uplink and downlink, the dedicated Physical Control CHannel (PCCH) and the dedicated Physical Data CHannel (PDCH).

3.1.1. DEDICATED PHYSICAL DATA CHANNEL (PDCH)

Service: 1 2 3 4

1

The dedicated physical data channel is used to transmit data generated at layer 2 and above, i.e., the dedicated control and traffic channels (DCCH and DTCH)

I

Pilot Symbols ' (160 symbols) ' ... R I I ... I I R Power Control

b . . . . a PCCH (Control)

Symbols

Fig. 1 - FMA? Uplink Multir~tr transmission with variable rate PDCH and constant rate PCCH.

326

160 x Pilot Code

Pilot Broadcast

Synchc Channel

Channels

User Data

I

I I (Control) I I ... ... * PCCH

(16Symbols) -' Fig. 2 - FMA2 downlink multirate transmission with common pilot and constant rate PCCH and variable rate PDCH.

of section 2. Fig. 1 shows the principle frame structure of the PDCH for the uplink and Fig. 2 for the downlink. Each PDCH frame on a single code carries 160 + 2k bits (16 . 2k kbitsls), where k = 0, 1...8, corresponding to a spreading factor of 25612k with the 4.096 Mchipsls chip rate. Multiple parallel variable-rate services (=dedicated logical traffic and control channels) can be time multiplexed within each PDCH frame. The overall PDCH bit rate is variable on a frame-by-frame basis.

In most cases, only one PDCH is allocated for each link and services are jointly interleaved sharing the same PDCH. However; multiple PDCHs can also be allocated e.g. if the needed symbol rate is higher than could be sup ported by a single PDCH with spreading ratio of 4.

As the rate of the PDCH may vary on a frame-by- frame basis, the power of the PDCH may also vary on a frame-by-frame basis. Then the C/I measurements needed for the fast power control algorithm can not be based on measurements on the received power of a variable- rate PDCH. Therefore, in the FMA2, it is based on the measured received power of the dedicated physical control channel (PCCH) instead.

3.1.2. DED~CATED PHYSICAL CONTROL CHANNEL (PCCH)

The dedicated physical control channel is used to transmit control information generated at layer 1, i.e., pilot bits for coherent detection, transmit-power-control (TPC) commands, and a Frame-Control Header (FCH) with rate information. Fig. 1 shows the principle of the frame structure used on the PCCH. The pilot bits, TPC commands, and FCH are time multiplexed within a slot structure within each PCCH frame, with the typical unit slot length of 0.625 ms. Longer slots are then multiples of the unit slot, making possible power command rates from 1.6 kHz downwards.

Since the PCCH rate and power are constant, the power measurements for the fast power control are based only on the received PCCH power for connections with a variable-rate PDCH. Compared to CII measurements from the PDCH the measurements from

PCCH are-naturally noisier due lower power level, although based on known pilot symbols. Parallel service are controlled together with the slow outer loop adjustment taking into account the quality for both services.

Although the PCCH frame structure is fixed during a connection, it may vary between different environments. As an example, in indoor environments, with semi-stationary terminals, the pilot power and the rate of the power control commands may be significantly reduced, with a reduced overhead and increased capacity as a consequence.

3.1.3. SPREADING OF PDCWCCH

Fig. 3 shows the uplink PDCWPCCH multiprexing and spreading for the most common case of one data channel per each link. A combination of code and I-Q multiplex is used, where the PDCH and PCCH are spread by different channelization codes and mapped to the I and Q branch respectively. The complex I + jQ signal is then scrambled by a short code. A short scrambling code is used in order to simplify the future imple- mentation of advanced receiver structures, e.g. multiuser detectors. As an option, long-code scrambling may take place, e.g. in the case when the BS employs ordi- nary RAKE reception. The power differences between PDCH and PCCH is adjusted with the gain factor G.

The spreadinglchannelization codes of the Fh4A2 are based on the Orthogonal Variable Spreading Factor (OVSF) technique originally proposed in [ 5 ] . This

Channelisation Codes (OVSF)

COS (Ot) pDca , , Complex

c, Multiply

U

Fig. 3 - Uplink spreading and scrambling for the c:w of one data channel per connection.

