LTE White Paper

3GPP Long-Term Evolution: fit or flawed?


Technical White Paper Technical White Paper April 2007 April 2007

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Contents 1 Introduction 3 2 Broadband Anyone? 4 3 Technology Race 4 4 LTE Technology 5

4.1 Evolution? 5 4.2 Downlink PHY 6 4.3 Uplink PHY 7 4.4 MAC Layer 8 4.5 MIMO 9 4.6 Enhanced Base Stations 9 4.7 Performance 10 4.8 Timelines 10

5 Evolutionary Lifeline 11 5.1 History Lessons 11 5.2 Predicting the Future 11 5.3 Right Technology, Right Time 12 5.4 picoArray™ Solution 13

6 Conclusions 15 References 17 Glossary 17

Executive Summary With LTE, UMB and WiMAX vying for supremacy in the race to mobilise data, it is no wonder that most observers are left scratching their heads.

LTE, or Long-Term Evolution, is the next big technology push for the 3GPP technologies, following closely after High-Speed Packet Access (HSPA). It is a response to the growing wireline broadband business, but the timing is all about countering the new mobile technology: WiMAX. With first LTE deployments penciled in for 2010, the pressure is on to bring it to market and achieve a 1000-fold increase in data throughput in 10 years – all without increasing end-user cost.

This paper takes a closer look at the LTE technology and asks the question, is it fit for purpose? On paper at least the answer is yes, and when coupled with the equivalent enhancements to HSPA, also referred to as HSPA+, the evolutionary path is clear.

The latest enhancements in the base station, which include a new PHY using Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple-Input, Multiple-Output (MIMO) antenna techniques, exceed the capabilities of today’s technology at the right price point. As before, technology and engineering ingenuity have a habit of catching up, but the clock is ticking.

A technology lifeline does exist: a dense array of hundreds of processors on one or more devices can replace existing baseband designs and offer a software-based solution. Second-generation devices and software reference designs meet the technology needs for HSPA and WiMAX today. With next generation devices planned for 2008 that will target LTE and MIMO, it is now down to the ingenuity of the baseband and RF engineers to develop the algorithms and products that will change the future of mobile data.

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1 Introduction Nobody can say that the world of mobile technology is dull. Relentless maybe, or even exciting depending on your involvement. For the consumer it must appear confusing, and that is before anyone has even mentioned to them what the industry has in store.

The truth is that acronyms and industry jargon mean very little, which leaves the network operators with the task of branding and marketing new technologies to create an emotional appeal: Vodafone Live!, Orange World and O2 Active to name but three.

Acronyms do slip through, especially on written phone specifications. Not easy for the consumer – remember WAP. And if technologies in the same family cause bewilderment, what will happen when new technologies such as WiMAX come along?

“The challenge? Achieving up to a 100Mbps data connection to a mobile device. 3GPP, the body responsible for GSM and its successors, has responded with Long-Term Evolution (LTE) plans for its 3G technology”

Parallels can be drawn with the DVD industry and the emergence of two competing formats for storage of high definition (HD) films. On the one hand is the modestly named HD DVD. On the other is Blu-ray Disk. Each is backed by big industry names, but does the consumer really care? All they want is a low-cost solution that performs. Past experience suggests that most will sit back and wait until the dust settles.

The mobile world is more tolerant of multiple standards, but the challenge to the industry is keeping pace with the broadband data bandwagon. The two leading mobile technologies have been battling it out for more than a decade now. With its genesis in Europe, the GSM family has gone from strength to strength to become the leading mobile standard around the world. With its roots in the US, CDMA has held its own and is the leading standard in some countries.

While vying for supremacy with each other, both have been forced to face the broadband data challenge sooner than anticipated by a new mobile technology: the IEEE 802.16e specification under the WiMAX banner. The challenge? Achieving up to a 100Mbps data connection to a mobile device. 3GPP, the body responsible for GSM and its successors, has responded with Long-Term Evolution (LTE) plans for its 3G technology, WCDMA and High-Speed Packet Access (HSPA). The CDMA camp has come up with Revision C of its 3G Evolution-Data Optimised (EV-DO) standard, recently branded as Ultra Mobile Broadband (UMB).

“This paper takes a closer look at the technology being considered for LTE and discusses whether it is up to the task.” LTE versus UMB versus WiMAX. This paper takes a closer look at the

technology being considered for LTE and discusses whether it is up to the task. It also looks at the challenge of developing solutions for multiple standards and introduces a baseband processing technology that could offer developers an alternative to the mainstream ASIC, DSP and FPGA route.

