Saturday, 12 July 2008

Advanced 3GPP Interference Aware Receivers

Receiver structures in UEs and Node-Bs are constantly being improved as products evolve and more complex features are added to HSPA. The result is improved system performance and higher user data bit rates. This trend is reflected in constantly changing UE receiver requirements in 3GPP. In 2006, 3GPP has studied further improved minimum performance requirements for UMTS/HSDPA UEs. These enhanced performance requirements are release-independent (i.e. apply also to a Rel-6 terminal with advanced receivers).

Interference aware receivers, referred to as type 2i and type 3i, were defined as extensions of the existing type 2 and type 3 receivers, respectively. The basic receiver structure is that of an LMMSE sub-chip level equalizer which takes into account not only the channel response matrix of the serving cell, but also the channel response matrices of the most significant interfering cells. HSDPA throughput estimates were developed using link level simulations, which include the other-cell interference model plus Orthogonal Carrier Noise Simulator (OCNS) models for the serving and interfering cells based on the two network scenarios considered.

This type of receiver attempts to cancel the interference that arises from users operating outside the serving cell, which is also referred to as other-cell interference. Interference models/profiles were developed for this other-cell interference in terms of the number of interfering Node Bs to consider, and their powers relative to the total other cell interference power, the latter ratios referred to as Dominant Interferer Proportion (DIP) ratios. For the purposes of this study item it was determined that five interfering Node Bs should be taken into account in the interference models. DIP ratios were defined based on three criteria: median values of the corresponding cumulative density functions, weighted average throughput gain, and field data. Of these criteria, the one based on the ‘weighted average’ was felt to offer a compromise between the conservative, median value criteria and the more optimistic field data criteria. In addition, two network scenarios were defined, one based solely on HSDPA traffic (HSDPA-only), and the other based on a mixture of HSDPA and Rel-99 voice traffic (HSDPA+R99).

HSDPA throughput estimates were then developed using link level simulations, which included the othercell interference models plus OCNS models for the serving and interfering cells based on the two network scenarios considered. The two-branch reference receiver, referred to as a type 3i receiver, was found to offer significant gains in throughput primarily at or near the cell edge. Link level results were developed for a wide range of operating conditions including such factors as transport format, network scenario, modulation, and channel model. For example, the gains for the DIP ratios based on the weighted average ranged from a factor of 1.2 to 2.05 for QPSK H-SET6 PB3, and from 1.2 to 3.02 for VA30 for network geometries of -3 and 0 dB. This complements the performance of existing two-branch equalizers (type 3), which typically provide gain at high geometries, and thus, the combination of the two will lead to a much better user experience over the entire cell.

In addition, a system level study was conducted that indicated that a type 3i receiver provided gains in coverage ranging from 20-55% for mildly dispersive channels, and 25-35% for heavily dispersive channels, the exact value of which depends upon user location. A second system level study divided the users into two different groups depending on their DCH handover states, where the first group collected users in soft handover (between cells), and the second group collected users in softer handover (between sectors of the same cell). The results of this second study indicate that the Type 3i receiver will provide benefits for users in these two groups, increasing their throughput by slightly over 20%. With regards to implementation issues, it was felt that the type 3i receiver is based upon known and mature signal processing techniques, and thus, the complexity is minimized. With two-branch, equalizer-based receivers already available in today’s marketplace, it appears quite doable to develop a two-branch equalizer with interference cancellation/mitigation capabilities. Given all of the above, 3GPP concluded that two-branch interference cancellation receivers are feasible for HSDPA, and a work item has been created to standardize the performance requirements with type 3i receiver.

More on this topic is available in the following:
  • 3GPP TR 25.963 V7.0.0: Feasibility study on interference cancellation for UTRA FDD User Equipment (UE)
  • Signal Processing for Wireless Communications By Joseph Boccuzzi
  • Simulation results can also be obtained from reports here.

1 comment:

S. Sendil Kumar said...

Can you please put some reference on Type-M receivers for HSDPA. All the other receivers Type-1/2/3i talks only about receiver diversity. I am interested about the STTD equalization