VOl. 9, NO. 4 July - August 1998 327

allows to change the spreading factor and to maintain the orthogonality between different spreading codes of different lengths when the codes are picked from the code tree in Fig. 4.

A PDCH that is to be transmitted on the I (Q) branch may use a certain code in the tree if and only if no other physical channels to be transmitted on the same I (Q) branch are using a code that is on an underlying branch or on the path to the root of the tree. For a PCCH the restriction is that a certain code may be used if and only if no other physical channel is using a code that is on an underlying branch or on the path to the root of the tree. The reason for stronger restrictions for the PCCH is that physical channels transmitted with the same channelisation codes on the I and Q branches respectively can not be separated before the PCCH has been detected and channel estimates are available.

L. Fig. 4 - Code tree for the channelisation codes.

The terminal unique short scrambling code is allocated from the set of Extended Very-Large Kasami codes of length 256 chips. Since this set of codes includes more than one million different codes, no extensive code planning is required. A subset of these codes is allocated to each cell for usage by active mobile stations within the cell and because of the high number of codes the code can remain the same in the handover process.

The optional long code is a 10 ms segment of a Gold code of length Z41 - 1. The long code to be used by an active mobile station is derived from the short scrambling code assigned to that mobile station. As the scrambling code has the length of 256 always when short scrambling

codes are used, the cross-correlation between consecutive symbols will have similar averaging effect than a long scrambling code for higher data rates. For the cross-correlation between base stations, the difference is in practise minimal between the 256 chip scrambling code and the 10 rns scrambling code with 40960 chips when including the fading channel effects adding to the averaging.

For the general case of multiple PDCHs (multi-code transmission), additional PDCHs can be mapped to either the I or the Q branch. In general, each PDCH should be allocated to the I or Q branch in such a way that the overall envelope variations are minimised. The Z-Q imbalance is avoided with the complex scrambling operation which makes the amplifier constellation similar to the case where I and Q branches are with equal power. The signal after complex scrambling for the k-th chip of the i-th symbol for a user with single PDCH and PCCH can be written as

where cDe and cck present the k-th chips in channelisation codes, and c$ are the k-th chip of the complex- valued scrambling code and the i-th data symbol on the PDCH is presented by bLDCH, the i-th PCCH data symbol by bLCCH and the G is the PCCH gain factor. Obviously there is no unbalance between I and Q if PCCH and PDCH are at the same power level. The worst case in terms of the unbalance would take place when the PCCH power level approaches zero (G = 0). However, as we can see from eq. ( l) , by using complex- valued scrambling, the unbalance is avoided.

The FMA2 uses square-root raised-cosine TX chip shaping with a 0.22 roll-off factor. The achievable adja- cent channel attenuation depends then on the carrier spacing used, which may vary from 4.4 MHz with steps of 200 kHz coming from the GSM compatible carrier raster. With this kind of raster the guards bands between systems can be optimised depending whether any CO-

ordination among systems is used or not. The downlink spreading and scrambling in Fig. 5 is

very similar to that of the uplink, except that all down-

Common Control

Fig. 5 - FMAl downlink spreading and scrambling.

Channdisation BS Scrambiing

Control Channel

Fig. 5 - FMAl downlink spreading and scrambling.

318

link connections of a base-station share a common set of short OVSF channelisation codes and are jointly scrambled by a short base-station unique scrambling code. The sector unique short scrambling code is allocated from the set of orthogonal Gold codes with length of 256 chips. This is used by the unmodulated pilot channel which has the channelisation code with all ones.

TPC Symbols

3.2. Physical channels for downlink common control

The FMA2 defines two downlink physical channels to carry the downlink common control logical channels (BCCH, PCH, and FACH, the primary and secondary Physical CHannels for Common Control (PCHCC).

3 . 2 . 1 . PRIMARY PHYSICAL CHANNEL FOR COMMON CONTROL

The primary physical channel for common control carries the BCCH. It has fixed rate and it uses the same short channelisation code in all cells. A mobile terminal can thus always find the BCCH, once the BS unique scrambling code has been detected during the initial cell search.

In addition to the BCCH, the Primary PCCCH also includes the Frame Synchronisation Word (FS W) that is used to find frame synchronisation. The FSW consist of two different 26 bits words (FSWO and FSWI) that are transmitted in odd and even frames respectively. The mobile uses these synchronisation words to measure the timing between different base-stations to facilitate the Rake combining in soft handover with several asynchronous base-stations.