• Section 2 sets the scene by introducing the coming mobile data age

• Section 3 briefly outlines the leading mobile data technologies

• Section 4 focuses on the 3GPP Long-Term Evolution (LTE) technology

• Section 5 talks about the challenges facing baseband developers and presents a potential technology lifeline

• Section 6 concludes with some final thoughts for mobile operators and manufacturers facing some tough technology decisions



2 Broadband Anyone? In the good old days, everything had a specific job to perform. This may have led to a cumbersome array of electronic gadgets, but at least the customer knew what they were getting: a phone, a camera, a personal digital assistant and a music player. Not anymore. One device can do all of this and tell you where the nearest petrol station is located.

To the purist, there is the inevitable argument that all of this integration must compromise performance. Although most people probably don’t even notice, if they did they would say that it is a small price to pay for the convenience.

“Spurred on by declining voice revenues, competition from fixed line providers and the market potential for data, the mobile industry can at last offer megabit performance to a mobile device.”

The same is happening in home entertainment, with digital now firmly in the vocabulary of the average consumer. First it was the telephone followed by the music system and then the radio, and now it is the turn of television. Integration will follow and, if communications providers have their way, it will all be delivered over a single broadband connection on the Internet.

Spurred on by declining voice revenues, competition from fixed line providers and the market potential for data, the mobile industry can at last offer megabit performance to a mobile device. In Europe this is the HSPA enhancement to the 3G WCDMA standard [1]. In the US it is the EV-DO enhancement to the 3G CDMA2000 standard.

This is not bad given the vagaries of the mobile environment and the physical limits on performance. But already this is seen as insufficient to meet the perceived market needs in the future. The fixed line world is ahead, with the latest DSL technology planned for 2007 offering symmetrical 100Mbps connections over existing copper wires.

The 3GPP response for the 2009/2010 timeframe is aggressive [2]:

• Scalable bandwidth from 1.25MHz up to 20MHz

• Peak downlink data rate of 100Mbps in 20MHz bandwidth

• Peak uplink data rate of 50Mbps in 20MHz bandwidth

• 3-4 times the spectral efficiency of HSDPA in a loaded cell

• 2-3 times the spectral efficiency of HSUPA in a loaded cell “The fixed line world is ahead, with the latest DSL technology planned for 2007 offering symmetrical 100Mbps connections over existing copper wires.”

• User plane latency of < 5ms and control plane latency < 20ms

To put this into perspective, it equates to a 1000-fold increase in data throughput in 10 years without increasing end-user cost. So what technologies are available?

3 Technology Race Race might not be the right term since it is not a winner-takes-all event, but the accelerated development of the three main competing mobile technologies to meet the long-term goals is remarkable. This is highlighted in Figure 1. Put simply:

1. GSM evolution starting with GPRS before moving to EDGE and WCDMA, and now HSPA before the next big step to HSPA+ and LTE

2. CDMA evolution starting with CDMA2000 1xRTT before moving to EV-DO Revision A and Revision B before the next big step to Revision C (also known as UMB)



3. WiMAX revolution starting almost from scratch with a fixed wireless access specification (IEEE 802.16d) before developing a mobile version (IEEE 802.16e)

Figure 1: Evolution of mobile radio technologies [3]

The long-term goals for each are very much the same: all-IP networks with 100Mbps peak on the downlink and 50Mbps peak on the uplink. Unsurprisingly, LTE, UMB and WiMAX are adopting a similar air interface technology – Orthogonal Frequency-Division Multiplexing (OFDM) – and claim comparable performance [3].

Worldwide, mobile networks are almost exclusively divided between the GSM and CDMA technology families, with GSM having the largest share. The two have evolved from their 2G digital voice origins early in the 1990s to embrace the coming age of mobile data with their 3G offerings. While the network and air interface technologies have had to evolve, each retains a common thread, balancing support for legacy systems with the need for change.

“Unsurprisingly, LTE, UMB and WiMAX are adopting a similar air interface technology – Orthogonal Frequency-Division Multiplexing (OFDM) – and claim comparable performance.”

WiMAX is the newcomer, unconstrained by legacy systems and with an emphasis on data applications from the start. An enviable position, and one that has arguably stimulated the two incumbent 3G standards to quickly respond. The following section describes LTE in more detail, highlighting the many changes that are planned along with the close links to HSPA.

