Wcdma Hspa and the Evolution of 3g2231

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TheevolutionofWCDMAandthe3rdgeneration

cellularcommunicationsystemsReviews of articles within the course TSDT74

Amanda Hasselberg, amaha829, 820302-4982

Linus Lund, linlu992, 820610-1910


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The

evolution

of

3G

The Wideband Code Division Multiple Access (WCDMA) technique is used in the 3rdgenerationmobile communication systems (UMTS). Since the first release of the standard R99 numerousimprovements have been done. This paper intends to present some basic principles of techniquesused along the evolution of the WCDMA standard and to glimpse somewhat towards the futureof the 3G long term evolution. Shown in Figure 1are the highlights of advancements within the

WCDMA standards and also the aspects that we will focus on.

Figure 1: The developing of WCDMA

WCDMA

R99

The original forerunners of what now is called WCDMA are military system that were designedto be wideband and employing direct sequence (DS) techniques to get multiple access capability.

The forerunners of the present CDMA and WCDMA were designed in the late 1980s when thecommercial sector became interested in cellular-type communication. The first type of WCDMAspread over 5 MHz and the WCDMA in the future will spread over either 10 or 20 MHz.(Milstein, 2000, p 1345)

WCDMA is a wireless communication based on and developed from Code Division MultipleAccess (CDMA). CDMA is a technology where all users use the same frequency and transmit atthe same time, separated by unique codes. ("Basic concepts in WCDMA Radio Access Network"2001, p 4) WCDMA uses a wide radio signal at 5 MHz and a chip rate at 3.84 Mcps. The basicconcept is to multiply the radio signal with a spreading signal which constructs a signal that seemsto be random. The receiver will then, with the help of a specific code, reverse the process and theoriginal signal will appear.

The WCDMA needs to possess several functions to bee able to control the radio network andtheir users. Some of the functions that are essential for WCDMA are described below.

Power

control

The power control regulates the transmission power of the terminal and the base station. Thisaims to lessen interference and enable more users on the same carrier. ("Basic concepts in

WCDMA Radio Access Network" 2001, p 5) The power control in WCDMA is used in both up-link and down-link to ensure good performance. The aim of power control is to establish goodquality over the time by ensuring that the power the base station receive from all handset are thesame regardless of the distance from the base station. In WCDMA a Signal-To-InterferenceRatio based (SIR-based) power control is used. This means that the base station compares thereceived SIR with a SIR target value and on the basis of that demands the transmitter to increaseor decrease the power. The fact that the received quality of the signal compares with the requiredquality of the signal results in a closed-loop power control. (Dahlman 1998, p 1111) A higherpower from one handset than needed causes an excessive quality, unnecessary interference andtakes too much resources meanwhile too low power will cause poor quality. The power control is

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a way to escape this and keep the power at a suitable level and thereby also get a suitable qualityover the time. ("Basic concepts in WCDMA Radio Access Network" 2001, p 5)

Another effect of the power control is the phenomenon cell breathing. Cell breathing is thetrade-off between coverage and capacity and it means that the cell differs in size depending onthe number of users. A low number of users can achieve good quality even at long distance

meanwhile a high number of users creates a high interference level and the users therefore needto get closer to the base station to get good quality. (Basic concepts in WCDMA Radio AccessNetwork" 2001, p 5)

Softandsofterhandover

There are two kinds of handover that can be used by WCDMA, soft and softer handover whichare shown in Figure 2. The one normally used in WCDMA is soft intrafrequency handover.Intrafrequency handover is a handover between WCDMA carriers on different frequencies thatoccurs in high capacity areas. (Dahlman 1998, p 1112) Soft handover means that the handsetsimultaneously is connected with two or more cells on the same frequency on two or more base

stations. Softer handover is a special case of soft handover when the handset is connected, notonly with two or more cells, but with two or more cells at the same base station. (Basic conceptsin WCDMA Radio Access Network" 2001, p 5)

Figure 2: Soft and softer handover (Basic concepts in WCDMA Radio Access Network 2001, p 5)