3 . 2 . 2 . SECONDARY PHYSICAL CHANNEL FOR COMMON CONTROL

The secondary physical channel for common control carries the PCH and FCH in time multiplex within the super frame structure. The rate of the Secondary PCCCH may be different for different cells and is set to provide the required capacity for PCH and FACH in each specific environment.

------I [ BER 1 e-3 Services

lBER 1 e-6

3 . 2 . 3 . SPREADING/SCRAMBLINC OF DOWNLINK PCHCC

The downlink physical channels for common control are spread in the same way as the downlink dedicated physical channels. For coherent detection of the PCHCC the common pilot channel is used similar to the dedicated channels. Note that this pilot channel can also be seen as a physical control channel, similar to the PDCH for dedicated physical channels, that is common for all PCHCC. Neither power-control commands'nor rate information need to be transmitted for the fixed rate PCHCC.

3.3. Super-frame structure

A super-frame consists of 60 frames, i.e., the length of the super frame is 0.6 s. This is equal to the length of five GSM super frames (120 ms). This length of the super- frame struc'ture has been chosen to simplify handover between UMTS and GSM. Each PCCCH frame within a superframe is tagged by a 6 bit Super-Frame Number (SFN) located within the BCCH. From the SFN, the mobile station can achieve super-frame synchronisation.

4. CHANNEL CODING AND SERVICE MULTIPLEXING

4.1. Channel coding

Fig. 6 illustrates the basic channel coding approach used in the FMA2. Standard services, with a BER requirement of BER = lo-) are convolutionally coded (rate 1/3 or 1/2) with constraint length 9. Outer Reed- Solomon coding is applied to services with lower-BER requirements (BER = 10"). It is also possible to apply service-specific coding only, e.g. for speech codecs with multiple bit-classes with different error-sensitivity. In that case, the FMA2 air-interface does not apply any error correction. In the performance evaluation 113 rate coding was used for speech service as it results to lower EJN, requirements and the number of code channels

Bitsl 1

lnterleaver 1

Repetition Modulation . PCCH

Vol. 9. NO. -I July - A L I ~ U S ~ 1998 31-9

available is not a limiting factor for capacity with the low rate services, such as speech. For the high rate services the resulting spreading factor and amount of code channels available becomes more important and therefore 1/2 rate coding was used there as the inner coding.

Also the study for more advanced coding schemes is in progress within FRAMES, to study the possibilities of using coding schemes other than convolutional coding. Among the studied solutions are both rate compatible punctured convolutional codes [6] and turbo codes [7].

4.2. Frame Control Heading coding

The Frame Control Header (FCH) indicates the transmission rate for the current rate on the PDCH. The coding principle for FCH is biorthogonal coding, where mapping to Walsh functions takes place. The coding for the 6 bit FCH is mapping to biorthogonal Walsh functions of length 32 which represent the 64 different Val- ues for the FCH. The FCH data is interleaved and multiplexed over the entire PCCH frame. For multiple variable-rate services, the FCH indicates the rate for each service. As the FCH is interleaved over the whole frame, the PCCH frame needs to be detected first before PDCH can be decoded.

The actual code rate for the FCH depends on the amount of different combinations of transmission rates to be supported with services as a subset of the maximum of 64 code words which can be used when fewer FCH val- ues are needed. The supported number of different data rates or their combinations is defined by a higher layer protocol during initial service negotiations at connection set up and may be adjusted with service negotiation when a new service is added or the properties of the current service are changed during connection.

Slotted ALOHA technique with the random access burst structure shown in Fig. 7. Before the transmission of a random-access request, the mobile terminal should carry out the following tasks:

- Achieve chip, code synchronisation, and frame synchronisation to the target BS.

- Retrieve information from BCCH about the random access code(s) used in the target cellhector.

- Estimate the downlink pathloss, which is used together with a signal-strength target in the terminal and base station sector loading information to calculate the required transmit power of the random access request.