4 LTE Technology

4.1 Evolution? It could be argued that evolutionary changes should come as incremental steps to a successful technology rather than complete re-designs. But when faced with fierce competition, survival instincts kick in and more radical measures are required.

Survival for the wireless players in the communications industry means keeping pace with the fixed line service and the multimedia applications that are driving it forward. The latest DSL technology is aimed at supporting triple-play (voice, television and data) services over a single connection. Offering 100Mbps on the uplink and downlink, very-high bit rate DSL 2 (VDSL2) is an upgrade to existing DSL infrastructure and is set for deployment in 2007.

With voice revenues declining, wireless data offers salvation to mobile network operators, but requires a major technology shift. One is already



happening: the adoption of a packet-switched infrastructure and IP-based networks as the world moves towards an Internet-driven economy. This change is more gradual, and will hardly be noticed by mobile consumers.

The most tangible difference will be the raw data performance defined by the throughput and latency of the wireless connection: 100Mbps downlink in a 20MHz bandwidth with less than 5ms of latency.

One approach would be to scale the existing 5MHz air interface either as four contiguous 5MHz channels or four times the chip rate in a single 20MHz channel. While theoretically possible, the determining factor is handset complexity.

The result is a two-pronged strategy:

1. Continuous improvement to HSPA in 5MHz channels (HSPA+)

2. Long-Term Evolution (LTE) in channels from 1.25MHz up to 20MHz

“Sometimes referred to as HSPA+, the goal is to enhance HSPA technology to meet LTE performance targets in a 5MHz channel.”

The first is a commitment to the existing 2G/3G infrastructure and the investment made to date, but also provides an upgrade path to LTE in the future. Sometimes referred to as HSPA+, the goal is to enhance HSPA technology to meet LTE performance targets in a 5MHz channel. This includes both the core and radio networks, easing the transition to LTE. The following sections focus on the LTE technology, its performance and expected timelines.

4.2 Downlink PHY Orthogonal Frequency-Division Multiplexing (OFDM) is the preferred physical layer (PHY) technology for a number of reasons:

• Support for different channel bandwidths

• Improved performance in frequency selective channels

• Frequency domain adaptation

• Support for Multiple-Input, Multiple-Output (MIMO) processing

OFDM works by splitting data into a large number of lower bit-rate streams that are modulated onto individual subcarriers. Downlink modulation schemes include QPSK, 16-QAM and 64-QAM, and subcarrier frequencies are equally spaced by the symbol rate. This separation makes them orthogonal by removing any cross talk. Multiple user access is achieved by allocating different subcarriers or timeslots: Orthogonal Frequency-Division Multiple Access (OFDMA).

By using a 15kHz subcarrier spacing, the clock frequencies are common with HSPA allowing for simpler dual-mode handsets. But it is the relative narrowband 15kHz subcarrier that is the key to the many advantages highlighted above. It is scalable to support different bandwidth allocations, including those highlighted for LTE and others as required up to 20MHz. Each subcarrier is also more robust against multipath fading and the granularity allows channel variations to be exploited by adapting the subcarrier assignments to the prevailing channel conditions. Support for MIMO is discussed in Section 4.5.

“LTE brings a frequency domain component to the processing, which is particularly applicable in frequency selective environments. For example, built up areas with plenty of multipath activity.” Exploiting channel variations by adapting the link and scheduling resources

is already a feature of HSPA [1]. Resources include the modulation scheme and level of error correction, which can be scheduled for multiple users every 2ms. LTE brings a frequency domain component to the processing, which is particularly applicable in frequency selective environments. For example, built up areas with plenty of multipath activity.



LTE also reduces the Transmission Time Interval (TTI) to 1ms to further reduce end-to-end processing delays. The result is a flexible set of resources in both frequency and time domains that can be allocated to multiple users according to the channel conditions and the needs of the data being transmitted. This is controlled by the MAC layer processing, which is discussed in Section 4.4.

A key parameter with OFDM is the guard period between symbols to combat multipath time dispersion and reduce inter-symbol interference. This is termed the cyclic prefix since it uses a repeat of the end of each symbol at the start of the symbol to improve signal reception. For most environments, a cyclic prefix length of 4.8μs is adequate although this can be extended to 16.7μs for environments with much greater time dispersion [4]. This might include support for very large cells with up to 120km radius and broadcast services such as TV. In this case, each base station is synchronised and transmits the same signal. The mobile device can exploit this by treating the signals as multipath from the same source.

