Reconfigurable cores, scalable alogithms propel DSPs into 4G wireless

Reconfigurable cores, scalable alogithms propel DSPs into 4G wireless
By Wei-Jei Song, Senior Architect, 3DSP Corp., Irvine, Calif., EE Times
February 21, 2002 (12:55 p.m. EST)
URL: http://www.eetimes.com/story/OEG20020221S0041

If you are looking for fourth-generation, or 4G, wireless to shorten the life span of previous generations you had better look elsewhere. Rather, expect it — unlike other wireless protocols — to be a technology unifier that will not snuff out earlier technology such as 2G (GSM, TDMA, IS95), 2.5G (GPRS) and the emerging 3G (WCDMA, UMTS).

With the proposed 4G-SIM (Subscriber Identity Module) card concept used to configure any handset, subscribers equipped with 4G wireless will be able to use any 4G-SIM cell phone regardless of the underlying technology. The 4G-SIM would also work as a phone card for 4G pay phones or to configure a 4G household phone. During travel, Internet Protocol version 6 (IPv6) with 4G capability will track the 4G-SIM, so as soon as the 4G-SIM is "parked" into a 4G handset in the hotel phone calls will not have to be forwarded.

But to achieve the all-digital, IP-based 4G network — projected to be ready by 200 6 — certain requirements must be met and problems solved.

Handoff. The 4G wireless network must support a common protocol to access a satellite-based network and another protocol for terrestrial networks. The 4G-capable devices thus interface with fixed wireless networks, satellite networks, wireless LANs and cellular networks.
Deliver data rates up to 100 Mbits/second. It disappointing that the 3G settles for the 300- to 400-kbit/s range. This seriously limits its ability to deliver video and voice over packet.
Indoor/outdoor connections. Current wireless solutions generally deliver either effectively, but not both.
Universal packet network. By utilizing the 4G-SIM concept and embracing the soft-switch-based gateway integration protocol such as H.248, the routing basis on personal presence is made possible.
Automatic configuration. The combination of GPS technology and SIM card devices makes it possible to automatically rout into the proper connection point . The connection point attaching to the wireless LAN (WLAN), access point or wireless base station depends on the planning and preconfigurable criteria.

The 4G device, as a DSP-based multi-feature gadget, can switch protocols on the GPS-directed command string. The optimal wireless protocol is determined based on geographical and network congestion. The DSP-based device should possess:

Downloadable PHY protocols.
Downloadable vocoders.
Common ASIC blocks for multiple wireless protocols.
Generic interfaces for the protocol stack layer.

Key enablers for the all-digital 4G wireless networks, besides soft switch, are the H.248 standard, the Internet Protocol SIP (Session Initiation Protocol) and Ipv6. The technologies needed to deliver 4G wireless are multi-carrier code division multiple access and coded orthogonal frequency division multiplexing (COFDM).

One way to provide universal connection is to have a bona fide software radio implemented on the device t o guarantee connection to W-CDMA, EDGE, WLAN and GPRS/GSM. The alternative is to have the Universal Access Point (UAP) deployed to limit the development and manufacturing cost for the mobiles. The mobile needs at least WLAN and 4G-modem functionality to stay connected and to hand off for large data rate transfer. COFDM is the clear technology of choice because of its superiority in modulating and demodulating high data rate. OFDM technology is already deployed in the IEEE 802.11a and will be the technology of choice for the 2.4-GHz-band IEEE 802.11g.

The underlying technology for 4G wireless is the new radio technology OFDM with wireless multimedia streaming applications. It is scalable in data rate and therefore scalable in power. In the single application that demands the highest data rate, HDTV at 15 Mbits/s, the data rate is proportional to the number of the subchannels activated. OFDM handles multipath fading well with the frequency orthogonality. The technology has been selected for the 5-GHz and 2 .4-GHz wireless LAN standard.

