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OFDM is a modem technology which offers higher data rate for the same bandwidth thus resulting in an optimum usage of bandwidth.

Although OFDM, in theory, has been in existence for a long time, recent developments in DSP and VLSI technologies have made it a feasible option. Many wired and wireless standards have adopted OFDM. This technology is fast gaining popularity in broadband standards and high-speed wireless LAN.

The process of modem design is unique as well as highly complex. This paper first lists various approaches to implementing an OFDM system and compares the advantages and disadvantages of each approach. It then describes the VLSI implementation of OFDM in detail.

Three choices are available for implementing an OFDM system:

	DSP based implementation
	A DSP based implementation of the OFDM modem offers the following advantages: Reduced development time and quick prototyping. Quick time to market. Flexibility. It can quickly adapt to changing or different standards as it needs only a software change. However, given the stability of standards like 802.11a and 802.11b, this advantage is rather diminished in importance now. Ideal for multi-mode Basebands where multiple standards are supported by the same device
	On the other hand, it suffers from the following two prominent disadvantages: High MIPS requirement. Not very optimum in terms of area and power consumption, thus higher cost. Given the above limitation regarding MIPS requirement, this implementation method is ruled out for OFDM modems. No known DSP processor supports OFDM due to this constraint. Thus, the only possibility remains to add Hardware accelerators to implement the modem in DSP.
	DSP based implementation with hardware accelerators.
	This method manages to overcome the MIPS limitation of the DSP implementation and yet retain the flexibility of software implementation through implementation of some transceiver blocks in H/W. One particular implementation shown in this paper, for example, reduces MIPS requirement by around 4000 MIPS. However, gate count, and hence area, as well as power requirement continue to be limitations in this method.
	VLSI implementation.
	This method takes care of both the disadvantages that DSP based implementations suffer from: This method gives us a lower gate count hence driving down costs. It also has a much lower power consumption. Thus, this implementation emerges as a logical choice for OFDM modem design. The same is described in detail in this paper with all the design considerations and trade offs of the same.

Authors
Aseem Pandey,
Shyam Ratan Agrawalla,
Shrikant Manivannan

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