After the random access procedure is completed, a dedicated transit channel is set up. A transit channel is a channel with fixed parameters such as data rate. This channel is used for service negotiation or for a transmission of a short packet. No separate access channel is used for packet traffic related random access, but all traffic shares the same random access channel. More than one random access channel can be used if the random access capacity requires such an arrangement. The performance of the selected solution is presented in [8].

Spreadin Factor = 2% Spreading Factor = 128

-4 * ..._._____...._.......... - ....... 1 Preamble I MSID I ~ q . s e ~ i c e j shortpacket [q 32 bits 16 bits 3 bits __.._..._.___._______...........

+ 9,". - Fig. 7 - Structure of FMA2 random-access burst.

6. PACKET ACCESS 4.3. Rate matching

After channel encoding, there can be an almost arbi- trary bit rate, depending on the number of parallel services, their instantaneous rates and channel-coding parameters. To match this to the limited number of the PDCH bit rates (16 . 2k kbit/s or 160 . 2k bitdframe), unequal repetition coding (or puncturing) is used as illustrated in Fig. 6. Puncturing is used if the reduction in the bit stream from puncturing is less than 20%, otherwise repetition is done to reach the next available PDCH bit rate.

5. RANDOM ACCESS

The need for a fast and efficient random-access scheme is more important for a UMTS proposal, when compared to 2-nd generation mobile-communication systems. The reason is the expected increase in packet- transmission, for which fast and efficient random access is a key requirement.

The FMA2 random access scheme is based on a

As indicated in the previous section, efficient packet access is a key feature of the future mobile-communication systems. Because of the very varying characteristics of packet data traffic in term5 of packet size and packet intensity, a dual-mode packet-transmission scheme has been chosen for the FMA2. With this scheme, packet transmission can either take place on a common fixed rate transit channel or on a dedicated variable rate channel, where the choice of mode is selected based on the packet traffic characteristics.

Fig. 8 illustrates the M A 2 packet access approach for the uplink case. Small infrequent packets are appended directly to a random access request with the transmission power level selected based on the open loop power control. For larger packets, or packets with a high intensity, a dedicated packet channel is set up. This dedicated packet channel is then active until it is explicitly released or a certain time-out time T,,,,,, has passed. For dedicated packets channels fast power control is used.

The downlink packet scheme is the same as the uplink scheme except that small infrequent packets are

330

Random-Access User Small Infrequent Request Packet

Random- Access user Request Packet

Common Channel Large or Frequent .

Random-Access User Request Packet

Packets

Dedicated Channel - : Link Maintenance

Fig. 8 - FMAZ uplink packet access.

appended to a Forward Access Request instead of a Random Access Request.

7. ADDITIONAL FEATURES

7.1. Support of adaptive antennas

Adaptive antennas is also a potential technology for achieving the high capacity and range gains needed for the extensive use of high-rate services in UMTS, while having wide coverage. Current commercial DS-CDMA standards do not support downlink adaptive antennas because of the use of a common pilot code for downlink coherent detection. On the other hand, with the use of connection-dedicated pilot bits, the FMA2 concept allows the use of adaptive antennas on both uplink and downlink. To rninimise overhead when adaptive antennas are not used, the pilot symbols in the downlink are inserted on the PCCH only when actually using adaptive antennas, otherwise channel estimation is done based on the pilot channel.

The connection dedicated pilot symbols are needed when the user data is transmitted via varying antenna radiation pattern and the common pilot channel is transmitted with a fixed antenna radiation pattern, typically either using a sectorised or an omnidirectional antenna. As the antenna patterns are not the same then the channel estimate derived from the common pilot cannot be used anymore and thus pilot symbols are needed to facilitate coherent detection. The achievable capacity with adaptive antennas has been studied in [9].

7.2. Support of advanced receiver structures

It can be expected that in the first phase of FMA2 deployment, conventional RAKE receivers will be used. However, to enhance the capacity and range performance, more advanced receiver structures should be developed, e.g. different types of Multi-User Detectors (MUD) [ 101 and interference-cancellation schemes. The complexity of such advanced receiver schemes is, in many cases. significantly reduced if short spreading and scrambling codes are used. For that reason, the FMA2 air-interface may operate in a short code mode, where the uplink long code

is removed and the scrambling is done only with the 256 chip short code. For the case when the conventional RAKE receivers are used,.the optional long code is included for the uplink and the system achieves all the benefits of long-code scrambling, such as improved interference averaging. Studies on different MUDS in fading channels have been carried out in the project [ l 11. The potential for the uplink capacity performance enhancement with MUDS has been studied in [12] and in [ 131 for the range performance enhancement.