One final point to consider is the Peak-to-Average (power) Ratio (PAR) of the transmitted signal. As with WCDMA and HSPA, the push for higher data rate services has increased this up to between 10 and 12dB. It is in the mobile device where PAR is perceived as being more critical since it has a significant effect on cost and power consumption. In a world dominated by cost, the impact of such a high PAR in the base station should not be overlooked. As with WCDMA and HSPA, work should continue to reduce energy use and lower hardware costs through the combination of signal processing to reduce the PAR and more efficient power amplifiers.

4.3 Uplink PHY Concerns over an excessive PAR in the mobile transmitter influenced the choice of technology for the uplink. Rather than using OFDM as on the downlink, LTE uses single-carrier FDMA (SC-FDMA) with dynamic bandwidth allocation. Offering similar performance to OFDM, the single carrier modulation has a lower PAR essential for mobile devices. Studies indicate a 2-6dB reduction [5], which has a huge impact on cost and power consumption.

As with the downlink, time and frequency resources are assigned to each user. Unlike the downlink, adapting the frequency domain allocation to the prevailing channel conditions is unlikely since it is difficult for each mobile to transmit a pilot signal across the complete frequency band. However, there are two frequency-domain scheduling schemes to meet different requirements (Figure 2):

1. Localised FDMA in which the user is assigned a single carrier with varying bandwidth

2. Distributed FDMA in which each user is distributed in the frequency domain to offer increased immunity to frequency-selective fading (multipath)



Figure 2: Uplink frequency-domain scheduling options

“Concerns over an excessive PAR in the mobile transmitter influenced the choice of technology for the uplink. Rather than using OFDM as on the downlink, LTE uses single-carrier FDMA (SC-FDMA)”

A cyclic prefix is also inserted to mitigate multipath effects along with frequency-domain equalisation. Transmission parameters that can be changed include the level of error correction and modulation format. Possible formats include π/2-BPSK, QPSK, 8-PSK and 16-QAM. Assigning and scheduling these resources is discussed further in Section 4.4.

Unlike the downlink, where all the subcarriers for each user are the inputs for an inverse FFT that generates the signal to be transmitted, the subcarrier mapping is performed in the frequency domain. This requires a DFT step as shown in Figure 3. Likewise, at the receiver, there is a corresponding inverse DFT after the frequency domain equaliser to generate the user data stream.

Figure 3: Comparison between downlink OFDMA and SC-FDMA signal paths

“Multiple antennas are an intrinsic part of mobile communications folklore.”

4.4 MAC Layer As with HSPA [1], the availability of a number of degrees of freedom in the PHY to optimise the data connection for each user requires greater sophistication and processing power in the MAC layer. This includes a scheduler for the downlink and uplink connections. A feedback channel on the uplink provides the MAC with channel quality indicators for the downlink.

A hybrid ARQ (HARQ) mechanism is also included, which is part of the overall Radio Link Control (RLC) protocol. As with HSPA, the HARQ processing is in the base station and handles radio transmission errors to and from the User Equipment (UE).

One of the key characteristics of LTE is the mapping of existing IP packets, which are variable in size, onto RLC Protocol Data Units (PDUs). This



simplifies the protocol by removing the need for segmentation and concatenation with fixed-length PDUs. The MAC in the base station can also make more efficient scheduling decisions since it is now handling individual IP packets.

“LTE recognises [the performance gains] and will include sufficient scope in the specifications to make it future-proof and allow different MIMO techniques to be used.”

4.5 MIMO Multiple antennas are an intrinsic part of mobile communications folklore. And this is not just the dual antennas used for receiver diversity in most base stations today. The idea of using multiple antennas in both transmit and receive to increase coverage, capacity and spectral efficiency has been the focus of extensive R&D for at least the last 15 years.

A number of bolt-on products have been tried in 2G networks, but have not enjoyed universal success. The added cost and complexity is one stumbling block, although the main issue is more fundamental: it has never been included in the main specifications. Until now that is.

3GPP has always made provision for both transmit and receive diversity, and the Release 7 specifications will include something called Multiple-Input, Multiple-Output (MIMO) for HSPA that exploits multiple antennas at both ends of the link. LTE will accommodate up to four antennas on the mobile and four at the base station in a 4x4 MIMO configuration.

Instead of an after thought for coverage or capacity reasons, MIMO is at last an integral part of the air interface to support the high data rates required in the future. Various MIMO techniques and algorithms exist. These include spatial division multiple access (SDMA) using adaptive and fixed beamforming, and spatial multiplexing (SM) using each antenna to convey a different data stream [6].