The purpose of using soft or softer handover is that it enables the handset to maintain the qualityof the connection while moving between the cells. The handset will adjust its power to the basestation that requires the lowest transmitted power and the best cell to be used can thereby change

very fast. (Basic concepts in WCDMA Radio Access Network" 2001, p 5)

To determine which cell to connect to the user equipment uses a cell-search technique. The cell-

search technique can be described as follows. The first step is to find the base station with thestrongest cell. Thereafter, the user equipment determines the code group and the frame timing tobe used and the step after that is to identify all the scrambling codes to the code group. After thisthe broadcast on the channel can be read. The user equipment always searches for new cells touse but it only searches through neighbouring cells broadcast from the network. In soft handoverthe uplink signals are combined in the network while the downlink signals are combined in theuser equipments RAKE receiver. For the softer handover the combining can instead be done inthe base station and it thereby gets a more efficient uplink combining. To eliminate the numbersof user equipments in soft and softer handover the thresholds are not absolute in WCDMA butrelative. (Dahlman 1998, p 1112)

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Admissioncontrol

The admission control is a way to avoid reduction of the cell breathing and to avoid overloadingthe system. The interference will always increase when admitting a new call and to prevent thecoverage to decrease when the interference increases admission control is used. (Dahlman 1998,p 1115) The admission control works by either admitting or denying the new call depending onthe load of the network. (Basic concepts in WCDMA Radio Access Network" 2001, p 6) If theload is high and a risk for call dropping occurs the new call will be denied in order to keep thealready ongoing calls stable. The admission control is used in both uplink and downlink becauseof the systems capability of serving different services with different claims for capacity andquality. (Dahlman 1998, p 1115)

Congestioncontrol

Even with use of an admission control, is it possible for the system to be overloaded. This ismainly caused by user equipment that moves from one area to another. (Basic concepts in

WCDMA Radio Access Network" 2001, p 6)The congestion control is activated when the

threshold of the congestion is exceeded. In case of an overload there are four actions that can betaken which all aim to degrade the quality of the users in the overloaded cell until the congestionis solved. (Dahlman 1998, p 1115)The first step for the congestion control is to reduce the bitrate of services that are insensitive for increasing delays. If this doesnt work the next step to dois an interfrequency handover i.e. move some users to other less loaded frequencies. The thirdstep, if interfrequency doesnt help, is to move some users to GSM and if thats not enough thelast step is to remove some of the connections in aim to keep the quality on the remainingconnections. (Basic concepts in WCDMA Radio Access Network" 2001, p 6)

WCDMArelease5

HSPA is an abbreviation for High Speed Packet Access and was introduced in the downlink withWCDMA release 5, then called High Speed Downlink Packet Access (HSDPA) and presentedmore thoroughly below.

HSPDA

Traditional cellular systems have typically allocated resources in a relatively static way, where thedata rate for a user is changed slowly or not at all. This is efficient for services with a relativelyconstant data rate such as voice. However for high-speed data access data typically arrives inbursts, posing rapidly varying requirements on the amount of radio resources required. Together

with the low delays required for good end-user experience a fast allocation of shared resources ismore efficient approach (Parkvall 2006, p 68).

HSDPA solves this by introducing a shared channel, called the High Speed Downlink SharedCHannel or HS-DSCH. The HS-DSCH corresponds to a common channelization code resource,shared primarily in the time domain. An illustration of this is found in Figure 3where the HS-DSCH resource consists of a number of codes of spreading factor 16, the other codes are usedfor other purposes e.g. voice services (Parkvall 2001, p 28).

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Figure 3: The HS-DSCH (Parkvall 2001, p 28)

The HS-DSCH is as mentioned above primarily shared in the time domain and allocation of theresource is done on a 2 ms transmission time interval (TTI). A short TTI brings some advantagessuch as reduced over-all delays, improved possibility to track channel variations used by the linkadaptation and makes channel-dependent scheduling easier (Parkvall 2006, p 69). It is however

possible to use code multiplexing to allow more than one user transmitting data over the HS-DSCH if for example one user cannot fill the entire channel due to radio conditions (Parkvall2001, p 28). This is illustrated in Figure 4where in the first slot User #1 transmits using the

whole bandwidth but shares the next time slot with User #4.

