TMS320C5510实现16-QAM OFDM基带处理系统详解

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本文档是 EECS452 课程的最终项目报告，由 Awais Kamboh、Krispian Lawrence、Saradwata Sarkar 和 Eric Williams 四位作者共同完成，他们隶属于美国密歇根大学电气工程与计算机科学系。该报告聚焦于使用 Texas Instruments TMS320C5510 数字信号处理启动套件实现 OFDM（正交频分复用）基带处理系统。OFDM 是一种数字通信技术，特别适用于解决多径传播问题，如建筑物、墙壁和金属物体对无线信号的影响，它通过多个载波同时工作来减少符号间干扰(ISI)并对抗多径效应。报告详细介绍了 OFDM 的基本原理，包括如何利用 16 点的正交幅度调制 (16-QAM) 实现数据传输。编译时可配置的 OFDM 调制器设计使得系统具有灵活性。为了实现全双工操作，报告阐述了采用的相关技术手段，并对所构建的 OFDM 调制解调器性能进行了评估。报告还讨论了潜在的改进方向，旨在提供一个全面理解 OFDM 基带处理系统实现的平台。报告的主要内容包括： 1. 引言：介绍 OFDM 技术在现代通信中的重要性以及该项目的具体目标。 2. 正交频分复用：详细解释 OFDM 的工作原理、优势和适用场景，以及它如何通过频率分片来应对多径干扰。 3. 实现方法：展示如何在 C5510 DSP 上构建 OFDM 调制解调器，包括硬件和软件设计，以及信号处理流程。 4. 全双工操作：描述如何在不损失数据传输效率的情况下实现收发信号的同时进行通信的技术。 5. 性能评估：报告中包含实验结果，如误码率（BER）、吞吐量等关键性能指标，以及对比分析。 6. 改进与未来研究方向：提出当前系统的局限性和可能的优化策略，以提升系统的稳定性和效率。这份报告为读者提供了一个实际应用 OFDM 技术的案例，展示了 DSP 在无线通信领域的应用，同时也为其他研究者或工程师提供了宝贵的参考资源。

spectrum is then converted to its time domain representation using an Inverse Fast Fourier Transform. A

Fast Fourier Transform, at the receiver end, does the reverse during demodulation.

Consider the block diagram of a sample OFDM system shown in Figure 2.

2.2.1 Buﬀering and Block Processing

The input serial data stream is formatted into blocks of the size required for transmission. The data is

then transmitted in parallel by assigning each data word to one carrier in the transmission. There must be

enough data available before this process starts, because the data is consumed in blocks, not a byte at a

time. Similarly, the data from the IFFT must be transformed from a block into a serial set of data.

2.2.2 Quadrature Amplitude Modulation (QAM)

To increase the data rate, the 16-point quadrature amplitude modulation scheme (16-QAM) is used on each

sub-carrier. 16-QAM maps four bits onto one complex-valued symbol. Gray coding is also used, making

adjacent symbols diﬀer by only one bit. This makes it optimal for a minimum Euclidean distance receiver.

A sample gray-coded 16-QAM constellation is shown in Figure 3 and Table 1.

−3 −2 −1 0 1 2 3

x 10

−3

−2

−1

x 10

Figure 3: Sample 16-QAM constellation.

0000 (0) 0001 (1) 1001 (9) 1000 (8)

0010 (2) 0011 (3) 1011 (11) 1010 (10)

0110 (6) 0111 (7) 1111 (15) 1110 (14)

0100 (4) 0101 (5) 1101 (13) 1100 (12)

Table 1: A set of gray-coded 16-QAM constellation points.

2.2.3 Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT)

The IDFT performs the necessary transformation very eﬃciently; it provides a simple way of ensuring the

carrier signals produced are orthogonal.

Consider the N complex-valued symbols X(k), 0 ≤ k ≤ N − 1, modulated onto N orthogonal carriers using

the IDFT

x(n) =

N−1

k=0

X(k)e

+j2πn

(1)

Since the basis functions of the IDFT are orthogonal, orthogonal sub carriers are created.

The Discrete Fourier Transform (DFT) transforms a cyclic time domain signal into its equivalent frequency

spectrum. This is done by ﬁnding the equivalent waveform, generated by a sum of orthogonal sinusoidal

components. The amplitude and phase of the sinusoidal components represent the frequency spectrum of

the time domain signal. The IDFT performs the reverse process, transforming a spectrum (amplitude and

phase of each complex frequency component) into a time domain signal. The IDFT converts a number of

complex data points into the time domain signal of the same number of points. Each data point in the

frequency spectrum used for an IDFT is called a bin. The orthogonal carriers required for the OFDM signal

can be easily generated by setting the amplitude and phase of each bin, then performing the IDFT.

To achieve a real-valued output from the IDFT, a special packing technique is used. By appending a reversed

and complex conjugated copy of the N symbols to themselves, the output from the IDFT will be real-valued

(speciﬁcally, it will contain zero-valued imaginary components). By using this technique, the number of

points in the IDFT calculation will be increased from N to at least 2N + 2. In this case, the complex

frequency bins with equal but opposite frequencies then contain conjugate coeﬃcients.

An property that is important in the reception of OFDM signals is the close relationship between the DFT of

a signal and the DFT of the same signal circularly rotated. Taking the DFT of signal rotated by R samples,

y(n) = x((n + (N − R)) mod N ) (2)

Y (k) = e

−j2πk

N −R

X(k) (3)

The coeﬃcient is simply a linear phase shift. This follows directly from the Fourier shifting property.

2.2.4 Guard Interval

One of the most important properties of OFDM transmission is the robustness it provides against multipath

delay spread. This is achieved by having a long symbol period, which minimizes the Inter-Symbol Interfer-

ence (ISI). The level of robustness can be increased even more by the addition of a guard period between

transmitted symbols. The guard period allows time for multipath signals from the previous symbol to die

away before the information from the current symbol is gathered. If the end of the symbol waveform is put

at the start of the symbol during the guard period, this eﬀectively extends the length of the symbol, while

maintaining the orthogonality and periodicity of the waveform.

A technique for employing the guard interval is to use a cyclic preﬁx. The cyclic preﬁx is, a copy of the M

of the last samples prepended, making the signal appear as periodic over M + N samples with period N .

The received signal, consisting of the sent signal and the cyclic preﬁx, is demodulated using the FFT.

The sent signal can be written as:

s(n) =

(

x(n + N ) , −M ≤ k < 0

x(n) =

N−1

k=0

X(k)e

+j2Πn

, 0 ≤ k ≤ N − 1

(4)

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