Essay: Differences between WiMax and HSPA

Essay: Differences between WiMax and HSPA
April 12, 2011 Comments Off on Essay: Differences between WiMax and HSPA Academic Papers on Information Technology,Sample Academic Papers admin

Sample Essay

WiMAX

One of the main differences between WiMAX and HSPA is the difference of signal format in the physical layer. The WiMAX standard uses Orthogonal Frequency Division Multiplexing (OFDM) while the HSPA is based on Direct-Sequence Spread Spectrum (DS-SS), similar to WCDMA. The use of OFDM in WiMAX provides it with robustness to multipath.

The key feature of this technology is the use of narrow band tones combined with cyclic prefix. This cyclic prefix not only provides protection against inter-symbol interference but also ensures that propagation in a multipath channel only imposes a scalar distortion on each tone, simplifying the process of equalization.  When the tones are properly synchronized and have protection by a cyclic prefix, they remain orthogonal to each other, even after passing through a channel offering multipath propagation. The use of cyclic prefix offers a disadvantage though, as it increases the overhead which results in reduced bandwidth efficiency.

The ability to maintain orthogonality by WiMAX signals, even under multipath, results in a system that is very much suitable for high-speed transmissions and is also free from intra-cell interference. However, the problem of inter-tone interference still exists in WiMAX due to Doppler shifts in OFDM signals. Therefore when OFDM is employed for multiple accesses in uplink, the WiMAX base stations are required to fine tune the terminals to bring frequency errors to an acceptable level and also minimizes the total interference by using power control.  In addition to this, the OFDM signals of WiMAX have a comparatively large peak-to-average ratio (PAPR), which means that for a provided average power, the power amplifier must be able to handle large power peaks without putting distortion in the transmitted signal (The WiMAX Forum, 2006).

HSPA

HSPA, on the other hand, uses CDM code aggregation (orthogonal Walsh codes) to provide a high-speed downlink channel and DS-SS multiple access (CDMA) for providing services in the uplink.  Though this approach is less affected by Doppler spread, it introduces intra-cell interference, when used in the time-dispersive channel due to loss of orthogonality and thus prevents the use of higher order modulations for high-speed data transmission. This interference is overcome by using generalized RAKE receivers which provide advanced signal process but at the cost of additional receiver complexity. In comparison to WiMAX OFDM signal, the HSPA signals have a lower PAPR ratio, which facilitates the use of a power amplifier that is less complex (Ericsson, 2009).

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