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TEMS News No. 1, 2006

Figure 1. Basic principles for HSDPA

Shared channel transmission Dynamically shared in time & code domain

Higher-order modulation 16QAM in complement to QPSK for higher peak bit rates

Short transmission time intervals (2ms) Reduced round trip delay

Fast hybrid ARQ with soft combining Reduced round trip delay

Fast link adaptation Data rate adapted to radio conditions on 2 ms time basis

Fast radio channel dependent scheduling Scheduling of users on 2 ms time basis

User #1

Figure 2. Shared channel transmission

Channelization codes allocated for HS-DSCH transmission 8 codes (examples)

Shared channelization codes

TTI

User #2 User #4

User #3

SF=1

SF=2

SF=4

SF=8

SF=16

High Speed DownlinkPacket Access (HSDPA)

Anders Wnnstrm, Ph. D., provides a technical overview of HSDPA, the next step in the evolution of WCDMA.

By Anders Wnnstrm

HSDPA has been an integral part of 3GPP (3rd Generation Partnership Project) since early 2002. It is the first step of WCDMA evolved (the next step is an enhancement of the uplink). HSDPA is improving the downlink in the following ways:

Higher bit rates up to 14 Mbps Higher capacity two to five times

increase in downlink capacity Reduced delay leading to

shorter response times and other improvements, for interactive ap-

plications. The basic principles used for HSDPA are summarized in Figure 1 below.

Part I of this article discusses the basic principles in more detail, and Part II indicates how the TEMS product portfolio can help in planning and optimizing of HSDPA.

Shared channel transmission The HS-DSCH (High Speed Down-link Shared Channel) is, as the name indicates, shared among all users that are using HSPDA for their in-

teractive/background radio access bearer (see Figure 2 below).

This shared transport channel can be mapped onto one or several physical channels (also known as codes) all

using spreading factor 16. Users can take turns on a 2 ms time basis (the significance of this will be clear lat-er), but typically one user gets many consecutive TTIs (Transmit Time In-tervals). Since code multiplexing is expected to be rare, it is assumed in the rest of this article that sharing is only in the time domain.

Each code (physical channel) is called an HS-PDSCH and has one of two formats (Figure 3 on next page).

With a channel bit rate of 960 kbps (for 16 QAM) and fifteen codes used,

next step

in the evolution

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Figure 6. Short 2 ms transmission time interval (TTI)

10 ms20 ms40 ms80 ms

Earlier releases

Release 5 (HS-DSCH) 2 ms

Figure 4. HS-DSCH Power Utilization

3GPP Release 99 3GPP Release 5Power

Power usage with dedicatedchannels

HS-DSCH with dynamic power allocation

No need for extra spectrum/carrier Voice and data on same carrier

t

Unused power HS-DSCH (rate controlled)

Tota

l cel

l pow

er

Dedicated channels (power controlled)

Common channels

Power

Tota

l cel

l pow

er

Dedicated channels (power controlled)

Common channelst

Figure 5. Higher-order modulation (16QAM)

2 bits 4 bits3.84 Mcps 3.84 Mcps

Spreading Spreading

QPSKmodulator

16QAMmodulator

16 QAM may be used as a complement to QPSK

16 QAM allows for twice the peak data rate compared to QPSK

16 QAM more sensitive to interference

=> Higher data rate in good radio channel conditions (high C/I, little or no dispersion, low speed) i.e. close to cell site & micro/indoor cells

An HS-PDSCH may use QPSK or 16QAM modulation symbols. In the figure above, M is the number of bits per modulation symbols i.e. M=2 for QPSK and M=4 for 16QAM. The slot formats are shown below.

Figure 3. The HS-PDSCH (High Speed - Physical Downlink Shared Channel)

Slot #0 Slot #1 Slot #2

DataNdata1 bits

1 subframe: Tf=2 ms

Tslot= 2560 chips, M*10*2k bits (k=4

Slot format Channel Channel SF Bits/HS- Bits/Slot Ndata #1 Bit Rate Symbol DSCH (kbps) Rate (ksps) subframe

0 (QPSK) 480 240 16 960 320 320 1 (16QAM) 960 240 16 1920 640 640

the maximum possible physical channel bit rate that can be achieved becomes 15*960=14400 kbps. In practice this is not equal to the user payload rate since many of those bits are overhead due to, for example, channel coding.

If, for example, five codes are used, then the maximum physical chan-nel bit rate is equal to 5*960 = 4800 kbps. Due to overhead this corresponds to a (maximum) pay-load rate of 4.32 Mbps (still including e.g. TCP/IP headers) or 4.536 Mbps (adding RLC header sixteen bits) or 4.605 Mbps (adding MAC-hs header) and so on.

(The moral of this little excursion is that it is possible to quote many dif-ferent bit rate values depending on the choice of measurement point and coding overhead).

