X-Trend Architecture

Purpose

Complete guide to implementing the X-Trend (Cross Attentive Time-Series Trend Network) architecture, combining LSTMs, attention mechanisms, and few-shot learning for trend-following strategies.

When to Use

Activate this skill when:

• Implementing X-Trend or similar attention-based trading models
• Building encoder-decoder architectures for finance
• Using cross-attention mechanisms
• Implementing Deep Momentum Networks (DMNs)
• Creating interpretable trading predictions
• Combining forecasting with position-taking

Architecture Overview

Input: Target sequence x[t] + Context set C
       ↓
[Encoder]
   - LSTM for sequences
   - Entity embeddings
   - Variable Selection Network (VSN)
   - Self-attention over context
   - Cross-attention: target ← context
       ↓
[Decoder]
   - LSTM with encoder output
   - Dual heads: Forecast + Position
   - PTP (Predictive distribution To Position)
       ↓
Output: Trading position z[t] ∈ [-1, 1]
        + Forecast distribution (μ, σ) or quantiles

Core Components

1. Input Features

The model takes 8-dimensional feature vectors combining:

• Normalized returns at multiple timescales (5 features): 1, 21, 63, 126, 252 days
• MACD indicators at multiple (S,L) pairs (3 features): (8,24), (16,28), (32,96)

Normalization formula for returns:

r_hat[t-t', t] = r[t-t',t] / (σ[t] * sqrt(t'))

IMPORTANT: Use EWMA (exponentially weighted moving average) for volatility calculation:

volatility = prices.pct_change().ewm(span=60).std()

See [IMPLEMENTATION.md](IMPLEMENTATION.md#input-features) for full code.

2. Variable Selection Network (VSN)

Learns to weight different input features dynamically:

• Transforms each feature with dedicated FFN: v[j,t] = FFN_j(x[j,t])
• Computes attention weights: w[t] = softmax(FFN_weight(x[t]))
• Produces weighted sum: VSN(x[t]) = Σ w[j,t] * v[j,t]

Purpose: Automatically determines which features (returns vs MACD, short-term vs long-term) are most relevant at each time step.

See [IMPLEMENTATION.md](IMPLEMENTATION.md#variable-selection-network-vsn) for PyTorch implementation.

3. Entity Embeddings

Learn asset-specific representations:

• Each asset gets learnable embedding vector
• Automatically clusters similar assets (equities, commodities, etc.)
• Added to sequence representations for few-shot learning

Important: Exclude entity embeddings for zero-shot learning (unseen assets).

See [IMPLEMENTATION.md](IMPLEMENTATION.md#entity-embeddings) for code.

4. Sequence Encoder

LSTM-based encoder with skip connections:

1. VSN for feature selection
2. LSTM for temporal modeling
3. LayerNorm + skip connections for stability
4. FFN with entity embedding
5. Final residual connection

Architecture Pattern: x → VSN → LSTM → (+skip) → LayerNorm → FFN(+entity) → (+skip) → LayerNorm

See [IMPLEMENTATION.md](IMPLEMENTATION.md#sequence-encoder) for full implementation.

5. Cross-Attention Mechanism

Target sequence attends to context sequences:

Attention(Q, K, V) = softmax(QK^T / √d) V

where:
- Q (queries): From target sequence
- K (keys): From context sequences
- V (values): From context sequences

Multi-head attention (4 heads recommended) allows model to focus on different aspects simultaneously.

See [IMPLEMENTATION.md](IMPLEMENTATION.md#cross-attention-mechanism) for code.

6. Self-Attention Over Context

Context sequences attend to each other before cross-attention:

• Identifies similarities between different context sequences
• Helps model understand relationships within context set
• Uses same attention mechanism as cross-attention (Q=K=V=context)

Flow: Context → Self-Attention → Cross-Attention with Target

See [IMPLEMENTATION.md](IMPLEMENTATION.md#self-attention-over-context) for implementation.

7. X-Trend Encoder

Combines all encoder components:

1. Encode each context sequence (LSTM + VSN + entity embedding)
2. Apply self-attention over context encodings
3. Encode target sequence
4. Apply cross-attention: target queries context
5. FFN with residual connection

Output: Enriched target representation informed by context patterns.

See [IMPLEMENTATION.md](IMPLEMENTATION.md#x-trend-encoder) for complete code.

8. Decoder

Produces trading signals and forecasts:

Inputs: Target features + Encoder output Outputs: Position z[t] ∈ [-1,1] + Forecast (μ, σ) or quantiles

Architecture:

1. VSN on target features
2. Concatenate with encoder output
3. LSTM processing
4. Dual prediction heads:

- Forecast head: Predicts return distribution - Position head: Either PTP (uses forecast) or direct Sharpe

Three Variants:

• X-Trend: Direct Sharpe optimization (no forecast)
• X-Trend-G: Joint Gaussian MLE + Sharpe
• X-Trend-Q: Joint Quantile Regression + Sharpe

See [IMPLEMENTATION.md](IMPLEMENTATION.md#decoder) for implementation details.

9. Complete Model

The full X-Trend model combines encoder and decoder:

class XTrendModel(nn.Module):
    def __init__(self, input_dim=8, hidden_dim=64, num_assets=50,
                 forecast_type='gaussian', num_heads=4):
        self.encoder = XTrendEncoder(...)
        self.decoder = XTrendDecoder(...)

    def forward(self, target_features, target_asset_id,
               context_features, context_asset_ids, use_ptp=True):
        # Encode with attention over context
        encoder_output, attention_weights = self.encoder(...)