The performance gains are significant and warrant the recent upsurge in R&D activity. LTE recognises this and will include sufficient scope in the specifications to make it future-proof and allow different MIMO techniques to be used. This includes providing a framework for signalling and passing measurements between the base station and the UE. The challenge as always will be managing the cost and size of the added complexity.

“Faced with the need for improved performance and lower cost, LTE will adopt a much simpler architecture with fewer nodes and interfaces.”

4.6 Enhanced Base Stations Until now, the mobile radio network architecture has been split between the Radio Access Network (RAN) and the Core Network. The RAN includes the base station and Radio Network Controller (RNC), with each RNC managing multiple base stations. The Core Network includes the Mobile Switching Centre (MSC) for voice services and Serving GPRS Support Node (SGSN) for data services, with separate gateway nodes providing access to external networks such as the PSTN and Internet.

Faced with the need for improved performance and lower cost, LTE will adopt a much simpler architecture with fewer nodes and interfaces. The result is a reduction in both the processing overhead and latency. The approach is to combine the time-critical functions from the RNC into the base station while integrating all of the routing and internetworking functions to a single node: the access Gateway (aGW) [7]. The architecture change is highlighted in Figure 4. Also note that there is an interface between base stations for those that need to communicate, e.g. to support handovers.

Enhanced base stations (referred to as Node Bs by 3GPP) are already becoming a feature of HSPA networks, performing more of the MAC layer processing and radio resource management. For LTE, the base station will also be designed to support different spectrum allocations from 1.25MHz up to 20MHz. This additional flexibility is backed up by the means to operate in both paired and unpaired spectrum using Frequency-Division Duplex (FDD)



and Time-Division Duplex (TDD) respectively. These can also be combined to allow TDD operation in paired spectrum. By operating the UE in a half-duplex mode, but transmitting and receiving on a pair of frequency channels, the mobile device can exploit cheaper RF filters and operate with a variable frequency duplex separation.

Figure 4: LTE network evolution

4.7 Performance Can the objectives for LTE be met with this new technology? Published simulation results clearly show that LTE with 2x2 MIMO on the downlink, receive diversity on the uplink and enhanced receivers can achieve the 3GPP targets [8]. Interestingly, similar performance is shown with enhanced HSPA using MIMO technology. MIMO techniques are also attractive since they can extend coverage, allowing for greater reuse of existing 2G infrastructure.

“Published simulation results clearly show that LTE with 2x2 MIMO on the downlink, receive diversity on the uplink and enhanced receivers can achieve the 3GPP targets.”

So what is so special about LTE that justifies the investment? At the physical layer it is probably the extra spectrum flexibility combined with the ability to adapt to the radio channel in the frequency domain. However, the changes are more far reaching, extending beyond the physical layer to a new network architecture and enhanced protocols. Since many of these changes apply to HSPA and LTE, both are part of the 3G evolution.

4.8 Timelines The timing of LTE is a direct response to the rise of broadband data use on wired networks, but this is not the whole story and does not completely explain the pace of change: initial deployments planned two to three years from now in 2009/2010. The catalyst is arguably a competing technology called WiMAX defined by the IEEE 802.16e specifications.

Starting off life as a wireless alternative to fixed line connections, the mobile variant for WiMAX quickly took over and has spurred the wireless industry into action. With many similarities to LTE, the first deployments are expected later in 2007 placing it head-to-head with HSPA and EV-DO networks.

The 3GPP response is to continue the development of HSPA, sometimes referred to as HSPA+, while bringing LTE quickly to the fore. In fact, the



overlap between the two technologies is expected to be such that HSPA networks will provide the ideal springboard for LTE in the future.

Figure 5 shows the approximate timelines from the finalised specification to first handsets for 3GPP releases to date along with expectations for Release 7 and LTE. The first observation is that development timelines for each new release are getting shorter. This is probably a combination of the changes being incremental and the pressure to bring products to market faster. What is not so obvious is the associated pressure to start developments earlier in the specification phase, possibly before all have been finalised or frozen. With this comes the added risk of later specification changes and interoperability issues. Adopting a more flexible development platform can reduce the risk, especially one that is more programmable to handle any late modifications. Meeting the aggressive timelines for Release 7 and LTE almost makes this a prerequisite.

Figure 5: 3GPP release timelines to first handsets

5 Evolutionary Lifeline

5.1 History Lessons Telecommunications history is littered with technologies and companies that have held promise, but for one reason or another have not succeeded: too expensive, ahead of their time, overtaken by competition are just some of the epitaphs. Hindsight gives today’s protagonists the opportunity to learn from these hard-fought lessons, but with rich prizes at stake and fierce competition, risks will always be taken.

