Saturday, 5 July 2008

LTE And WiMax Together?

In my last blog I talked about LTE and WiMax finally finding a peace in each other and the early signs of the two having a future together. As I said before I have always believed that the two technologies as a basic are not very much different. I certainly support the notion that the industry can benefit a lot from the two working side by side.

But as always when I was discussing this with some of my friends in the industry they questioned about the similarity between the two technologies.

So how much similar or different they are?

Whenever the similarity between LTE and WiMax is discussed we conclude that the single most important similarity between LTE and WiMax is orthogonal frequency division multiplex (OFDM) signalling. Both technologies also employ Viterbi and turbo accelerators for forward error correction. From a chip designer's perspective, that makes the extensive reuse of gates highly likely if one had to support both schemes in the same chip or chip set. From a software-defined radio (SDR) perspective, the opportunity is even more enticing. Flexibility, gate reuse and programmability seem to be the answers to the WiMax-LTE multimode challenge and that might spell SDR.

So to start with I just concentrated on OFDMA and did some research to find out how much similar the two technologies are in terms of OFDMA or are they?

Most of the articles and discussion shows that LTE and WiMax may be two peas in an OFDM pod, but they are not twins. Here are three significant differences:

1. Both use orthogonal frequency division multiple access (OFDMA) in the downlink. But WiMax optimizes for maximum channel usage by processing all the information in a wide channel. LTE, on the other hand, organizes the available spectrum into smaller chunks.
WiMax pays a price for high channel utilization, however, because processing that much information might require a 1,000-point fast Fourier transform. LTE can get by with a 16-point FFT. This translates into higher power consumption, because it's difficult to design fixed-function WiMax hardware that is also efficient in LTE designs. An architecture that exploits the principles of SDR, however, could reconfigure its FFT function for better power efficiency.
2. LTE uses single-carrier frequency division multiple access (SC-FDMA) for uplink signaling, while WiMax sticks with OFDMA. A major problem with OFDM-based systems is their high peak-to-average power ratios. The average power spec cited in marketing presentations does not show the whole picture. Unfortunately, the system's power amplifier has to be designed to handle peak power--and the PA is the single-largest power consumer in a handset.
LTE opted for the SC-FDMA specifically to boost PA efficiency. "If you can improve the efficiency from 5 percent up to 50 percent simply by changing modulation schemes, then you save a lot of battery time," said Anders Nilsson, principal system architect at multimode specialist Coresonic AB. WiMax's OFDMA has a peak-average ratio of about 10 dB, while LTE's SC-FDMA's peak-average ratio is about 5 dB.
The difference also affects the baseband chip, Nilsson added, because of the need to support two modulation schemes in the uplink. Programmable solutions are flexible enough to reuse gates and keep power low in LTE mode.

Regarding the PAPR issue (Peak to Average Power Ratio), I found the following tutorial interesting

3. Although both the IEEE 802.13e standard and the evolving LTE standard support frequency division duplexing (FDD) and time division duplexing (TDD), WiMax implementations are predominantly TDD. LTE seems to be heading in the FDD direction because it is true full-duplex operation: Adjacent channels are used for uplink and downlink. LTE can therefore quote a better spec for downlink data rates, albeit at a cost of placing very severe latency requirements for forward error correction. The bottom line is that the WiMax radio is much simpler
These differences make designing a chip or chip set to support both standards more difficult, but they also have network infrastructure consequences that might be more easily resolved by harmonization instead of competition. Certainly, from the handset designer's perspective, there is no clear winner.
The battery life and power efficiency of the chip or chip set are critical to market success, said Fannie Mlinarsky, an independent consultant specializing in wireless testing and design. Power is a big issue for WiMax and LTE because megabit-per-second capability means running the DSP hard and making the chips more power hungry.

1 comment:

Emmanuel said...

Thanks for this very interesting post. Indeed I did not realize that Wimax and LTE were so similar.