The possibility to achieve very high bit rates using this structure depends primarily on two features of HSDPA: efficient power utilization and time multiplexing rather then code multi-plexing. Figure 4 below explains how power allocation to HSDPA is performed.

The shared channel can be given all the remaining carrier power (after com-mon control channels and any R99 channels have taken what they need). This makes it possible to provide very

high C/I (signal-to-interference) values to a single user (in the absence of code multiplexing the user doesnt have to share this high C/I with any other user) and hence achieve very high peak rates (the user perceived throughput depends on how often the shared resource is available, a problem discussed later).

Higher order modulation Since it is possible to achieve such high C/I values the capacity of the HS-DSCH can easily be exhausted i.e. due to the channel structure it is not possible to send at higher bit rates even if the radio environment allows it. Therefore a higher order modulation scheme is introduced in HSDPA (see Figure 5 below) that enables even higher bit rates to be achieved under favorable circum-stances.

Although used as a complement to QPSK, 16 QAM is therefore manda-

tory if really high bit rates are to be achieved.

Short transmission time interval The transmission time interval (TTI) is 2 ms (see Figure 6 below).

This enables some HSDPA features to operate at up to 500 times per second. These key features are:

Fast hybrid automatic repeat re-quest (H-ARQ) with soft combin-ing

Fast link adaptation Fast (radio channel dependent)

scheduling

Making these features so fast has implications for the implementation since these functions must be run-ning in the base stations and not in the radio network controller.

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Figure 7. Fast hybrid automatic repeat request (H-ARQ) with soft combining

Conventional ARQ (discard erroneous transmission)

Hybrid ARQ (store erroneous transmission and soft combine)

Transmitter

Receiver

NAC

K

ACK

ACK

NA

CK

AC

K

AC

K

NA

CK

NA

CK

Transmitter

Receiver

NAC

K

NAC

K

High code rate

Figure 8. Link adaptation (note that high code rate little coding and vice versa)

Low code rate

High data rate

Figure 9. Fast channel-dependent scheduling

Low data rateTime

User 1Scheduleduser

User 1

User 2

#1 #1 #1 #1#2 #2 #2

What may happen in this situation?

Fast hybrid automatic repeat request (H-ARQ) In H-ARQ the terminal is required to store soft information of blocks that are not decoded correctly. In Figure 7 on this page, the difference between traditional retransmission schemes and H-ARQ is illustrated.

The advantage with H-ARQ is that the terminal has a higher probability to decode the second transmission since the first transmission is stored and reused/combined in the new attempt. This implicit link adap-tation can for example be used to reduce the C/I requirements for op-eration at the same block error rate. In order to not put too much demand on the storage capacity of the termi-nal it is important that the scheme is fast and hence implemented in the base station. It is also important to combine H-ARQ with link adaptation despite the shortened TTI, since beginning at an inappropriate coding/modula-tion scheme could otherwise lead to very long delays under poor radio conditions.

Fast link adaptation Radio channel conditions are chang-ing very rapidly. In traditional WCDMA (R99) this is mitigated by using very fast inner loop power control to en-sure that all users always have the C/I they need for their service. This means that if the radio channel is poor high transmit power is neces-sary, but if the radio channel is good then much lower transmit powers are needed.

In HSDPA, however, the data rate is adjusted so that at good radio conditions the data rate is high (the user data rate is high, the amount of overhead/coding is lower) whereas in poor radio conditions the data rate is lower (the user data rate is low, the amount of overhead/coding is higher) (see Figure 8). Remember that each HSDPA user will be given all avail-able power, unless the maximum rate the terminal can deal with is already achieved at a lower power.

Indeed, rate control is more efficient if varying rates are acceptable.

Link adaptation is implemented by adjusting the channel coding rate the modulation scheme used Note that link adaptation can track very fast variation in the radio chan-nel since the TTI is only two milli-seconds.

But how does the base station know how to adapt the coding and modulation schemes? To put it an-other way, how does the base station know the radio channel conditions at the terminal? Well, the terminal is required to report channel quality information (CQI) on a two millisec-onds basis, information that is used to assess which rate can be used for sending1.

Fast (channel-dependent) scheduling In order to utilize the shared resource in an optimum way, it is important to implement fast scheduling algorithms in the base station. Figure 9 below illustrates two different scenarios.

The base station needs to determine which user can use the shared re-source. As can be understood from Figure 9, if the base station always gives the resource to the terminal with the best C/I, then the average rate used on the shared channel is maximized (and hence the cell ca-pacity, often called the cell through-put).

1 This is a simplification indeed, the ter-minal reports the requested rate taking its capabilities and the radio channel condi-tions into account.

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However, since this may lead to some rather unfair situations where some terminals are very unlikely to be receiving any data whatsoever, this capacity maximizing schedul-ing algorithm is normally adjusted (in Figure 9 compare the two user 1 cases). It becomes fairer by allow-ing the probability to get access to the resource to be proportional to the waiting (queuing) time. In this man-ner, even terminals with very poor C/I can get access to the shared channel and, even if not a very good average throughput, at least a non-zero one.