“Adopting a more flexible development platform can reduce the risk, especially one that is more programmable to handle any late modifications.”

And for the many that don’t make it, there are also the success stories. Ask any venture capitalist and they expect one in ten investments to be big winners along with three or four good earners. Since the mobile data market looks set to provide a rich vein of opportunity, the big questions about what technology and when still need to be answered to avoid early consignment to the history books.

5.2 Predicting the Future At one level there is selecting which air interface technology will succeed. Even though market forecasts are uncertain, it is probably fair to say that the three main technologies highlighted in Section 3 will secure market share, at least in the medium term. It is also a fair assumption that GSM and its successors, including HSPA+ and LTE, will continue their dominance for now.

The bigger challenge for the mobile industry is developing and deploying the infrastructure that will truly unwire communications. Mobile networks today are dominated by large macrocell base stations originally designed to provide a voice service. These networks alone cannot meet the challenges set by high rate, low latency data services.



The evolution of the air interface technologies is part of the solution, and will provide operators with the right tools. But how to put these to best use? For the network planners, it is all about link budget: the maximum path loss that can be tolerated between the transmitter and the receiver for a given service. This translates into the difference between the transmitter output power and the minimum signal level required at the receiver for the service to work.

For data services, the link budget is much lower than for voice and reduces with data rate. Even with a new air interface, maintaining only a macrocellular network is a short-term solution at best. For a start, it does not address the huge link budget impact in buildings, which can add 15dB or more of additional signal loss.

One approach is the hierarchical base station model: larger macrocells overlaying a network of smaller micro or picocells. With voice services until recently dominating the minds of network planners, this was seen as one solution to meeting capacity needs in city centres rather than addressing link budget constraints. The use of this approach has been modest, with the cost of the wired network connection often being the stumbling block.

The increasing use of the Internet and a shift in this direction by the mobile standards provides network planners with an alternative backhaul connection. This opens up the potential for wider spread use of smaller cells to meet high-speed data needs and the realisation of a hierarchical base station strategy as summarised in Table 1.

Table 1: Different base station classifications in a hierarchical architecture

Classification Antenna Location

Range Application Possible Antenna Technology

Macro Above rooftops

> 1km National coverage in all areas

Spatial Division Multiple Access (SDMA) using fixed/reconfigurable beamforming to maximise capacity and extend cell-edge data performance where needed

Micro Below rooftops at street level

< 1km High capacity public spaces, e.g. sports stadium, airports, shopping malls, etc.

Spatial Multiplexing (SM) to exploit increased multipath activity and meet higher data throughput needs

Pico In buildings, ceiling or wall mounted

< 300m High capacity enterprise use, e.g. large offices

As above

Femto In buildings, free standing or wall mounted

< 200m Low capacity personal use, e.g. homes, small offices

Possibly dual-antenna receive diversity (optional)

5.3 Right Technology, Right Time When it comes to implementing changes to the air interface, it is the baseband processing that is most affected. This includes the physical (PHY) layer, which transmits and receives the data, and the Media Access Control (MAC) layer, which manages the PHY resources between multiple users.

“With any new technology, it is invariably time pressure that dictates early design choices, leading to the use of more flexible DSP and FPGA solutions.”

Baseband implementations typically use a mixture of ASIC, DSP and FPGA, partitioning the design according to performance, time and cost needs. With any new technology, it is invariably time pressure that dictates early design choices, leading to the use of more flexible DSP and FPGA solutions. The rationale behind this usually includes a longer-term route to ASIC once volumes and design stability have been achieved.



The challenge for every design team is managing the interfaces between the different development environments and the added overheads these introduce. This also includes accommodating very different programming methodologies, from the abstracted higher-level software languages on the DSP to the hardware description languages used by FPGAs (and ASICs). Operating with a small number of high performance DSPs can also lead to inefficiencies, especially when it comes to coding algorithms to support multiple users.

An alternative approach, which has already been discussed in the context of HSPA, uses an array of hundreds of processors on a single device [1]. Instead of a few general-purpose DSPs that must time-multiplex tasks, it is now possible to dedicate resources on a per-user or per-function basis, greatly reducing latency.

Another advantage of a highly integrated parallel architecture is that the implementation is within the same design environment. This minimises integration issues and allows for the rapid development of software for algorithm improvements. This could be brought about by new features in the standard, the inclusion of new intellectual property or feedback from deployed systems. The ability to scale the design for different types of base station is also crucial as highlighted in Table 1.