        # Decode to position + forecast
        position, forecast = self.decoder(...)

        return position, forecast, attention_weights

See [IMPLEMENTATION.md](IMPLEMENTATION.md#complete-x-trend-model) for full code.

Training

Loss Functions

Sharpe Loss:

L_Sharpe = -sqrt(252) * mean(returns) / std(returns)

Gaussian MLE Loss:

L_MLE = -log p(r | μ, σ)

Joint Loss:

L_joint = α * L_forecast + L_Sharpe

Where α = 1.0 for Gaussian, α = 5.0 for Quantile.

Training Protocol

1. Episodic Learning: Train in episodes, not mini-batches
2. Sample target (asset, time) from training set
3. Sample context set causally (before target time)
4. Forward pass through encoder + decoder
5. Compute joint loss balancing forecasting and trading
6. Backward pass with gradient clipping (max norm = 10.0)

Hyperparameters

{
    'input_dim': 8,
    'hidden_dim': 64,
    'num_assets': 50,
    'num_heads': 4,
    'context_size': 20,
    'seq_len': 126,        # 6 months
    'learning_rate': 1e-3,
    'alpha': 1.0,          # Joint loss weight
    'dropout': 0.3,
    'target_vol': 0.15
}

See [TRAINING.md](TRAINING.md) for complete training guide including:

• Full training loop implementation
• Validation procedures
• Optimizer configuration
• Common training issues and solutions

Interpretation

Attention Weights

Visualize which context sequences the model attends to:

_, _, attention_weights = model(target, context)
# Shape: (batch, num_heads, seq_len, num_contexts)

# Average across heads and time
avg_attention = attention_weights.mean(dim=(0, 1, 2))

# Top 3 most important contexts
top_k = torch.topk(avg_attention, k=3)

Use for:

• Understanding model decisions
• Debugging unexpected predictions
• Validating that model uses context meaningfully

See [IMPLEMENTATION.md](IMPLEMENTATION.md#interpretation) for visualization code.

Best Practices

DO:

✅ Use LayerNorm + skip connections for stable training ✅ Entity embeddings for few-shot but exclude for zero-shot ✅ Multi-head attention (4 heads is good default) ✅ Joint loss training balances forecasting and trading ✅ Episodic training mimics test-time usage ✅ Gradient clipping (max norm = 10.0) ✅ EWMA for volatility (span=60) not simple rolling std ✅ Monitor attention weights to ensure meaningful patterns

DON'T:

❌ Don't skip warm-up - first 63 predictions are unstable ❌ Don't use entity embeddings in zero-shot - model hasn't seen asset ❌ Don't mix training data - use episodes, not mini-batches ❌ Don't ignore attention interpretation - helps debug ❌ Don't forget dropout (0.3-0.5) for regularization ❌ Don't use simple rolling std - use EWMA for volatility

Performance Expectations

Based on X-Trend paper results (2018-2023):

Few-Shot Learning:

• Baseline: Sharpe = 2.27
• X-Trend-Q: Sharpe = 2.70 (+18.9%)
• vs TSMOM: Sharpe = 0.23 (10× improvement)

Zero-Shot Learning:

• Baseline: Sharpe = -0.11 (loss-making)
• X-Trend-G: Sharpe = 0.47 (profitable)
• 5× improvement over baseline

COVID-19 Recovery:

• Baseline: 254 days to recover
• X-Trend: 162 days (2× faster)

Implementation Checklist

When implementing X-Trend:

• Input features with EWMA volatility normalization
• Variable Selection Network for feature weighting
• Entity embeddings (few-shot) or exclude (zero-shot)
• LSTM encoders with LayerNorm and skip connections
• Self-attention over context set
• Multi-head cross-attention (4 heads)
• Decoder with dual prediction heads
• Joint loss function (α * Lforecast + LSharpe)
• Episodic training loop
• Gradient clipping (max norm = 10.0)
• Dropout regularization (0.3-0.5)
• CPD-based context sampling for best performance
• Attention weight visualization for interpretation

Architecture Variants

X-Trend (Base)

• Objective: Direct Sharpe optimization
• Heads: Position only (no forecast)
• Best for: Pure trading performance

X-Trend-G (Gaussian)

• Objective: Joint MLE + Sharpe (α=1.0)
• Forecast: Gaussian (μ, σ)
• Best for: Balanced forecasting + trading

X-Trend-Q (Quantile)

• Objective: Joint Quantile + Sharpe (α=5.0)
• Forecast: 13 quantiles
• Best for: Robust predictions, tail risk

Related Skills

• financial-time-series - Input features, returns, momentum
• few-shot-learning-finance - Episodic training, context construction
• change-point-detection - CPD for context set improvement

Reference Files

• [IMPLEMENTATION.md](IMPLEMENTATION.md) - Complete PyTorch implementations of all components
• [TRAINING.md](TRAINING.md) - Training loop, loss functions, hyperparameters, best practices

References

• Attention Is All You Need (Vaswani et al. 2017)
• Deep Momentum Networks (Lim, Zohren, Roberts 2019)
• Attentive Neural Processes (Kim et al. 2019)
• X-Trend: Cross-Attentive Time-Series Trend Network (Wood, Kessler, Roberts, Zohren 2024)

Last Updated: Based on X-Trend paper (March 2024) Skill Type: Architecture + Implementation Line Count: ~370 (under 500-line rule ✅)