Figure 4: Sharing within time slots (Parkvall 2007, p 69)

There are some architectural impacts using a HS-DSCH. The HSDPA technique relies on rapidadaptation which requires functionality such as scheduling and retransmissions to be placed inthe Node B (the base station). These functions were earlier handled by a radio network controller(RNC) which in its turn handled a number of Node Bs. In other words the base station must bemore complex than in networks not supporting HSDPA (Parkvall 2001, p 28). As a direct effectfrom the architectural impacts soft handovers are not possible with the HSDPA scheme becauseinter-node-B soft handovers are impossible. Since the HS-DSCH is not power controlled but ratecontrolled it is possible to allow the power remaining after serving other channels, such as voice

channels, to be used for HS-DSCH transmissions. This enables efficient use of all the availablepower for a base station (Parkvall 2006, p 69) and has been implemented by Ericsson (Bark,WCDMA/HSPA, 2007).

The HS-DSCH controls the Eb/N0ratio by adjusting the data rate while keeping transmissionpower constant, which is also know as link adaptation. The link adaptation is implemented byadjusting the channel-code rate selecting between e.g. QPSK and 16-QAM, where 16-QAMmakes more efficient use of bandwidth but requires higher received Eb/N0. Hence when aterminal is within good radio conditions the link adaptation features selects the higher-ordermodulation and vice versa. To provide node B with information about the channel conditionseach terminal transmits a channel quality indicator (CQI) containing a recommended data rate asoften as every 2 ms making the data rate independently selectable in every TTI (Parkvall 2006, p

70).

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The demands on short TTIs and fast channel adaptation makes the schedulervery important itdetermines to large extent the overall performance of the downlink (Parkvall 2006, p 70). Thescheduler is responsible for selecting which user is to send at what data rate in every TTI(Parkvall 2001, p 30). The scheduler can actually make use of one of the greatest problems withinradio communications the channel fading. Since radio conditions for the users typically vary

independently there is almost always a user, in each point of time, whose channel quality is nearits peak. By using this channel-dependent schedulingand schedule the users with good radio conditionsto transmit one obtains larger gains with larger channel variations, effectively making fadingdesirable (Parkvall 2006, p 70). This is illustrated in Figure 5where the dotted line represents thechannel as seen by node B which is a rather good channel effectively rendered by theindependent variations of User 1 through 3.

Figure 5: Channel-dependent scheduling (Parkvall 2006, p 71)

However if only the terminals with good radio conditions are allowed to transmit there is apotential for large variations in the service quality among the user population (Parkvall 2001, p30). A practical scheduler strategy needs to maintain some degree of fairness between the users,but in principle the larger the long-term unfairness the higher cell-capacity (Parkvall 2006, p 70).So there is a trade off between fairness and channel utilization and there are different strategiesavailable for a scheduler, e.g. one implemented by Ericsson is called Proportional Fair where thechannel is assigned to the user with best relative channel quality (WCDMA/HSPA, 2007).

The last feature of the HSDPA technique is the hybrid ARQ with soft combiningwhich allows theterminal to rapidly request retransmissions of erroneous transport blocks the terminal reports asuccessful or failed decoding of a block within 5 ms from reception (Parkvall 2006, p 70).Incremental Redundancy (IR) is used as basis for soft combining meaning that the

retransmissions may contain parity bits not contained in the original transmission (Parkvall 2001,p 30). IR is well known to be able to provide significant gains when the code rate for the initialattempt is high, thus IR is mainly useful in band limited situations, e.g. when the terminal is closethe Node B and the number of channelization codes limits the achievable data rate.

WCDMArelease6

With the 6threlease of the WCMDA standard two features were introduced, namely theEnhanced Uplink (EUL) and Broadcast/Multicast (MBMS). These two concepts are presentedbelow.

