Preparations beyond HSDPA

With 3G failing to deliver on the data side with its promises of 2-Mbps data rates, W-CMDA operators needed a fix to their R99 (Release 99) systems and needed it fast.  Not only was there the pressure of cdma2000 operators taking business away, there was (and still is) the pressure felt from the impending launch of WiMAX systems, also providing broadband data rates.

The answer from 3GPP was Release 5 featuring HSDPA, which offers several significant improvements for R99 networks, and network operators pushed for its implementation before the ink on the Release 5 specification was barely dry.  Furthermore, the operators are now pushing for the uplink portion of the fix, Release 6, featuring HSUPA (high speed uplink packet access).
HSDPA is tagged as being a software update.  But this is just like Microsoft's Vista OS being just a software update.  Many operators will find that if their network isn't 'HSDPA ready', then they will need to update/expand the hardware capabilities to be able to handle the added data volume.

Testing will be more than a software update as well.  When integrating system components or load testing HSDPA/HSUPA-capable UTRAN devices, manufacturers and operators alike will face an increase in the data traffic to be analyzed by one order of magnitude compared to R99-based systems.

But independent of whether adding HSDPA is merely a software update or a software/hardware update, the benefits for the operators are several fold.  The headline spec for HSDPA is that it will support peak data rates of 14.4 Mbps in one cell, e.g., nine video clips can be sent via HSDPA in the same time required to send one on R99.  While realistic data rates for HSDPA may actually only be a few megabits per second or less, the actual quantity and number of users achieved in a cell will improve significantly over R99 networks.

For example, instead of limiting high-speed data access to fewer than five users in a cell, HSDPA can deliver 384-kbps rates to up to possibly 30 users.  However, test engineers need to be careful to not treat HSDPA as just a faster Release 99.  There are key differences between the two, and these differences need to be considered closely.

As its name implies, HSDPA is meant for downlink data traffic only.  For uplink data traffic improvement, HSDPA is needed to bump up the maximum theoretical data rate to 5.6 Mbps. HSUPA gains much of its data-rate improvement by defining a new transport channel - E-DCH (enhanced dedicated channel).  Though adding HSDPA capability to a network creates more than a few testing challenges, adding HSUPA will not impact testing requirement as severely.

Testing the testers

The introduction of HSPA, i.e., the common operation of HSDPA and HSUPA, is not the last step in the evolution chain, however. WiMAX and other technologies are keeping the pressure on for further improvements.  In response, 3GPP is developing a specification labeled UTS LTE (UMTS long-term evolution) with the improvements required to keep network operators competitive for applications such as broadband mobile radio or mobile TV.  The UMTS LTE specification is not scheduled to be finalized until September 2007; however, it is now stable enough for designers to begin work on system components.

UMTS LTE's goal is to achieve data rates of up to 100 Mbps in the downlink (an increase in capacity of three to five times compared with HSDPA in the same bandwidth) and up to 50 Mbps in the uplink.  Like WiMAX, UMTS LTE will use OFDMA (orthogonal frequency division multiple access) technology for the downlink.  This switch from W-CDMA to OFDMA will be a big change at the very lowest level of the radio communications portion of a system and achieving synchronization will be a major challenge.  For the uplink, UMTS LTE will use SC-FDMA (single carrier frequency division multiple access technology).  Plus UMTS LTE is adding MIMO antenna technology to the mix.


With UMTS LTE scheduled to be launched in 2009, test and measurement companies are already providing testing solutions for the designers.  For example, at the 3GSM World Congress last month, Rohde & Schwarz introduced first-time solutions for analyzing and generating UMTS LTE signals in the downlink and in the uplink by using the R&S FSQ signal analyzer and the R&S SMU200A signal generator.

Agilent Technologies showed off EDA software solutions for 3GPP system and circuit design, featuring dedicated libraries for UMTS LTE in connection with its signal generators and analyzers to perform prototype verification and interference analysis.  Anritsu also displayed equipment.
Aeroflex says it is designing a pair of test products:  the Aeroflex TM500 LTE and the Aeroflex 6401 LTE.  These products are targeted at supporting physical layer testing of networks and mobile devices.  With the switch from W-CDMA to OFDMA, developers will need to see into the very lowest layers of the radio modem to diagnose the actual cause of a synchronization problem rather than just knowing that synchronization has failed.  These products should help gain that visibility.

