Meeting IMT-Advanced Requirements: A Look Under the Hood of Next Generation Wireless Networks.

IMT-Advanced LogoThe International Telecommunication Union (ITU) has just approved LTE-Advanced and WiMAX-Advanced (aka WiMAX 2.0) as part of the IMT-Advanced standards. Aside of marketing catch phrases like “putting fiber optical speed on your mobile phone,” the statements about efficiency – being able to transfer higher data rates in lesser bandwidth – are what the industry will be grappling with.

You see, the problem is that we are reaching the limits of what the physical layer can actually provide in terms of throughput. Capacity targets of IMT-Advanced, figures like 1 Gbps peak downlink throughput are reached by using 8×8 MIMO techniques in 100 MHz of bandwidth using “carrier aggregation” techniques. But increasing the amount of spectrum used does not improve spectral efficiency. Using more antennas has practical limitations: Can you imagine a mobile device with 8 antennas supporting multiple frequency bandwidths? Add to this the requirement to support legacy technologies and you get the picture of the complexity involved, not just for consumer devices but also at the base station side.

Declining Incremental Spectral Efficiency of Radio Access Technologies (Source: Qualcomm)

Declining Incremental Capacity of Radio Access Technologies (Source: Qualcomm)

Headline news on capacity is those for peak rates. But the real deal is how to increase average capacity which is what’s used to plan and dimension wireless networks (see table blow). So, in reality the burden of improving data rates and squeezing more capacity out of available spectrum will be on intelligent techniques that coordinate transmissions in the network and reduce interference.  This is the role of a whole slew of techniques that LTE-Advanced incorporates which include: enhanced inter-cell interference coordination (eICIC), coordinated multipoint transmission/reception (CoMP), and relay and small cell technologies. These solutions require coordination of communication with a single mobile between neighboring base stations in an attempt to reduce interference. Intelligence is then happening at the network level which requires fast communication between base stations and ability for tight synchronization between all the network elements.

LTE

LTE-Advanced

Peak Data Rate (Mbps)

Downlink

300

1000

Uplink

75

500

Peak Spectral Efficiency (bit/s/Hz)

Downlink

15

30

Uplink

3.75

15

Spectral Efficiency (bit/s/Hz/cell)

Downlink

2 x 2

1.69

2.4

4 x 2

1.87

2.6

4 x 4

2.67

3.7

Uplink

1 x 2

0.74

1.2

2 x 4

2.0

Cell-edge User Throughput (bit/s/Hz/cell/user)

Downlink

2 x 2

0.05

0.07

4 x 2

0.06

0.09

4 x 4

0.08

0.12

Uplink

1 x 2

0.02

0.04

2 x 4

0.07

It will not be ‘headline’ technologies such as MIMO that will lead us to greater capacity as these techniques are pushing on what can be possible from practical perspective that’s bounded by the laws of physics. Rather, intelligent network-level algorithms, inter-base station communication and coordination, and other solutions that aim to reduce transmitted power and reduce interference will be the corner-stone of future mobile networks. That is unless some comes up with novel techniques to increase the capacity of the physical layer!

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About Frank Rayal
Telecom industry management consultant.

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