The challenges and solutions for OFDM technology are:

Nonlinearity. Digital pre-distortion proves to be a possible solution.
Timing synchronization. Symbol and sample level timing synchronization is critical to receiver performance. The classical cross-correlation-based symbol timing estimate provides satisfactory results. The theory that maximum length channel impulse response is typically shorter than the cyclic prefixes does not introduce intersymbol interference assuming that the symbol timing is fixed to the first sample.
Carrier frequency offset. Interchannel interference spills over when carrier frequency offset is not detected and corrected and performance is severely degraded. A high-performance DSP-based OFDM engine facilitates fine tuning of the carrier synchronization algorithm.

The flexibility, configurability and scalability of "anywhere, anytime, all IP" 4G wireless networks is distributed between UAP and the wireless terminal . A real world phone is possible through a mobile device equipped with flexible software architecture to configure and download in the air. Legacy phones work well with 4G UAP-guided networks.

OFDM, one of the multicarrier modulations proposed as the 4G modulation technique, has already been successfully implemented in IEEE 802.11a to deliver 54 Mbits/s. It is also the technology of choice for IEEE802.11g to deliver 36 Mbits/s and for the widely deployed legacy IEEE 802.11b devices up to 54 Mbits/s with enhanced radio.

The building blocks of OFDM technology are the fast Fourier transform and Viterbi engine. The superior performance in handling of intersymbol interference and the avoidance of single-frequency jamming simplifies the design. The well-known higher peak-to-average ratio demands a higher degree of linearity in the power amplifier while the additional guard band in the form of cyclic extension lowers the effective throughputs slightly.

The software radio is geared to handle the RF and baseband challenges come with 4G. The secured IP connection and technology download ability, as well as configurability on the fly, make it possible to keep the minimum required video and audio coders on the silicon. This design feature enables the configuration of language-based code book, country-based voice activity detector parameters and the desired video and audio coders that the hosting UAP or basestation wants or can handle.

Although the baseband modules will be activated one at a time, media processing is designed to be able to deliver any of the following combinations: voice only, simultaneous voice and data and videoconferencing. When simultaneous voice and data are delivered, the voice is configured to provide higher quality of service.

Besides the power-efficient next-generation programmable DSP core, the intelligent DMA to build the system-on-chip and the interface and control of the RF unit, the extension of Ipv6 and SIP or a new remote function control (RFC) is needed. This will addr ess the protocol in setting up the universal link, the monitoring mechanism and the actual downloading and configuration. The chosen baseband module and the coder determine the QoS.

There has been a constant battle between ASIC and DSP solutions for baseband implementation. The ASIC was the only solution when IS-95 came along because of its MIPS requirement and the lack of a high-performance DSP core designed for baseband processing. The high-performance baseband processor offloads the RF requirement and complements the RF.

The vocoders and encryption algorithms in the WLAN are evolving. The DSP solution enables software radio implementation, flexibility and scalability. Being able to configure the DSP core would be a real plus in this application. For example, configurability would allow new instructions to be added to enhance baseband performance, to adapt memory space to optimize the silicon real estate and to tailor the subsystem interfaces to speed up the system design.

The advantage of the ASIC vs. high-performance DSP is primarily in the area of power consumption, but this can be addressed by employing scalable algorithms to provide the right performance for the application. For example, with fractional OFDM channels activated, the frame error rate drops exponentially and the power decreases. The identity of line of sight also can alleviate the requirement in handling multipath, thus saving power.

Besides the baseband and RF areas, 4G media streaming algorithms such as audio, video and low-bit-rate vocoders are all but impossible to implement in ASICs. The DSP provides integral solution to the presentation layer and cuts down the high-throughput communication requirement as well as power consumption and additional silicon space associated with it.

The 4G building blocks are the 4G-mobiles as well as the 4G-UAP. Although the algorithms on them are similar and sometimes identical, the complexity, power consumption requirement and proliferation are quite different. The DSP-based solut ion enjoys much greater configurability, thus a higher degree of sharing between the two.

The fourth-generation wireless network will have a WLAN infrastructure and mature IP standards to deliver wireless multimedia by the time trials begin. With OFDM technology already proved and embraced, 4G development will gain momentum. Over the next few years, any slowdown in 3G deployment will not slow 4G implementation, since 3G is not a stop but a passenger for the 4G-technology train.

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