Though OFDMA is a known technology (it is currently being used in WiMAX and WLANs), it is not known how to implement the technology in cellular networks and how the technology will really work in an real-world environment.  As a result, there is not yet an agreed basis for test specifications.

UMTS operates up to 3 GHz; OFDMA operates up to 8.  So right away, it's obvious that test instruments will require higher frequency support.

Technically, OFDMA operates using a large number of sub carriers to transport the data.  The data traffic is split up into 512 or 1,024 subcarriers across the frequency spectrum.  The benefit is that the system can pick the subcarriers with the best characteristics to send the data.  This is complemented with MIMO - beam forming, active antennas with the smarts to dynamically react to application service profiles.  The result is a very dynamic operating environment.  In other words, a very complex measurement environment for test equipment.

Test equipment suppliers need to figure out what are the important factors in OFDMA signal behavior, interfaces, etc. in a cellular network and start to develop new protocols and new ways to characterize and manage networks.

So while some delegates on the UMTS LTE committee feel that the September 2007 date for finalizing the specification is ambitious and could slip, there is clear agreement in the industry that LTE product development is happening fast and is happening now; thereby, putting additional pressure on test equipment vendors.

Beyond conformance testing

Release 99 is largely built on the traditional CDMA principle of separating users in the code domain.  Each user is assigned a dedicated physical channel (DPCH).  The network operator manages the users in a cell by using a fast power-control technology to ensures that each user gets a fair share of resources and allows for maximum cell capacity.

While this configuration works well for moderate data rates, it doesn't do a good job of supporting high-data-rate users.  The R99 network must allocate a large portion of the code space to the high-data-rate users, which diminishes the available cell capacity for other users.


Data traffic is often bursty.  That is, you may download a video file at a very high data rate, but then not need the data link for a significant period of time after that.  In other words, the physical-layer resources in a network may not be in use all the time.  From the operator's point of view, this is an inefficient use of resources.  Unfortunately, R99 doesn't do a good job of quickly reallocating resources to support the instantaneous demands of the users in a cell.

HSDPA changes the rules of the game within a cell.  First, the network allocates one big, fat pipe for high-speed data and the remaining cell resources for voice and low-rate data traffic.  The big, fat pipe is known as the HS-DSCH (high-speed downlink shared channel) and is shared among multiple users.  This is different from the concept of each user assigned to a dedicated physical channel.

HSDPA then uses three new technologies to optimize the resources available in the big, fat pipe.  These include adaptive modulation and coding (AMC), including a choice of two modulation schemes - 16QAM and QPSK; a protocol to handle retransmissions and to guarantee error-free data transmission; and a fast packet scheduling algorithm, which allocates the pipe's resources such as time slots and codes to different users.

For these three technologies to do their jobs, some of the functions needed to be moved to different locations within an R99 network.  Specifically, some functions, previously found in the RLC protocol layer and the serving RNC are moved down into the MAC protocol layer as well as into the base station, node B.  But because there's no standard to define how the node B implements scheduling, an effective test plan requires a certain amount of flexibility.

The 3GPP standards community has defined RF and protocol test cases for HSDPA, providing a basis for RF and protocol performance testing..  However, several key aspects of HSDPA are outside the scope of traditional conformance testing.  To design a truly comprehensive HSDPA test plan, the real-world environment and network interoperability scenarios must be factored in as well.

And the resulting testing needs to move beyond conformance testing only.  Network operators are nervous about putting new handsets on their networks.  If services aren't delivered as promised, it's the operator that gets blamed, not the handset supplier.

On the other side of the fence, cdma2000 operators faced a similar scenario when introducing EV-DO services to their basic 3G networks.  Two cdma2000 handsets may have both passed the conformance test, but they often varied greatly in performance and in the demands that they made on network resources.  Engineers found that the test standards weren't as comprehensive as needed.  The early revisions of the new standards were based on visible needs and were written without the benefit of real-world experience.

History is repeating itself.  The complexity of the HSDPA standard is far greater than the basic R99 protocol.  The gap between just-meeting-the-conformance-test requirements and performance-to-customer expectations has widen as well. Network operators need to develop additional performance testing requirements to minimize the chance of bad customer experiences with the latest 3G handsets.