Also the capabilities of downlink advanced receivers are being studied in the FRAMES project. In the downlink the viable solutions are the adaptive, single-user detectors which do not require the knowledge of the spreading codes used by other users. Such schemes have been studied for example in [14, 15, 161. The single-user detectors are critical in their convergence and therefore benefit from the FMAZ downlink solution where the base-station specific scrambling codes are short with 256 chips. Overview of the of the different possible methods can be found for example in [ 171 and [ 181 and the references therein. In [ 191 the BER performance of the conventional RAKE receiver and the LMMSE receiver was studied in the FMA2 downlink.

7.3. Support of intra- and inter-frequency handovers

For a high capacity cellular system efficient handovers are needed to reduce the interference among cells. In the FMA2 the intra-frequency handover is a soft handover, where a mobile receives data from more than one base- station combining the transmissions from different sources with the RAKE receiver. To allow soft handover with asynchronous base stations the mobiles measure the timing difference among different base-stations with the help of the synchronisation words on the synchronisation channel. If the downlink would have long code sequence then naturally the respective information could be obtained from the long code phase as well, but then usage of better receiver solutions in the mobile would be more difficult.

When establishing a connection to a new base-station, the timing of the base-station on a symbol level is adjusted to allow combining in the RAKE receiver on symbol level. Different users in the downlink are thus

Vol. 9. NO. J July - August I998 33 I

symbol but not frame synchronised and retain the orthogonality among them achieved with the use of the code three structure as the adjustment period corre- sponds to the longest spreading ratio (256) used.

The inter-frequency handover is supported in a way that a MS is able to carry out measurements on other carriers during connection. For FMA2, two techniques have been considered, namely dual receiver solution and slotted mode solution. The dual receiver has additional receiver branch which carriers out measurements on other carrier during a connection while with slotted mode for part of the frame the transmission is periodically stopped for the MS to perform measurements on other carriers.

Both solutions have their own benefits. The dual receiver approach fits very well together to the case when the mobile has antenna diversity while the slotted mode is more suited for the case of a simple mobile terminal where a single receiver RF chain is desirable. Thus, the slotted mode in FMA2 is for speech and low rate data terminals without antenna diversity, while the more advanced high rate terminals with antenna diversity shall use dual receiver solution. The different slotted mode options have been studied in [20].

Tap I Indoor A

8. PERFORMANCE

Outdoor To Indoor A

Extensive performance evaluation has been carried out on the FMA2, both for use within the project and to be used for the ETSVSMG2 evaluation in the UMTS air- interface selection process. Four different channel-models have been assumed for the link-level simulations, Indoor A, Outdoor to Indoor A and Vehicular A and B [211. The mobile speeds for the two former models have been 3 km/h and for the Vehicular A model 120 k d h . With Vehicular B mobile speeds 120 kmlh and 250 k d h were tested to demonstrate the performance with larger delay spread and velocity as well. The receiver used was a conventional four-branch RAKE receiver in the Vehicular

1

2

3

4

5

6

~

Delay Aveg. Power Delay Aveg. Power (ns) (dB) (ns> (dB)

0 0 0 0

50 -3 .O 110 -9.7

110 -10.0 190 -19.2

170 -18.0 410 -22.8

290 -26.0

310 -32.0

channel and a two-branch RAKE in other channels, no MUD or other advanced receiver solutions were used in the evaluation. The channel models are described in Table 1 and Table 2, where Indoor A channel taps had flat Doppler spectrum while other channels had classicrtl Doppler spectrum for modelling the fading of channel taps. The link level results given for the different service are simulated against Additive White Gaussian Noise (AWGN) in different channels, while in the system level simulations the interference between mobiles and base stations has been modelled.