5.4 picoArray™ Solution “An alternative approach, which has already been discussed in the context of HSPA, uses an array of hundreds of processors on a single device.”

The PC202, PC203 and PC205 are a family of multi-core DSP chips from picoChip [9-11]. Based on the successful PC102 picoArray processor, this next generation of devices integrates additional functionality to meet the needs for improved performance and lower cost with evolving wireless standards. All devices include the proven multi-core picoArray – 248 DSPs interconnected by a high-speed 32-bit picoBus and programmable switches offering a fully deterministic processing core (Figure 6). Added to this are a number of function accelerators making it one of the most powerful baseband processors on the market today.

To meet the needs for tighter integration in some applications, the PC202 and PC205 include an embedded ARM926EJ processor that could implement the MAC functions (Figure 7). When combined with an on-chip Ethernet MAC, the PC202/5 is the complete solution for low-cost, high-performance base stations, access points and customer premises equipment (CPE). The PC205 is a higher performance version of the PC202, with faster function accelerators for higher bit-rate applications. Multiple devices can also be interconnected and a number of different system configurations are possible.

The PC203 has the same picoArray subsystem, but does not include the ARM926EJ subsystem. It does have the same processor interface for access to the picoArray by an external processor, which might include the ARM926EJ on a PC202/5, and so is ideally suited to larger base station developments.



Figure 6: picoArray core concept

Px

Switch Matrix

Inter-picoArrayInterface

IpI P2P1

P3

P3

IpI

IpI

P3

P3

IpI

P1P1 P1 P2 + P2

P1P1P1 P1

XP1P1 P1

P1P1P1 P1

P2P2 P2 P2

P2P2 P2 P2

P2P2 P2 P2

P1

P2

IpI

Array Processing Element (AE)

Figure 7: PC202/5 with integrated microprocessor

This second generation of devices uses 90nm technology and is ideally suited to the development of the existing wave of wireless technologies [1,12]. This includes the first deployments of WiMAX and the evolutions of HSPA and EV-DO.

“The complexity of the algorithms is increasing faster than the capability of the processors.” The baseband processing demands to meet mobile data requirements in the

2009/2010 timeframe are significant and cannot be met by today’s technology at the right price point – the complexity of the algorithms is increasing faster than the capability of the processors.

This is not a new phenomenon, and occurs whenever there is a major technology shift. And each time, the underlying technology and engineering ingenuity have caught up to meet the demands. The picoArray silicon roadmap shown in Figure 8 provides the next step in the evolution of this technology towards an OFDM future using MIMO and a hierarchical base station network.

The PC302 sees the development of a cost optimised device for indoor femtocell applications. The core picoArray in the PC203 is also shrunk to 65nm technology and optimised for more advanced base stations that exploit MIMO and other future enhancements. Added to this is the suite of software reference designs for the picoArray, all of which are scalable and flexible to meet future development needs. The combination is a valuable



lifeline to manufacturers wishing to be competitive in the coming technology race.

Figure 8: Preliminary picoArray silicon roadmap

6 Conclusions Is LTE fit for purpose? On paper at least the proposed new air interface meets the requirements for peak data throughputs, spectral efficiency and latency on the up and downlinks. In fact, the chosen technology has much in common with WiMAX and the EV-DO Revision C standard UMB, all being based on OFDM in bandwidths up to 20MHz.

Unlike WiMAX, LTE has a different approach on the uplink: single-carrier FDMA (SC-FDMA). The argument is based around handset complexity and the need to keep the Peak-to-Average (power) Ratio (PAR) of the transmitted signal low. This translates into a smaller power amplifier for a given output power, reducing power consumption and extending battery life.

“On paper at least the proposed new air interface meets the requirements for peak data throughputs, spectral efficiency and latency on the up and downlinks.”

The key for LTE, in common with the other standards, is optimising the air interface for multiple users requiring different data applications in various locations. The emphasis is on the MAC signal processing, which must schedule resources efficiently based on the prevailing channel conditions and various Quality of Service (QoS) metrics. These include requirements for the various services such as end-to-end delay and data rate, as well as the overall network.

Reducing latency has been one of the biggest drivers, and along with the push to an all-IP network, has led to a collapsed architecture. Put simply, the use of multiple network nodes is replaced by enhanced base stations connected to a single node: the access Gateway (aGW).