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EUL

The Enhanced Uplink (EUL) tries to achieve reduced delays, increased availability of highbitrates and an increased capacity. Low delays are critical to numerous applications, especially

TCP based applications where the higher the bitrates offered the lower the delays must be inorder for TCP to fully benefit from the bitrates. A lower delay in the uplink will also benefit thethroughput in the downlink when regarding TCP performance as it is dependent on an end-to-end round trip time (Parkvall 2005, p 1411). The implementation of the EUL is based on thesame principles to those of the HSDPA, but with a number of differences that will be presentedbelow (Parkvall 2006, p 71).

The EUL uses an enhanced dedicated channel (E-DCH) that is non-orthogonal. Hence fastpower control is necessary to handle the near-far problem. Within the uplink the shared resourceis no longer channelization codes but rather the maximum tolerable interference, which dependson the combined power resource of all terminals, further emphasizing the ability to perform fastpower control (Parkvall 2005, p 1411).

In order to handle interference generated from neighbouring cells soft handover is supported to

allow power control from multiple cells. This also allows reception at multiple cells providingmacro diversity gains (Parkvall 2005, p 1412). The HSDPA required certain functionality to belocated in the Node B, but since duplicate detection and macro-diversity combining is required inthe EUL some of this functionality is moved to the radio network controller (RNC).

Similarly to HSDPA EUL uses a hybrid ARQ to provide robustness against unpredictableinterference but also to receive an early termination gain a kind of implicit link adaptation. Toexplain the implicit link adaptation consider a terminal that wants to transmit with a data rate of xMBit/s. The terminal may do so by transmitting x MBit/s using a transmission power to target alow error possibility. But it may also do so by transmitting using an n time higher data rate withan unchanged transmission power and use multiple hybrid ARQ retransmissions that means

deliberately transmit with a higher probability of error. It can be shown that the latter caseactually averages a lower Eb/N0(Parkvall 2005, p 1413).

The amount of common uplink resources a terminal uses depends on the required transmissionpower, which usually follows from transmitting at a higher data rate. Packet data is typicallybursty with large and rapid variations in their resource requirements. The scheduler is stillresponsible for determining which users should transmit when and at what data rate, but nowfocuses on allocating a large fraction of the shared resources to the users momentarily requiringthe highest data rate. The scheduler also has to make sure that the system is stable by avoidinglarge interference peaks (Parkvall 2006, p 71).

Unlike the downlink scheduler, the uplink scheduler typically schedules multiple users to transmit

in the same time slot due to the fact that most terminals are not able to use all the available powerwithin one slot. The scheduler algorithm is not standardized but the framework forcommunication is. This framework is based on scheduling grants, sent by Node B, andscheduling requests sent by the terminal to request resources. There are two different schedulinggrants used to set the upper limit on the data rate the terminal may use. The absolute grant istransmitted on a dedicated channel and are used for large infrequent changes, e.g. at the time forbearer setup. The relative grant is transmitted on an individual control channel and tells theterminal whether its transmitting with too low, too high or sufficient power. When a terminal iscommunicating with several Node Bs one of them is the serving cell. The terminal only listens tothe serving cells absolute grant and only the serving cell may ask the terminal to increase itstransmitting power. The other cell may however tell the terminal to decrease its power if its

interfering with their transmissions (Parkvall 2005, p 1414).

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The scheduling requests are emitted by the terminal and contain its current status, includinginformation on buffer status, traffic priority and power availability. These requests are sent on theE-DCH and are allowed to be sent even when terminal lacks a scheduling grant (Parkvall 2005, p1414).

Broadcast

and

multicast

Broadcast and multicast differs from the traditional cellular point-to-point communication in thesense that one transmission should be receivable by many users. Traditional broadcast servicessuch as radio and TV have focused on covering very large areas and offered a very limitedpossibility to target individual users, but in WCDMA release 6 these two concepts are combinedin a single network (Parkvall 2006, p 72).