Table 1 - Vehicular channel model delay profiles 1211

The E,/N, value includes all overhead from pilot bits, power control commands and also the effect of actual transmission power has been taken into account due to fast 800 Hz rate power control with 1 dB step size used in the simulations. The effective code rate including the channel encoder code rate is defined as

ki R c = , where ki is the information bit rate and n is the number of bits from the channel encoder unit including rate matching. As the modulation used is dual channel QPSK with branches having different power levels, the data channel is thus treated separately as a BPSK channel with the constellation size M being 2. Now as the overhead from the PCCH has to be included in the definition, the factor F is defined as

(3)

and the effect of power control is introduced to scale the average energy of a data symbol given by

F = Edata

Edntn + Eoverheod

E ( ld" , I2 ) (4)

with the real value a,, which is measured during the simulation run. The overhead in the case of dual channel QPSK is easily defined from the power ratio between I I I I I I

332 ETT

PCCH and PDCH as all the overhead except one from channel encoding is carried on PCCH.

If the discrete representation of the channel is presented by vector h(kab) modelling the multipath channel per diversity branch, then the average symbol energy over diversity branch kd is given as

Vehicular B, 250 kmh

( 5 )

6.0 I 8.2

Thus the total average received symbol energy per bit is given by

Direction

Antenna diversity:

The noise power in the receiver input is defined as in [22] , given by

Uplink Downlink

Yes No

where 3, is the user bandwidth and No/2 is the double sided spectral noise power density. Thus the average Eb/No is given by

384 kbitsls Vehicular A, 1 1 1 2 0 k d h

No log, (M) . RC . F 9 No

3.1 15.6

The Eb/No per antenna diversity branch is then given as (Eb/Ka)/Np

8.1. Speech pelfonnance

The speech-service simulations assume an 8 kbitsls speech codec with a BER = 10-3 requirement. Interleav- ing over two frames (20 ms) has been used, but longer

4.096 4.096

Convolutional code rate

1 Rate matching I 9/10 I 9/10 I Interleaver

PCCH - PDCH power ratio (dB)

Table 4 - Speech service link level results

Indoor office A. 3 kmh 3.1 16.4

Outdoor to indoor and 3.3 16.7

Vehicular A.120 km/h

1 Vehicular B, 120km/h I 4.9 I 7.7 I

inter-frame inner interleaving can be applied to services for which longer delay is ,allowed. The simulation parameters for the speech service are listed in Table 3 and the EJN, results are given in Table 4.

8.2. High rate data pelformance

The results presented here are for 384 kbitsls and 2 Mbitsls data services with the quality target requirement of BER = 10-6. In addition to the convolutional coding also Reed-Solomon coding with additional outer interleaver has been used and larger interleaving depth ( 1 20 ms) has been applied because of less constraints on the delay requirements. The EbINo results are given in Table 5 and the simulation parameters are listed in Table 6. In the high bit rate link level simulations, the downlink modelling has been identical to the uplink modelling without antenna diversity in terms of parameter selection.

8.3. Performance conclusions

From the performance studies it can be concluded that all the UMTS requirements can be provided with the FMA2. The bandwidth of 5 MHz is even sufficient to provide the 2 Mbitsls, which was studied in the Indoor channel models, because although the resulting spreading

Table 5 - 384 kbitds and 2 Mbirs/s link level performance results

2.1 15.1

3.0 16.0

VOI. 9, NO. 4 July - August IY98 333

Table 6 - Data service simulation parameters ~~ ~

Service

Channel type

Antenna diversity:

Chip rate (Mchipds)

PDCH Spreading factor

Reed Solomon Code

Convolutional code

Rate matching

Interleaver

PCCH Spreading factor

PCCH - PDCH power ratio (dB)

384 kbitds 2 Mbitsls

Indoor A, Outdoor 3 kmlh &

120 km/h

Yes/No Yes/No

4.096 4.096

1 x 4 5 x 4

1921240 192/240

1 / 2 1 /2

603/640 20 1 /200

120 ms 120 ms

256 256

-I0 -10

to Indoor A Vehicular A 3 M

factors are small, the Indoor channel model is basically a one tap model as the other taps are greatly attenuated.

The 384 kbits/s and 2 Mbits/s services show clearly better performance than the low rate speech service. In addition to the better coding this is also because of the easier channel model which due to the low mobile speeds and fast power control, especially in the uplink, approaches the non-fading AWGN channel performance level. Also the overhead due to pilot and power control is significantly smaller compared to the speech service with 8 kbits/s data rate, where overhead from the PCCH is in the order of 1.5 dB.