The LTE plan looks good, but what about its implementation? Multiple antenna techniques under the MIMO umbrella are key to its success, as is the adoption of a hierarchical network of base stations. The majority of data use is indoors, and so smaller specialised base stations for public, business and home use should be considered. The overlap with HSPA and its continued development with HSPA+ will also ease the transition, building on existing investments into 3GPP technology.



Handling all of this additional complexity and scalability is the baseband processing in the new enhanced base stations. It is probably not the most expensive line item in the bill of materials, but certainly requires the greatest investment. Most adopt a processing platform that is a mixture of ASIC, DSP and FPGA, the exact mix often determined by legacy designs and long-term supplier relationships.

With ASIC development costs rising, the balance of power is shifting towards more programmable platforms. Good news for the DSP and FPGA vendors, but is this the right mix for LTE and other advanced standards? Multi-core processors are now coming to the fore, and picoChip have a baseband processor family that includes an array of over two hundred processors on a single device. These remove the need for a mixed DSP/FPGA design, and are ideal for the multi-user, multi-function signal processing algorithms in MIMO and other LTE enhancements.

Being completely programmable and easily scalable, baseband software development for multiple standards on a common hardware platform is now possible. This is aided by the significant overlap between the competing technologies. NEC Electronics has responded to the battle over DVD formats by coming up with a device that supports both. The picoArray running different software could do the same for LTE, UMB, WiMAX and any other new technology.



References 1. picoChip White Paper, “Betting on High-Speed Packet Access?”, April 2007.

2. 3GPP Technical Report TR 25.913, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)”.

3. 3G Americas White Paper, “Mobile Broadband: EDGE, HSPA & LTE”, Rysavy Research, September 2006.

4. H Ekström et al, “Technical Solutions for the 3G Long-Term Evolution”, IEEE Communications Magazine, March 2006.

5. H G Myung et al, “Peak-to-Average Power ratio of Single Carrier FDMA Signals with Pulse Shaping”, 17th International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06).

6. B Allen at al, “Demystifying MIMO”, IET Communications Engineer, December/January 2006/07.

7. 3GPP Technical Report TR 25.912, “Feasibility study for evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN)”.

8. E Dahlman et al, “The 3G Long-Term Evolution – Radio Interface Concepts and Performance Evaluation”, IEEE Vehicular Technology Conference, 7-10 May 2006.

9. picoChip Datasheet, “PC202 – Wireless Communications Processor”.



12. picoChip White Paper, “The Case for Home Base Stations”, September 2006.

Glossary 3GPP Third-Generation Partnership Project MSC Mobile Switching Centre aGW Access Gateway PAR Peak-to-Average (power) Ratio ARQ Automatic Repeat Request PDU Protocol Data Unit ASIC Application Specific Integrated Circuit PHY PHYsical layer BPSK Binary Phase Shift Keying PSTN Public Switched Telephone Network CDMA Code Division Multiple Access OFDM Orthogonal Frequency-Division

Multiplexing CPE Customer Premises Equipment OFDMA Orthogonal Frequency Division Multiple

Access DFT Discrete Fourier Transform QAM Quadrature Amplitude Modulation DSL Digital Subscriber Line QoS Quality of Service DSP Digital Signal Processor QPSK Quadrature Phase Shift Keying DVD Digital Video Disk RAN Radio Access Network EDGE Enhanced Data rates for GSM Evolution RLC Radio Link Control (protocol) EV-DO Evolution-Data Optimised RNC Radio Network Controller FDD Frequency-Division Duplex RTT Radio Transmission Technology FDMA Frequency Division Multiple Access SC-FDMA Single Carrier FDMA FFT Fast Fourier Transform SDMA Spatial Division Multiple Access FPGA Field Programmable Gate Array SGSN Serving GPRS Support Node GPRS General Packet Radio Service SM Spatial Multiplexing GSM Global System for Mobile communications TDD Time-Division Duplex HARQ Hybrid ARQ TTI Transmission Time Interval HSDPA High-Speed Downlink Packet Access UE User Equipment HSPA High-Speed Packet Access UMB Ultra Mobile Broadband HSUPA High-Speed Uplink Packet Access VDSL2 Very-high bit rate DSL 2 IP Internet Protocol WAP Wireless Application Protocol LTE Long-Term Evolution WCDMA Wideband Code Division Multiple AccessMAC Media Access Control WiFi IEEE 802.11 wireless technology MIMO Multiple-Input, Multiple-Output WiMAX IEEE 802.16 wireless technology


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