The deployment of HSPA in W-CDMA networks and handsets heralds the availability of truly broadband mobile services that have been promised so long by 3G. Wireless broadband will allow customers to have mobile access to the “Triple play” services that they have become accustomed to in the home. Mobile broadband access will continue to track the fixed access capabilities offering voice-calls, high-speed internet access and television (Video on demand and broadcast), but in order to do this requires the continued evolution of the mobile network.
Higher bandwidths, lower latency, and the move to a packet-based IP network are all seen as essential to enable broadband mobile access. IEEE 802.16e is widely recognized as the most mature standard aimed to meet these criteria, and picoChip is the leading chipset supplier to this market (link) . Consequently, some operators have already announced their intentions to launch mobile WiMAX services by 2008.
Partially fuelled by this competition from WiMAX, the 3GPP standards are setting a very aggressive agenda to standardize their next generation UTRAN architecture. Referred to as LTE (standing for Long Term Evolution), work began in 2005 to set requirements (now finished and documented in TR25.913) and the intention is to conduct feasibility studies and have a complete, ratified standard by the end of 2007.
Timescales for uptake of successive releases of the 3GPP UTRAN standard show a strong trend towards ever faster deployment after standardization. It is anticipated that the roll-out of LTE infrastructure equipment could be as early as 2009.
What’s new about LTE?
Despite its name (“Evolution”), LTE is a much bigger change to the current W-CDMA 3GPP standard than the previous enhancements to support HSPA and is more akin to the change from GSM to WCDMA.
True, some aspects of LTE do offer evolutionary enhancements such as the introduction of advanced QoS support for VoIP and multi-media services first introduced in Release 5. However, LTE also introduces radical changes to the air interface to improve spectrum efficiency and bandwidth, as well as a new collapsed RAN architecture to reduce latency. These more revolutionary changes have come about in order to meet the ambitious requirements for LTE to compete with the performance and services offered by fixed line broadband.
LTE Requirements
The headline requirements for LTE are:
- Scalable bandwidth 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz
- Peak data rate of 100 Mbits/s in the Downlink and 50 Mbits/s in the uplink (20 MHz bandwidth)
- 3-4 times spectral efficiency of HSDPA with in a loaded cell
- 2-3 times the spectral efficiency of HSUPA in a loaded cell
- Support latency of < 5 ms in the user plane and < 20ms in the control plane.
- Support for MIMO
- Uplink (1 Tx and 2 Rx antennas)
- Downlink (2 Tx and 2 Rx antennas)
Physical Layer Technology - OFDM & MIMO the way forward
OFDM offers an attractive technology to meet the requirements for scalable bandwidths with high data rates and is well suited for MIMO processing. LTE has chosen OFDMA in the downlink which offers frequency adaptation for dynamic channel allocation to overcome changes in the channel quality as well as channel dependent scheduling as is done in current systems such as HSDPA.
In the uplink, concern over power consumption in the user-terminal has lead to a different choice of air interface. Single Carrier FDMA offers reduced peak to average power ratio when compared to OFDM at the expense of higher inter symbol interference (ISI). SC-FDMA has a very similar Layer 1 processing chain to OFDMA and is often referred to a DFT-Spread OFDMA as it introduces a DFT function to Layer 1 processing chain prior to sub-carrier mapping.
The DFT and mapping functions provide frequency domain scheduling functions that can either be localized (allowing channel dependent scheduling in the frequency domain) or distributed (which distributes each user in the frequency domain for fading robustness).
Multi Antenna Solutions -
To meet the coverage, capacity and data-rate requirements, LTE supports various multi-antenna schemes. Proposals for both beam forming (for increased coverage and capacity) and MIMO (for increased data-rate) are both being considered in the standards.
Collapsed RAN
Combining the functionality of RAN nodes removes interfaces and reduces both the processing requirements and latency. LTE offers a simpler architecture moving time critical functions from the RNC to the Node-B and combining routing and internetworking functions to a single node (the Access Core Gateway ACGW). Interestingly, this trend towards moving the complexity to the edge of the network has already been seen in wired networks where Network NAS and intelligent classification and security switches have offloaded processing to the edge where the data-rates are lower, leaving the core functions to deal will routing functions.
This pattern is already emerging in Radio Access Networks in the Femtocell model where small, smart Node-Bs service fewer users but do additional function like MAC processing and Radio Resource management (see link to Femtocell page). picoChip is already developing reference designs for collapsed RAN Node-Bs incorporating RNC functions for Femtocells. The LTE architecture standardises this approach as a way to balance the processing requirements between high bandwidth gateway processing and low bandwidth protocol and control processing.
picoChip and LTE
It is clear that there is much in common with mobile WiMAX in Physical layer functionality and WCDMA in higher layer and core network architectures. picoChip has extensive experience in both these markets and is leveraging its expertise in these standards in developing LTE solutions to map to its programmable picoArray architecture. picoChip’s programmable multi-core silicon enables pre-standard reference designs to be developed as the standards are refined.