FMA2 uplink performs better than downlink due to the antenna diversity in the uplink and since the power control in the uplink can and must use higher dynamics and thus is able to compensate the fading in the case of slow moving mobiles. In the downlink more restrictions to the dynamics limits the ability for tracking deep fades in such environments where little diversity is available. In the downlink the total power control dynamics must be limited to around 15 to 20 dB depending how much of the orthogonality between different parallel channels is main- tained in different environments. The effects of such limi- tations are not that much visible in the link level simulations but rather on the system level capacity simulations.

The use of either antenna diversity also in the ‘downlink and/or more advanced receiver solutions needs clearly to be considered especially if the downlink is expected to be the direction with the highest load. Although the exact UMTS services are not exactly known today, with the services such as WWW-brows- ing, the uplink traffic load can be only a fraction of the

downlink load. The other side of such enhancements for the terminal is naturally the resulting complexity.

The resulting capacity figures for the speech service from the ETSI evaluation are ranging from 80 to 190 kbits/s/MHz/cell and for the data services from 85 to 250 kbits/s/MHz/cell. A great deal of details and results in the performance aspects of wideband CDMA can be found in [23].

9. NETWORK ASPECTS

The FMA2 has been designed to facilitate fully asynchronous network operation. The motivation for the asynchronous operation has been:

- Independence from the external systems, such as GPS or other external clock signals via radio bea- con etc. Thus easy deployment in locations where external timing reference not available, such as in underground facilities.

- No time domain consideration in the form of code phase design in the cell planning

- Micro cell deployment in the presence of high rise buildings can be done without concern for available satellite visibility.

10. CONCLUSIONS

This paper has presented the FMA2 Wideband CDMA solution developed within the European ACTS FRAMES project. The FMA2 physical layer service multiplexing and the multinte concept has been designed to be able to respond to the service needs of UMTS, especially varying transmission capability and simultaneous service operation. Packet data based traffic functionality is assured with efficient random access mechanism and with the defined channel structure supporting packet based tnffic.

The flexible system deployment with the asynchronous network operation and support for hierarchical cell structures has been taken into account in the system design with the FMA2. For the future capacity expan- sion schemes such as multi-user detection and interference cancellation as well as the adaptive antennas have been made possible.

The work in finalising the FMA2 concept is progress- ing at the moment with interaction with the UMTS stan- dardisation activities in ETSI SMG2. The FMA2 was major contribution to the ETSI SMG2 wideband CDMA concept group, which was selected as a basis for the UMTS air interface for paired frequency band (FDD band) by ETSI and will form the FDD part of the ETSI IMT-2OOO contribution to ITU.

The main parameters of the FMA2 are summarised in Table 7, where the parameters given are valid for the ETSI wideband CDMA with the exception that the ETSI wideband CDMA uses pilot symbols instead of

334 ETT

Table I - FRAMES FMA2 parameter summary

Duplex method

Basic chip rate

1 FRAMESFMA2

FDD

4.096 Mchipsls

Frame length

Multi-rate I variable-rate

Coherent detection UL

DL

Inter-BS synchronisation 1 Asynchronous ~

10 ms

Multi-code + variable spreading

factor

Pilot symbols Pilot

channellsymbols

Layer 1 control (Power control commands, pilot bits etc.)

I-Qlcode multiplexed

Associated control (ACCH)

pilot code for both transmission directions and in the downlink direction layer 1 control information is time multiplexed instead of code multiplexed.

Time multiplexed (inband)

Acknowledgements

This work has been performed in the framework of the project ACTS AC090 FRAMES, which is partly funded by the European Union. The authors would like to acknowl- edge the contributions of their colleagues from Siemens AG, Roke Manor Research Limited, Ericsson Radio Systems Al3, Nokia Corporation, Technical University of Delft, University of Oulu, France Telecom CNET, Centre Suisse d’Electronique et de Microtechnique SA, ETHZ, University of Kaiserslautern, Chalmers University of Technology, The Royal Institute of Technology, Instituto Superior Ticnico and Integxacion y Sistema.

Manuscript received on December 18, 1997.

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Vol. 9. NO 4 July -August 1998 335

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Frames FMA2 wideband-CDMA for UMTS