It should be clear that random (or take-turn) scheduling doesnt give any performance benefits (this scheduling is in fact rather poor from a cell capacity point of view). However, it should be equally clear that intelligent scheduling only gives performance benefits in a cell where there are more then one user ask-ing for the shared resource. But in a case with many users, since the cell throughput increases with intelligent scheduling, for most users also the perceived user bit rate is increasing since they are sharing a larger bit pipe (compared to a case with many users in the cell but no intelligent scheduling).

There are a quite a number of chal-lenges facing operators who want to implement HSDPA in their networks, including issues involving traffic man-agement. However, we will here fo-cus on some performance related challenges.

Peak rates The peak rate a terminal can expect to achieve (and hence of course also a user) depends on two things: (1) the capability of the terminal (there are twelve different categories of terminals with different capabilities when it comes to bit rates (in turn depending mainly on the number of codes and modulation schemes supported)), and (2) the available C/I of the HS-DSCH..

While we will not be elaborating more on the terminal capabilities, it is worthwhile to discuss C/I further. The C/I available depends on the radio environment and the transmit carrier power available, which in turn depends on the amount of R99 users (if HSDPA shares carrier with R99 users) and the power allocation to common control channels.

Figure 10 below shows a typical C/I distribution assuming non-zero R99 traffic.

Not surprisingly, the plot shows that high SIR is obtained close to the base station antenna.

By choosing the relevant terminals it is straightforward to convert the C/I distribution into a bit rate plot. Operators who study this output can learn in which areas high peak rates can be expected and in which areas the peak rate is expected to be low. One example is given in Figure 11 below.

Of course, the achievable bit rates closely follow the C/I distributions.

Cell capacity However, peak rates are not enough (they correspond to the terminal per-ceived throughput if the terminal is the only HSDPA user in the cell). It is equally important to acquire an understanding of the capacity (cell throughput) of a given cell, in par-ticular for cells that are expecting to have a lot of HSDPA users. The cell capacity indicates the size of the bit pipe the users are going to share, but it depends on user distribution. One example is found in Figure 12 at right. To emphasize an important point again the size of the bit pipe de-

pends on the user distribution since users in favorable positions obtain higher peak rates. Indeed, with only one user connected to a cell, the size of the bit pipe of that cell cor-responds to the rate that particular user can achieve. With many users simultaneous connected (sharing the HS-DSCH) the cell capacity depends also on the scheduling algorithm em-ployed

Impact on existing services Any increase in load (power con-sumption) will impact the coverage on existing services. In a well-planned/tuned network, the introduction of HSDPA is not expected to have any effects that cannot be overcome by some simple parameter adjustments. Still, it is re-assuring to perform the predictions to see the impact and anticipate the changes necessary rather then just allow them to be discovered.

Figure 13 at right shows the pilot signal-to-interference ratio when only speech is available.

As can be seen (note the scale to the left only the non-orthogonal part of the interference is included), the pilot coverage is good in this relatively low traffic scenario. However, if the traffic

Figure 10 Signal-to-interference ratios for HS-DSCH Figure 11 Achievable bit rates for category six terminals

TEMS News No. 1, 2006

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Figure 15 TEMS Investigation data window for HSDPA

RadioEnviron-ment

DSCHCurrentThrough-put

Figure 12 Cell throughput Figure 14 Pilot coverage when more traffic has been generated

Figure 13 Pilot signal-to-interference ratios

increases (here by adding HSDPA users, but of course it could be any traffic increase), the amount of down-link interference is also increased and suddenly the pilot coverage is (al-most) jeopardized in a few places as illustrated in Figure 14 (note that the color coding is different).

This reduction in coverage can be accounted for by allocating more power to the pilot channel. How-ever, this can also be an indication that some areas may need re-tuning; in other words, an improved design may be called for.

Field measurements As a complement to planning, field measurements in trial systems can be used to obtain an understanding of the anticipated performance of HSDPA. Of course, field measure-ments are also used after deployment in order to verify performance and as input to further tuning activities. See the TEMS Investigation window in Figure 15 for an example.

Different throughput measurements are readily available as well as in-formation relating to the channel quality the terminal is experiencing

and reporting. As mentioned, these field measurements are valuable in-dicators of the subscriber perceived performance. Note, however, that the performance of schedulers can only be investigated under a high load of many users.

ConclusionFor the operator implementing HSD-PA, it is imperative to understand not only if HSDPA will have any impact on on-going services but also what the performance of HSDPA is expected to be. Only then can the proper ac-tions be taken regarding the both

the network itself, and the develop-ment of business plans (what rates can be offered to their customers, etc.). Although there are many chal-lenges associated with implementing HSDPA, the TEMS portfolio offers planning and optimization solutions that will ease the process.