Multicast and broadcast are in principle equal the same content is transmitted to multiple usersin a unidirectional way. There is a difference between them though. Broadcast serves all userssubscribing to the service simultaneously and users need not to notify the network beforereceiving the service. The service is distributed via a point-to-multipoint radio service set up in

each cell as a part of the MBMS broadcast area (Parkvall 2006, p 72). Multicast on the other handcould be set up both as point-to-multipoint radio service as well as a point-to-point radio service.When a user wants to take part in a multicast transmission it has first to request to join amulticast group and after being granted this request the users movements are tracked and radioresources are adjusted appropriately. In cells where only a few users are subscribing the service itmight be more appropriate to use point-to-point transmissions whereas if the cell has a largenumber of users the point-to-multipoint approach is better (Parkvall 2006, p 72).

The point-to-point transmission is handled over a HS-DSCH as described above and the point-to-multipoint transmission uses a forward access channel (FACH). Multiple services may betransmitted on the same channel using time multiplexing or transmitted through multiplechannels. In neither case no information is fed back from the terminal, the communication is

strictly unidirectional (Parkvall 2006, p 72).

The transmission of one signal to multiple receivers leads to an efficient use of radio resourcesthrough macro diversity. Since no adaptation of parameters to specific users conditions ispossible the service coverage is determined by the worst case scenario and it is of key importancefor MBMS services to maximize diversity without relying on feedback and with limited power.

The two main techniques used for this is a long 80 ms TTI to obtain time diversity andcombining multiple cells to obtain macro diversity. The long TTI will make the transmission lesssensible for fast fading and since MBMS services are not delay sensitive this will not be a problemfrom the users perspective (Parkvall 2006, p 73).

There are two different ways to obtain macro diversity by combining different transmissions,

namely soft combining and selection combining. The soft combining combines soft informationbits before decoding and provides diversity gains of 4-6 dB. However the soft bits from eachradio link needs to be buffered until the whole TTI is received from all transmitting links and thismay pose a problem since many WCDMA networks operate on different levels of synchronism.If the transmissions are synchronized within approximately 80 ms soft combining is possible,otherwise selection combining will be used (Parkvall 2006, p 73-74).

Selection combining gives less diversity gains than soft combining in the order of 2-3 dB. Theprinciple is to decode each signal received and for each TTI select one of the correctly decodedblocks. When using the strategy it sufficient to buffer the information bits from each link and, forlarge asynchronism, requires smaller buffers. The terminal will be informed about the level ofsynchronism and can select strategy accordingly (Parkvall 2006, p 73-74).

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The

future

of

3G

The 3G long term evolution (LTE) specifies a number of goals it wishes to achieve, among thoseare (Ekstrm, 2006, p 38)

High data rates more than 100 MBps downlink and 50 MBps uplink peak data rate and

improved cell-edge throughput

Low latency less than 10 ms round trip time

Spectrum flexibility enabling deployment in different spectrum allocations

High spectral efficiency

The developing of the 3G LTE is an ongoing process and many of the techniques introducedhave been considered for the 4thgeneration (Ekstrm 2006, p 39). A few of these techniques willbe touched upon briefly below.

It is proposed that the number of nodes along the data path should be reduced by substituting

entities like the radio network controller (RNC) and the GPRS support node (GGSN) with asingle central node called the access core gateway (ACGW). This reduction of nodes wouldreduce overall protocol-related processing and the number of interfaces which yields shorter callsetup times (Ekstrm 2006, p 39).

Further it is proposed that a classification scheme should be used to classify each packet with aquality of service (QoS) indicator. This indicator would then effect strategies concerning queuing,bandwidth allocation, scheduling and dropping of packets (Ekstrm 2006, p 40-41).

Orthogonal Frequency Division Multiplexing (OFDM) is an attractive choice to manage radioresources since it allows for a smooth migration from earlier radio access technologies, has aflexible spectrum allocation and performs well in frequency-selective channels. It is also possible

to operate in different spectrums by varying the number of subcarriers while keeping thesubcarrier spacing unchanged. This will effectively support transmissions in spectrum allocationsof 1.25, 2.5, 5, 10 and 20 MHz (Ekstrm 2006, p 41).

The proposed downlink structure is to use a subcarrier spacing of 15 kHz and time slots as shortas 500 s. The proposal intends to take the exploitation of channel variations used through linkadaptation with HSDPA one step further and also allow fast adaptation of parameters in thefrequency domain. The use of frequency domain adaptation can give large performance gains incases where the cannel varies significantly over the system bandwidth. The scheduler is nowresponsible, based on feedback information about the downlink channel quality, to schedule

which user should transmit when, at what frequency or frequencies and at what data rate. Theprinciple is illustrated in Figure 6(Ekstrm 2006, p 42).

Figure 6: Downlink and uplink principle structure (Ekstrm 2006, p 42, modified)

The uplink structure needs to allow for power efficient user-terminal transmissions to maximizecoverage. Therefore the preferred structure is to use a single-carrier FDMA, which can be seen in

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Figure 6. The base station scheduler assigns a unique time-frequency interval where the terminalmay transmit and thereby ensuring intracellular orthogonally. Primarily time domain scheduling isused, but it is possible to use frequency domain scheduling for terminals with limitations intransmitting power or the amount of data awaiting transmission. Frequency domain adaptation istypically not used as each terminal cannot continuously transmit a pilot signal covering the entire

frequency domain (Ekstrm 2006, p 42).In order to achieve high coverage, capacity and data rate it is necessary to consider multi-antennatechnologies as a well integrated part (Parkvall 2006, p 42-43). For example to achieve a highpeak data rate a multi-layer transmission could be used, to achieve a good coverage beam-forming antennas are suitable and to achieve a high capacity a combination of the two mentionedis suitable (3G Long Term Evolutions (LTE), 2007).

Conclusion

This paper has presented the basic principles of the WCDMA R99, the first WCDMA release. Ithas then covered some of the advantages made in release 5 and release 6 to increase data rates

and base station efficiency. Finally it has looked upon the current standardization work for 3GLTE, which is an ongoing project where we probably will see more efficient solutions emergingas we approach the 4thgeneration of cellular communication systems.

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References

Bark, Gunnar . 3G Long Term Evolution (LTE). Ericsson Research. Linkping. 28 Nov. 2007.

Bark, Gunnar . WCDMA/HSPA. Ericsson Research. Linkping. 28 Nov. 2007.

Basic Concepts of WCDMA Radio Access Network. (2001). Retrieved December 2, 2007, fromhttp://www.ericsson.com/technology/whitepapers/e207_whitepaper_ny_k1.pdf

Dahlman, E. Beming, P. Knutsson, J. Ovesjo, F. Persson, M. Roobol, C. (1998). WCDMA TheRadio Interface for Future Mobile Multimedia Communications. IEEE Transaction On VehicularTechnology, 47(4), 1105 1118

Ekstrm, H. Furuskar, A. Karlsson, J. Meyer, M. Parkvall, S. Torsner, J. (2006). TechnicalSolutions for the 3G Long-Term Evolution. IEEE Communicationz Magazine, 44(3), 38 45

Milstein, Laurence B. (2000). Wideband Code Division Multiple Access. (2000, August). IEEEJournal on selected areas in communication, 18(8), 1344 1354

Parkvall, S. Englund, E. Lundevall, M. Torsner, J. (2006). Evolving 3G Mobile Systems:Broadband and Broadcast Services in WCDMA. IEEE Communications Magazine, 44(2), 68 74

Parkvall, S. Dahlman, E. Frenger, P. Beming, P. Persson, M. (2001). The High Speed Packet DataEvolutions of WCDMA. 2001 12thIEEE International Symposium on Personal, Indoor and Mobile RadioCommunications, 2, 27 31

Parkvall, S. Peisa, J. Torsner, J. Sagfors, M. Malm, P. (2005). WCDMA enhanced uplink principles and basic operation. 61stVehicular Technology Conference, 3, 1411 1415

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Wcdma Hspa and the Evolution of 3g2231